# Running the Risk: Road-Crossing Behavior in Wild Chimpanzees (Pan troglodytes) in an Anthropogenic Habitat in Uganda

Marie Tellier, François Druelle, Marie Cibot, Johnmary Baruzaliire, Tom Sabiiti, Matthew Mclennan

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## To cite this version:

Marie Tellier, François Druelle, Marie Cibot, Johnmary Baruzaliire, Tom Sabiiti, et al.. Running the Risk: Road-Crossing Behavior in Wild Chimpanzees (Pan troglodytes) in an Anthropogenic Habitat in Uganda. American Journal of Primatology, 2025, 87 (2), pp.e70000. 10.1002/ajp.70000. hal-05191110

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RESEARCH ARTICLE

OPEN ACCESS

# Running the Risk: Road-Crossing Behavior in Wild Chimpanzees (Pan troglodytes) in an Anthropogenic Habitat in Uganda

Marie Tellier¹,² ID | François Druelle³,⁴ ID | Marie Cibot²,⁵ ID | Johnmary Baruzaliire² | Tom Sabiiti² |
Matthew R. McLennan²,⁶ ID

¹Oniris - Ecole Nationale Vétérinaire de Nantes, Agroalimentaire et de l'Alimentation, Nantes, France | ²Bulindi Chimpanzee and Community Project, Hoima, Uganda | ³UMR 7268 ADES, Aix-Marseille Université, CNRS, EFS, Marseille, France | ⁴Functional Morphology Laboratory, University of Antwerp, Antwerp, Belgium | ⁵Solâme, Lorient, France | ⁶Faculty of Humanities and Social Sciences, Oxford Brookes University, Oxford, UK

**Correspondence:** Matthew R. McLennan (mclennan.bccp@gmail.com)

**Received:** 24 October 2024

| **Revised:** 30 December 2024

| **Accepted:** 3 January 2025

**Funding:** Friends of Chimps; Jane Goodall Institute-Switzerland; Blair Drummond Safari Park; Born Free Foundation.

**Keywords:** anthropogenic habitat | behavioral flexibility | *Pan troglodytes* | risk perception | road crossing | roads

## ABSTRACT

Recent research highlights the behavioral flexibility of wild chimpanzees in response to human-induced changes in their environment, including agricultural and infrastructural development. The expansion of road networks threatens chimpanzee populations across Africa. Studying their road-crossing behavior, especially outside protected areas where road impacts are greatest, helps identify factors influencing their choices and flexibility. This study seeks to gain a deeper understanding of how chimpanzees navigate busy roads and assess the danger posed by roads. Such insights are needed to develop effective conservation strategies in regions facing escalating human impact, including recommendations for the design and management of traffic on existing and future roads. Using a dataset of 129 video-recorded road crossings spanning 38 months, we analyzed the behavioral adjustments of chimpanzees in Bulindi, Uganda, when crossing a recently paved, busy main road within their home range. Using generalized linear mixed models, we investigated chimpanzee risk perception, protective and cooperative behaviors, vigilance, and progression order during road crossings. We identified variations in their behavior according to age-sex of individuals, group composition, and level of risk. We found that Bulindi chimpanzees exhibit behavioral strategies to reduce risks of collision or close encounters with humans on the road, as previously described. However, they were less vigilant than expected. We suggest that the chimpanzees have developed tolerance of the risks presented by the road, owing to their long history of crossing it before it was tarmacked and widened, and their familiarity with local people and motor traffic. Our results provide further evidence of the flexibility of wild chimpanzees. However, road crossings remain highly risky for large mammals like great apes, necessitating measures to mitigate the impact of road development on this and other endangered species (e.g. speed bumps, police enforcement, public awareness raising).

**Abbreviations:** AIC, Akaike Information Criteria; ANOVA, Analysis of Variance; BCCP, Bulindi Chimpanzee and Community Project; BORIS, Behavioral Observation Research Interactive Software; GLMM, Generalized Linear Mixed Model.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *American Journal of Primatology* published by Wiley Periodicals LLC.
## Summary

* The expansion of road networks presents a major threat to wild chimpanzees.

* We studied chimpanzee behavior during 129 recorded road crossings at Bulindi, Uganda. We found behavioral patterns varied by level of risk, group composition and age-sex of individuals.

* Bulindi chimpanzees show behavioral strategies to reduce risks during road crossings. Yet the chimpanzees were less vigilant than expected, suggesting behavioral tolerance toward the road.

# 1 | Introduction

Human impact on the environment and the stability of ecosystems continues to expand through activities including logging, mining, farming and infrastructure development. In this context, most wild chimpanzee (Pan troglodytes) habitats in Africa have been human-transformed in past decades, leading to a drastic decline in their populations (Plumptre et al. 2010; Humle et al. 2016; Kühl et al. 2017). A marker of human encroachment on wildlife habitats is the expansion of road networks, and a 35%-60% increase over the 2010 global road network length is expected by 2050 (Dulac 2013). Roads adversely affect wildlife survival across multiple levels (Jaeger et al. 2005; Bennett 2017). Chimpanzees are no exception. For example, only 4.3% of the western chimpanzees' geographical range is currently unimpacted by the presence of roads (Andrasi et al. 2021), and the same can be imagined for other chimpanzee subspecies considering the rapid development occurring over much of tropical Africa (Arcus Foundation 2018). Therefore, while road construction supports economic development across the continent (Laporte et al. 2007; Fayissa, Nsiah, and Tadasse 2008; Weng et al. 2013), it is also a threat to endangered mammals like chimpanzees because roads divide and fragment their habitats (e.g. Garriga et al. 2019; McLennan et al. 2021), facilitate poachers' access to their territories (Laurance et al. 2006; Vanthomme et al. 2013; Perumal et al. 2021), increase health risks due to plastic and atmospheric pollution (Krief et al. 2020) and zoonosis transmission (Namusisi et al. 2021), and expose them to the risk of collision with motorized traffic (McLennan and Asiimwe 2016; Krief et al. 2020; McLennan et al. 2021). In that respect, it is necessary to understand how wild chimpanzees adapt behaviorally to road and traffic infrastructures in their home range to better characterize the impact roads have on this great ape species and to assess their long-term ability to persist in environments with roads. Such insights are urgently needed to implement or adapt conservation measures for this endangered species, as applied to traffic control and road design and construction (Laurance et al. 2014; Perumal et al. 2021).

Recent research has highlighted the behavioral flexibility of chimpanzees, including their responses to anthropogenic habitat changes (Hockings et al. 2015; McLennan, Spagnoletti, and Hockings 2017). For example, chimpanzees readily incorporate agricultural crops into their diets (e.g. McLennan 2013; Hockings, Anderson, and Matsuzawa 2012; Hockings and McLennan 2012; Couturier et al. 2022) and use exotic and

planted species for nest construction (McCarthy, Lester, and Stanford 2017; McLennan et al. 2021). In “risky” habitats or situations, chimpanzees may modify their grouping patterns by moving in smaller subgroups (Satsias et al. 2022), adjust their activity budget by reducing resting time (Hockings, Anderson, and Matsuzawa 2012), nest more cohesively (van Dijk, Cibot, and McLennan 2021), modify their vocal behavior by vocalizing less often in areas with high human presence (Wilson, Hauser, and Wrangham 2007; Hicks, Roessingh, and Menken 2013), or shift their crop-feeding activities to nighttime (Krief et al. 2014; Lacroux et al. 2022) to reduce the risk of encountering humans. These behavioral adjustments also include responses to novel situations that carry potential risks to health and survival such as increased contact with people (McLennan and Hill 2010) and domestic animals (Fryns et al. 2021), or the presence of roads to cross (Hockings, Anderson, and Matsuzawa 2006).

Paved and unpaved roads run through the ranges of various studied chimpanzee populations, in both protected (e.g. Cantanhez National Park in Guinea-Bissau: Hockings and Sousa 2013; Bersacola, Hill, and Hockings 2021, and Kibale National Park in Uganda: Cibot et al. 2015; Krief et al. 2020) and unprotected areas (e.g. Bossou in Guinea: Hockings, Anderson, and Matsuzawa 2006; Lawana in Sierra Leone: Garriga et al. 2019; and Bulindi in Uganda: McLennan and Asiimwe 2016). Chimpanzees cross roads to access dispersed food resources within their home ranges (Reynolds, Wallis, and Kyamanywa 2003; Hockings, Anderson, and Matsuzawa 2006; Cibot et al. 2015) or when females migrate between groups (McLennan et al. 2021). However, road-crossing behavior in chimpanzees has been analyzed at two study sites only, Bossou (Hockings, Anderson, and Matsuzawa 2006; Hockings 2011) and Sebitoli (Cibot et al. 2015), with both studies based on around 30 direct observations of crossings. These studies showed that chimpanzees perceive roads as dangerous and can adapt their behavior to reduce the risks posed by road users (including vehicle traffic and pedestrians), for example by looking right and left before and during crossing, having adult males occupy the riskiest positions, paying attention to conspecifics, and waiting before crossing (Cibot et al. 2015).

In western Uganda, current infrastructural development including road upgrades (Uganda National Roads Authority 2023) pose a threat to many wild chimpanzee communities, especially those living outside protected areas (McLennan et al. 2021). Here, we focus on the chimpanzees in Bulindi, Hoima District. The Bulindi chimpanzee community provides an opportunity to investigate potential resilience of chimpanzees to anthropogenic landscape change, as it has lived for decades in an unprotected and highly human-impacted area (McLennan and Hill 2012). Their home range is a mosaic of riverine forest fragments, croplands, village homes, and trading centers, bisected by a busy two-lanes-wide road linking two major urban centers. The chimpanzees have crossed this road for generations, despite increasing traffic in recent years (McLennan and Asiimwe 2016). This road poses a significant danger to chimpanzees and other wildlife, especially since it was upgraded in 2018 (widened and tarmacked), allowing vehicles to travel faster than before.

Our aim in this study was to analyze the choices and behavioral flexibility of wild chimpanzees when faced with the risk of crossing a busy main road, and to determine the factors that
influence their road-crossing behavior as a group or individu-
ally, using a 3-year database of video material. We tested the
following hypotheses:

**Hypothesis 1—Age-sex influences on crossing behavior:** road crossing behavior varies according to sex and age class. Specifically, as described in chacma baboons (*Papio ursinus*; Rhine and Tilson 1987) and Bossou and Sebitoli chimpanzees (Hockings, Anderson, and Matsuzawa 2006; Cibot et al. 2015), we assume that mature males are most likely to take up forward and rearward positions (considered the riskier positions in previous studies), especially when the proportion of vulnerable individuals (immatures, mothers with dependents, injured individuals) in the crossing group increases (1.1). Similarly, mature males should be most likely to display cooperation and attention towards more vulnerable individuals during road crossings, such as waiting for conspecifics (Cibot et al. 2015) or exhibiting reassuring or protective behavior towards them, to enhance group protection (1.2). Generally, mature male chimpanzees are described as being bolder (Hockings 2007; Wilson, Hauser, and Wrangham 2007; Bertolani and Boesch 2008; McLennan and Hill 2010; Satsias et al. 2022). Consequently, females and immatures should display more vigilance than mature males by road crossing less frequently, waiting longer before crossing, crossing at a faster gait, and showing more apprehensive and cautious behaviors [e.g. traffic checking, avoidance; (1.3)].

However, because females spend more time with males when they are sexually receptive, i.e. when they display swellings of their anogenital region (Matsumoto-Oda 1999; Pepper, Mitani, and Watts 1999), we expect their behavior to vary during their sexual cycle, and we hypothesize that females with anogenital swellings should behave in a similar way to males, i.e. less vigilantly (1.4).

**Hypothesis 2—Chimpanzee risk assessment:** because chim-
panzees can assess the risk of road crossings (Hockings 2011;
Cibot et al. 2015), they adjust the way they cross to the degree of
risk involved. Specifically, we predict that they should choose to
cross when there is less traffic (2.1) and at safer crossing points,
i.e. road sections with good visibility (low gradient and no bend),
high vegetal coverage on at least one side of the road (allowing
chimpanzees to hide before or disappear from view quickly after
crossing), and reduced vehicle speeds [due to the presence of
speed humps; (2.2)]. Additionally, as the level of risk presented by
the road increases (higher traffic intensity, reduced visibility of
oncoming traffic and detectability of chimpanzees; Hockings,
Anderson, and Matsuzawa 2006), chimpanzees are expected to be
more vigilant (2.3), i.e. individual waiting times before crossing
should increase, chimpanzees should cross faster and show more
cautious behaviors.

**Hypothesis 3—Social influences on crossing behavior:** chimpanzee behavior during road crossings varies with the size and composition of the crossing group (Cibot et al. 2015). Larger groups may take more time to organize themselves in potentially risky situations, such as crossing a road (Hockings 2011). Therefore, we hypothesize that as group size increases, waiting time before crossing will also increase (3.1). Furthermore, humans cross roads more rapidly in small groups than when many

people cross (Gates et al. 2006; Peters et al. 2015; Onelcin and Alver 2017). Thus, we expect the chimpanzees to cross faster in smaller groups to limit their exposure time on the road (3.2). We also predict that they will show more cautiousness in smaller crossing groups (3.3).

2 | Methods

2.1 | Ethical Note

These observations of wild chimpanzees were noninvasive; observers strived to maintain a minimum distance from the chimpanzees of 7 m and avoided interacting with them. All national and international guidelines were followed. This research adhered strictly to ethics guidelines detailed by the Association for the Study of Animal Behaviour (UK) and the American Society of Primatologists Principles for the Ethical Treatment of Nonhuman Primates. The study was approved by the Uganda Wildlife Authority and the Uganda National Council for Science and Technology.

## 2.2 | Study Site and Subjects

Bulindi (1°28′N, 31°28′E) is located in Hoima District, western Uganda, midway between the Budongo and Bugoma Central Forest Reserves (Figure 1). Since the 1990s, most forest outside the reserves has been converted to farmland (McLennan 2008; Mwavu and Witkowski 2008), and the human population has grown rapidly (156 persons per km² in 2014; Uganda Bureau of Statistics 2016). However, more than 260 chimpanzees comprising 10 or more communities survive in fast-changing human-dominated habitats outside the reserves, including the Bulindi chimpanzee community (McLennan 2008; McCarthy et al. 2015).

Chimpanzees at Bulindi were first studied in 2006–2008, when the community comprised about 30 unhabituated individuals (McLennan and Hill 2010). Research has been ongoing since 2012 and the chimpanzees were habituated by 2015 (McLennan et al. 2019). The present study covered the period from November 2018 to December 2021 (38 months). During this period, community size varied from 20 to 22 individuals (Table 1). We estimated the age class of individuals born before 2012 from body size and other physical and behavioral characteristics. At the start of the study, the community comprised 20 individuals including 3 adult males (≥ 12 years old), 5 adult females (≥ 12 years old or at first birth, if earlier), 4 sub-adults or adolescents (≥ 8 years old), 4 juveniles (≥ 4 years old) and 4 infants (newborn to 3 years old) (Sugiyama 2004). Some individuals transitioned to an older age class during the study (e.g. 3 juveniles entered adolescence; 3 subadult males became young adults; Supporting Information [SI]: Table S1). At the end of the study, community size was 22 individuals including 5 adult males, 6 adult females, 3 sub-adults, 4 juveniles and 4 infants. The alpha male at the start of the study (SL) disappeared after December 2019 and was replaced by MO, who was alpha male during the remainder of our study. For behavioral analyses, we chose to lump late-subadults (from 10 years old) with adults, hereafter referred to as “mature
<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | Location of Bulindi, Hoima District. (a) Map of Uganda showing the location of Hoima District in western Uganda. (b) Map (adapted from Cibot et al. 2019) showing the location of the Bulindi study site (encircled), midway between the Budongo and Bugoma forests in the north and south, respectively. (c) Satellite map of the Bulindi area (Bulindi: 1°29' N, 31°28' E, Hoima-Masindi Road) (map adapted from Google Earth). The chimpanzees' 20 km² home range consists of small fragments of riverine forest and papyrus swamps (visible in dark green on the map) that are surrounded by crop fields, exotic wood plantations, homes and village areas. The core of their home range is represented by the white dashed oval (ca. 4 km²). It is bisected by a major road (visible at the center) that connects Hoima city to Masindi town. Numerous secondary roads and village paths also run through the area. The portion of road crossed by chimpanzees during this study is framed in yellow.</figcaption></figure>

<table><caption>**TABLE 1** | Number of individuals in each age-sex class in the Bulindi chimpanzee community and community size per year (calculated at year-end) during the study period.</caption><thead><tr><th>YEAR</th><th>2018<sup>a</sup></th><th>2019</th><th>2020</th><th>2021</th></tr></thead><tbody><tr><td>MATURE MALES (10+)</td><td>3</td><td>6</td><td>5</td><td>5</td></tr><tr><td>MATURE FEMALES (10+)</td><td>5</td><td>5</td><td>6</td><td>6</td></tr><tr><td>IMMATURES (4+)</td><td>8</td><td>5</td><td>7</td><td>7</td></tr><tr><td>INFANTS (0-3)</td><td>4</td><td>6</td><td>3</td><td>4</td></tr><tr><td>COMMUNITY SIZE</td><td>20</td><td>22</td><td>21</td><td>22</td></tr></tbody></table>

individuals”, and young sub-adults (8–9 years old) with juveniles, hereafter referred to as “immature individuals”. Socially and behaviorally, late subadult males at Bulindi are prominent in social networks (Satsias et al. 2022), range independently of their mothers, and frequently lead travel parties, in the same way as adult males. This is not the case with young subadults. Table 1 provides information about the composition of the population during the study period.

Between 2006 and 2014, about 80% of forest in Bulindi was cleared entirely for farming (McLennan et al. 2020), shrinking main forest fragments used by the chimpanzees from ca. 122 to 26 ha and leading to an increase in their use of farmland for traveling and

foraging. Remaining forest occurs patchily on either side of a 10.5 m-wide major road (Figure 1) that connects Hoima city and Masindi town—two main urban centers with over 110,000 inhabitants each (Uganda Bureau of Statistics 2016). This road, which was widened and tarmacked in 2018 in response to infrastructure needs including oil sector development occurring regionally (Mawejje 2018; Ogwang and Vanclay 2019; McLennan et al. 2021), receives high volumes of motor vehicles (motorbikes, cars, taxis, buses, trucks, tourist vehicles) along with cyclists and pedestrians. Living in this highly human occupied area, Bulindi’s chimpanzees are confronted with the frequent need to cross this busy road to access food sources located on either side (McLennan and Asiimwe 2016). One fatality involving an adult female which collided with a taxi vehicle prior to the upgrading of the road has already been reported (McLennan and Asiimwe 2016). During our study, two speed humps were installed by the road authorities on the section of the road where this female died and where the chimpanzees often crossed (Figure 2) to slow down vehicles and reduce the risk of accidents. These humps were under construction from late-2018, including use of temporary humps made from sand, which necessitated vehicles to slow down; the permanent, single cement humps were completed by 2020.

## 2.3 | Data Collection and Analysis

### 2.3.1 | Fieldwork: Data Collection

Road crossing observations took place on a 1-km stretch of the Hoima-Masindi road. We divided the road into two sections

<footnote><sup>a</sup>Two subadult males who were classed as 'mature' from 2019, featured in a single video in November 2018 when they were still 'immature' according to our definition (≤ 9 years old). For convenience, we treated them as mature in that video clip.</footnote>
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | Map showing the 1 km stretch of road where Bulindi's chimpanzees habitually cross. Zone A is delimited by yellow lines and zone B by pink lines. Two speed humps, 180 meters apart, are marked with yellow asterisks. Notable bends affecting visibility are indicated by white arrows. Dark green patches to the north of the road and on either side of the road at the far right of the map are fragments of riverine forest, regenerating forest, and papyrus swamp (map adapted from Google Earth).</figcaption></figure>

which we perceived presented the chimpanzees with different
levels of danger (Figure 2):

- Zone A (in yellow): 200 m long; boundaries are the speed humps at the west and east sides. There is a road bend just beyond the western hump. We considered this section as a comparatively “safe” zone because traffic speed is limited by the two speed humps positioned 180 meters apart. The vegetation cover is dense on the northern side of the road (regenerating forest), and medium on the southern side (fallow fields and bushes), providing cover for the chimpanzees prior to and after crossings.

- Zone B (in pink): 730 m long; boundaries are the road bend and speed hump on the eastern side (separating zones B and A). On the western side, the boundary was set at the most westerly point where chimpanzees were observed crossing. We considered this section as a “high-risk” zone because, while vehicles are slowed down by the hump on the east side, they can accelerate quickly and vehicles can travel at high speed on the remaining section. Moreover, two road bends affect the visibility of oncoming traffic in this section (Figure 2). There is medium (bushes) to low vegetation cover (open croplands and gardens) on both sides of this section, making it difficult for chimpanzees to approach the road surreptitiously or stay hidden by the roadside.

During the study period, the chimpanzees were followed 5 to
7 days per week by field assistants JB and TS. Both assistants
were present during morning hours, while only JB worked
during afternoon and evening hours. Field assistants recorded
party (subgroup) composition and activity every 30 min (party
scans; see Satsias et al. 2022) and noted the time of road
crossings when these occurred between scans. Road crossings
were filmed opportunistically by the field assistants using a
digital camera (Table S3), sometimes with an alternate view
filmed by MRM. When a crossing seemed imminent (e.g. as
indicated by the direction of chimpanzees' travel towards the

road), where possible field assistants would try to get ahead of
the chimpanzees and film the crossing, usually by waiting at the
road near to where the chimpanzees seemed most likely to
cross. Sometimes the field assistants would actively intervene to
encourage drivers to slow down or advise pedestrians to wait so
that the chimpanzees could cross safely. Some videos did not
capture the entire crossing, as the assistants could not always
reach the road before a crossing began or because individuals
crossed at different locations on the road. These videos, repre-
senting 22 separate road crossings, were not retained in the
dataset for behavioral analysis. For these reasons, and due to
the fission-fusion social organization of chimpanzees which
meant the field assistants normally followed only a portion of
the community, many crossings were undoubtedly missed.
Crossings were recorded no earlier than 0700 h and no later
than 1900 h, corresponding to field assistants' working hours, as
well as the times chimpanzees typically leave their nests in the
morning and build their nests in the evening. Additionally, the
chimpanzees were not normally followed from 1200 to 1400 h,
and sometimes not relocated until 1430 h or later. Thus,
crossings during early afternoons were rarely observed. How-
ever, it is worth noting that Bulindi's chimpanzees are unlikely
to cross the road in the early afternoon as they usually rest in
undergrowth from late morning to about 1600 h (unpublished
data). Overall, our dataset represents a sample of crossings that
occurred during the study period; while these were recorded
opportunistically, we believe they are sufficiently representative
of the chimpanzees' road crossing behavior.

2.3.2 | Video Analysis and Behavioral Observations

Behavioral data collection from the videos was carried out using
the Behavioral Observation Research Interactive Software
(BORIS, v. 7.11.1). All behaviors and descriptions considered in
this study are listed in the ethogram in Table S2.

A "road-crossing event" was defined as the observation of at
least one chimpanzee crossing the road (Cibot et al. 2015).
"Total crossing-group size" was the total number of individuals
excluding infants that crossed during a road-crossing event. It
was considered a "single crossing-event" when two (or more)
subgroups of chimpanzees crossed the road in the same direc-
tion, on the same stretch of road, within 30 min (modified from
Cibot et al. 2015). A chimpanzee was considered to be in the
same subgroup as the preceding individual if they crossed the
road within 1 min of each other at the same location. Hereafter,
we will refer to the crossing group as a "(sub)group", depending
on whether the chimpanzees crossed as a single group or in two
or more sub-groups. The total duration of a crossing event—the
"crossing duration"— extends from the moment the first
chimpanzee (or leader) steps on to the asphalt to the moment
the last individual to cross leaves the asphalt. The "individual
waiting time" was the elapsed time between the start of the
crossing by the first chimpanzee in the (sub)group, and the start
of a subsequent individual's crossing (modified from Cibot
et al. 2015). Leaders were therefore excluded from this analysis.

For each crossing event we noted the date and time; location
(i.e. zone A or B); direction and extent of cover on both sides of
the road (i.e. low vegetation cover is open croplands or gardens;
medium cover is fallow fields or bushes; dense cover is re-
generating forest); any interventions by the field assistants (i.e.
vocalizing or gesturing to road users, or adopting a guarding
posture on the road); the total crossing-group size (excluding
infants, i.e. all individuals visible in the video that were
≥4 years old at the end of the video year, the age that first
independent road crossings of young chimpanzees are observed
at Bulindi) and subgroup size when the chimpanzees did not
cross all together; the composition of the total crossing-group
and subgroups; and the order in which group members crossed.
To gain a measure of the intensity of motor and non-motor
traffic at the time chimpanzees chose to cross the road, we
counted the number of motor vehicles (motorbikes, cars, taxis,
buses, trucks) and non-motor road users (cyclists and pedes-
trians) visible per minute during recordings (i.e. when chim-
panzees had approached the road and a crossing was
imminent). Road users that were clearly audible (i.e. close to
the camera and/or interacting with field assistants) but not
visible in the camera field were also taken into account, even
though the type of vehicle or the number of pedestrians could
not always be identified (the default value was 1). These values
were minimum estimates because we could not assess all traffic
out of the camera range. Time of crossing and composition of
(sub)groups were corroborated by the field assistants' scan data
and written notes. Each crossing individual was assigned an
age-sex class (mature female, mature male, or immature indi-
vidual under 10 years old). The anogenital (estrus) swelling
status of each mature female was scored daily for the duration
of the study: "0" = no swelling, "1" = partially swollen and
"2" = maximally swollen (Satsias et al. 2022).

An individual's behavior was analyzed from the moment they
appeared in the video (or from the beginning of the video if they
were already visible) until the moment they disappeared from
the camera field, or if applicable until the end of the recording.
For directed behaviors involving another individual, the iden-
ty of the receiver or emitter was also noted. Individual waiting
times could not be calculated for all videos and subjects because
the roadside where individuals crossed from was not always
visible in the camera field. However, the dataset of usable vi-
deos was sufficient to conduct a meaningful analysis of waiting
times (N = 66 videos representing 416 individual waiting times).

We defined two crossing gaits according to Cibot et al. (2015): fast gait (running, including intermediate speeds such as “ambling,” defined by Schmitt et al. [2006] as a fast walking gait) or slow gait (walking). Crossing gait was scored for each individual, and we noted any lameness caused by temporary injuries. For dependents (i.e. infants and young juveniles), we noted if they were carried by their mother for at least part of the crossing. We defined “vulnerable individuals” as immatures (slower, less experienced, less visible to drivers), females with dependents (slower, vigilance can be affected by the dependent) and injured individuals (individuals with temporary injuries that cause them to limp during locomotion and cross slower). We further identified the following behavioral categories (Table S2): protective and cooperative behaviors, such as reassurance gestures, individuals adopting guarding postures or waiting for conspecifics (excluding dependents waiting for their mother); apprehensive and cautious behaviors, such as traffic checking, retreating or avoidance; and reckless behaviors, such

as stopping in the middle of the road or charging displays
performed during the crossing.

2.3.3 | Statistical Analysis

2.3.3.1 | Evaluating the Extrinsic Factors. We first compared traffic intensity during crossings (i.e. the number of motor vehicles and non-motor road users per minute) in each video recording according to the time of day (5 time slots of 2 h between 0700 and 1900 h) to assess if traffic intensity during chimpanzee crossings varied significantly over the course of the day.

2.3.3.2 | **Testing Our Hypotheses.** According to *Hypothesis 1*—*Age-sex influences on crossing behavior*, we expect variation in road crossing behavior according to sex and age class. Overall, we expect that females and immatures will display more vigilance than mature males; hence, they should cross the road less frequently than mature males. We compared the rate of taking part in road crossing per crossing event for each category of individual (immatures, mature females, mature males), to test whether any category crossed more frequently than others. Because these data were not normally distributed (using Shapiro-Wilk normality test), we used analysis of variance (ANOVA) with permutation tests (non-parametric ANOVA). If the ANOVA was significant, we applied post hoc permutation tests for independent samples and corrected the p-values (P') with the Bonferroni correction (to avoid type I error, i.e. false-positives). We used the *aovp* function in the *lmPerm* package (Wheeler and Torchiano 2016) for the permutation tests.

As we differentiated the two crossing zones according to level of
danger, we compared the observed age-sex composition of
crossing groups in each zone with the expected theoretical
proportion (i.e. mean proportion over the study period of each
age-sex category in the total group) using Chi-square tests. In
the same way, we compared crossing rates of females in dif-
ferent anogenital swelling stages between the two zones, as we
expect their behavior to vary during their sexual cycle. Specif-
ically, we expect females to cross in a similar way to males, i.e.
less vigilantly, when maximally swollen.

Furthermore, we tested our different hypotheses by fitting gen-
eralized linear mixed models (GLMM) using the *glmer* function in
the R package *lme4* (Bates et al. 2015). Several models were fitted
including alternatively, as the dependent variable, an individual's
position [binary outcome: being in a risky position (1) or in a
protected position (0)], protective behavior [binary outcome:
showing protective behaviors (1) or not (0)], crossing gait [binary
outcome: traveling fast (1) or not (0)], cautious behavior [binary
outcome: showing apprehensive/cautious behaviors (1) or not (0)]
and waiting time (continuous variable; note that waiting time
varies from 1 s to 7 min, so we log-transformed this variable to
ensure normality and homoscedasticity assumptions). We chose
the Binomial or the Gaussian family depending on the nature of
the dependent variable.

In general, we expect that mature females and immatures will
wait longer before crossing, travel at a faster gait to limit their
exposure time on the road, and exhibit more cautious behaviors.
Individual chimpanzees and crossing events were both added as random effects to take into account possible correlations amongst observations of the same individual and amongst observations of individuals in the same crossing event (pseudo-replication). We compared the fit of the full model with that of the null model comprising only the random effects using the Akaike Information Criteria (AIC; Onyango 2009) coupled to an ANOVA. In addition, we expect that mature males will adopt riskier positions during crossings (i.e. forward or rearward positions) than other age-sex categories (fixed effect), and should display more protective behaviors towards vulnerable individuals (immatures, mothers with dependents, injured individuals; fixed effect), especially when the proportion of vulnerable individuals in the crossing group increases. We thus included an interaction between age-sex category and the proportion of vulnerable individuals. Here, we compared the fit of the full model, including age-sex category and the interaction with the proportion of vulnerable individuals, with that of the null model comprising only the fixed control effect (age-sex category) and random effects. We also tested the influence of anogenital swelling status (i.e. '0' = no swelling, '1' = partially swollen and '2' = maximally swollen) on the crossing behavior of females, and the effect of carrying a dependent. Additionally, we examined the effect of field assistant interventions on chimpanzees' cautious behavior.

According to *Hypothesis 2*—*Chimpanzee risk assessment*, we expect that chimpanzees adjust how they cross to the degree of risk involved. We expect that traffic intensity during crossings and crossing zone (fixed effects) will have a significant influence on individual waiting times (after the first chimpanzee to cross), as well as on crossing gaits and cautiousness of individuals. The individual's age-sex category was included as a fixed control factor. Individual chimpanzees and crossing events were added as random effects when testing the influence of traffic intensity, whereas only individual chimpanzees were added as random effects when testing the crossing zone because crossing events are nested within the crossing zone (i.e. crossing events always occurred in one zone only). The extrinsic predictors (traffic intensity or crossing zone) were tested on each dependent variable, i.e. an individual's waiting time, crossing gait and cautious behavior. We compared the fit of the full models with those of the null model comprising the fixed control effect (age-sex category) and random effects.

According to *Hypothesis 3*—*Social influences on crossing behavior*, we expect that chimpanzee behavior during road crossings varies with the size and composition of the crossing group. We expect that (sub)group size (fixed effect) and the proportion of mature males (fixed effect) will influence an individual's waiting time, crossing gait and cautiousness. Age-sex category was included as a fixed control factor and individuals were added as random effects. The crossing event was not included as a random effect as the fixed effects do not vary within crossing events. We compared the fit of the full model [including (sub)group size or proportion of mature males] with that of the null model comprising the fixed control effect (age-sex category) and the random effect.

All statistical procedures were performed using R (v.4.0.3; R Core Team 2022) and the significance threshold was set at *p* < 0.05.

## 3 | Results

The dataset comprised a total of 176 videos of chimpanzees crossing the road, recorded on 111 different days and representing 129 distinct road-crossing events. Among these events, we recorded 190 crossing (sub)groups. The dataset represents 6.25 h of video material and the duration of individual videos ranges from 14 s to 16 min with an average duration of 2.1 ± 2.4 (mean ± SD) minutes.

Over the 38-month study, we made video recordings of chimpanzees crossing the road an average of 4.2 ± 3.0 (range 0–12 crossing events) times per month. Road crossings were recorded mainly in morning hours between 0700 and 0900 (33.3% = 43/129) and in the evening between 1700 and 1900 (31.8% = 41/129). No recordings were made between 1300 and 1500 h.

### 3.1 | Extrinsic Factor: Traffic Intensity During Crossings

Mean traffic intensity per minute during crossings was 0.8 ± 1.4 (range 0–13.2) pedestrians, 0.2 ± 0.6 (range 0–5.1) cyclists, and 2.4 ± 1.8 (range 0–12.7) motor vehicles, with motorbikes being the most frequent vehicles representing 63.2% (592/936) of motor road users. Traffic intensity during crossings was highest between 9:00–11:00 and 17:00–19:00 but did not differ significantly depending on the time of day (nonparametric ANOVA: number of motor vehicles per minute: *F*<sub>4</sub> = 0.378, *p* = 0.824; number of pedestrians and cyclists per minute: *F*<sub>4</sub> = 0.820, *p* = 0.514; Figure 3). Most crossings occurred between 7:00 and 9:00 when traffic intensity was relatively low and 17:00–19:00 when it was relatively high (Figure 3). Thus, chimpanzees did not appear to adjust the time of their road crossings according to traffic intensity, but instead selected early morning and late afternoon and evening hours to cross.

### 3.2 | Overall Crossing Behavior

Overall, 34.9% of the mature males, 24.8% of mature females and 27.4% of immatures in the community crossed the road per event recorded. Mature males were recorded crossing significantly more often than females (Permutation test: *F*<sub>1</sub> = 12.72, *P*′ = 0.001) and immature chimpanzees (Permutation test: *F*<sub>1</sub> = 6.317, *P*′ = 0.025). Overall, the chimpanzees crossed preferentially in the safer zone A compared to the riskier zone B. Of 129 crossing events in the dataset, 49 took place in zone B (38.0%), which is 3.7 times longer in length than zone A (see Figure 2), and 80 crossing events took place in zone A (62.0%) (χ<sup>2</sup> test for given probabilities: χ<sup>2</sup><sub>2</sub> = 7.450, *p* = 0.006). Individuals waited an average of 39.4 (range 0.1–421.4) seconds before crossing after the (sub)group leader. We observed an average of 8.9 ± 7.3 cautious behaviors per crossing event, primarily consisting of traffic checking (1103/1143 occurrences; 96.5%). Remaining cautious behaviors comprised avoidance and retreat (40/1143 occurrences; 3.5%).

Over the study period, the average proportion of individuals in the different age-sex classes was as follows (excluding infants):
<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | Barplots showing the percentage of road crossing events in relation to traffic intensity (the average number of road users [motorized vehicles, cyclists, and pedestrians] per minute during crossings) at different times of day. Sample size of crossing events (N = 129): 7:00–9:00 = 43 events; 9:00–11:00 = 22; 11:00–13:00 = 12; 15:00–17:00 = 11; 17:00–19:00 = 41). No crossing events were observed between 13:00 and 15:00 h, thus information on traffic intensity was not available.</figcaption></figure>

29.9% were mature males, 33.3% were mature females and 36.8% were immatures. The observed distribution of these categories when crossing in zone B was 59.5% for males, 17.2% for females, and 23.3% for immatures. This distribution differed significantly from the average distribution of age-sex categories in the community ($\chi^2$ test for given probabilities: $\chi_2^2 = 94.37, p < 0.0001$), while the observed distribution of these age-sex categories when crossings in zone A did not differ from the community average (29.5% for males, 33.2% for females and 37.3% for immatures; $\chi^2$ test for given probabilities: $\chi_2^2 = 0.067, p = 0.967$). Thus, mature males were more likely to cross in the riskier zone B than females and immatures.

A summary of the following results from the GLMM analyses is shown in Table 2.

## 3.3 | Hypothesis 1—Age-Sex Influences on Crossing Behavior

Overall, there was a general effect of age-sex category on crossing gait, cautious behavior, and waiting time (null model comparison: $\chi^2 = 19.83$, df = 2; $p < 0.0001$; $\chi^2 = 22.27$, df = 2; $p < 0.0001$; $\chi^2 = 8.85$, df = 2; $p = 0.012$; respectively). According to the models, the likelihood of a mature male crossing the road at a fast gait (running or ambling) was 21%, while it was 61% and 85% in mature females and immatures, respectively. Thus, mature males crossed at a slower pace compared to mature females and immatures. The likelihood of an individual showing cautious behavior was 67% in immatures, compared to 81% and 88% in mature females and mature males, respectively (Figure 4). The proportion of vulnerable individuals in the crossing (sub)group had no influence on cautious behavior ($\chi^2 = 2.13$, df = 1; $p = 0.14$).

There was an effect of age-sex category on an individual's position when crossing (i.e. risky or protected; null model comparison: $\chi^2 = 19.07$, df = 2; $p < 0.0001$), but including the interaction with the proportion of vulnerable individuals in the

crossing (sub)group had no influence ($\chi^2 = 5.65$, df = 3; $p = 0.13$). According to the significant model, the likelihood of an individual crossing in a risky position was 11% in immatures, 30% in mature females, and 41% in mature males (Figure 4).

We observed a significant effect of age-sex category on protective behaviors (null model comparison: $\chi^2 = 47.18$, df = 2; $p < 0.0001$), but the proportion of vulnerable individuals in the crossing (sub)group had no influence ($\chi^2 = 0.19$, df = 3; $p = 0.98$). The likelihood of an individual exhibiting protective behaviors was 8% in immatures, 21% in mature females, and 36% in mature males (Figures 4 and 5). Mature males presented significantly more protective behaviors (estimate = 0.74, $z$ value = 3.012, $p = 0.003$) while immatures showed significantly less protective behaviors (estimate = -1.17, $z$ value = -3.568, $p = 0.0004$). However, there was also important individual variation; for example, among males, two individuals including MO, the alpha male for most of our study period, accounted for 67.9% [36/53] of occurrences of protective behavior.

## 3.4 | Influence of Females' Anogenital Swelling Status

Among all recorded road crossings, the proportion of females in the different anogenital swelling stages were as follows: 59.2% had no swelling (anestrous; scored “0”), 18.7% were partially swollen (scored “1”) and 22.1% were maximally swollen (i.e. in estrus; scored “2”). The observed distribution of these swelling stages when crossing in zone B was 38.5% for stage “0”, 28.2% for stage “1” and 33.3% for stage “2”. This distribution differed significantly from the average distribution ($\chi^2$ test for given probabilities: $\chi_2^2 = 6.805, p = 0.033$). Thus, females crossed more frequently in the riskier zone B when they had partial and full anogenital swellings. In contrast, the observed distribution of females in the different swelling stages when crossing in zone A did not differ from the average distribution (62.7% for stage “0”, 17.1% for stage “1” and 20.2% for stage “2”; $\chi^2$ test for given probabilities: $\chi_2^2 = 1.310, p = 0.519$). On average, 23.2% of female crossings
<table><caption>TABLE 2 | Summary of parameter estimates for the significant models (GLMM).</caption><thead><tr><th>Parameter</th><th>Estimate</th><th>SE</th><th>Z value or t value</th><th>p value</th><th>AIC full vs null</th><th>p value full vs null</th></tr></thead><tbody><tr><td colspan="7">Hypothesis 1: Age-sex influences on crossing behavior</td></tr><tr><td>Crossing gait ~ age-sex (AS) category</td><td></td><td></td><td></td><td></td><td>987 vs 1003</td><td>&lt; 0.0001</td></tr><tr><td>(Intercept)</td><td>1.76</td><td>0.37</td><td>4.77</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature female</td><td>-1.32</td><td>0.52</td><td>-2.53</td><td>0.0114</td><td></td><td></td></tr><tr><td>Mature male</td><td>-3.08</td><td>0.53</td><td>-5.79</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Cautious behavior ~ AS category</td><td></td><td></td><td></td><td></td><td>857 vs 875</td><td>&lt; 0.0001</td></tr><tr><td>(Intercept)</td><td>0.93</td><td>0.22</td><td>4.26</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature female</td><td>0.94</td><td>0.27</td><td>3.54</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature male</td><td>1.66</td><td>0.29</td><td>5.67</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Individual waiting time ~ AS category</td><td></td><td></td><td></td><td></td><td>1315 vs 1320</td><td>0.012</td></tr><tr><td>(Intercept)</td><td>2.44</td><td>0.17</td><td>14.07</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature female</td><td>-0.0005</td><td>0.15</td><td>-0.003</td><td>0.99</td><td></td><td></td></tr><tr><td>Mature male</td><td>0.47</td><td>0.16</td><td>2.97</td><td>0.0103</td><td></td><td></td></tr><tr><td>Position ~ AS category</td><td></td><td></td><td></td><td></td><td>547 vs 562</td><td>&lt; 0.0001</td></tr><tr><td>(Intercept)</td><td>-2.10</td><td>0.26</td><td>-8.23</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature female</td><td>1.24</td><td>0.33</td><td>3.74</td><td>0.0002</td><td></td><td></td></tr><tr><td>Mature male</td><td>1.76</td><td>0.35</td><td>5.04</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Protective behavior ~ AS category</td><td></td><td></td><td></td><td></td><td>515 vs 558</td><td>&lt; 0.0001</td></tr><tr><td>(Intercept)</td><td>-1.33</td><td>0.19</td><td>-6.98</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature male</td><td>0.74</td><td>0.25</td><td>3.01</td><td>0.0026</td><td></td><td></td></tr><tr><td>Immature</td><td>-1.17</td><td>0.33</td><td>-3.57</td><td>0.0004</td><td></td><td></td></tr><tr><td colspan="7">Hypothesis 2: Chimpanzee risk assessment</td></tr><tr><td>Crossing gait ~ AS category + traffic intensity</td><td></td><td></td><td></td><td></td><td>920 vs 987</td><td>&lt; 0.0001</td></tr><tr><td>(Intercept)</td><td>0.77</td><td>0.41</td><td>1.87</td><td>0.06</td><td></td><td></td></tr><tr><td>Mature female</td><td>-1.48</td><td>0.56</td><td>-2.66</td><td>0.008</td><td></td><td></td></tr><tr><td>Mature male</td><td>-3.44</td><td>0.57</td><td>-6.04</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Motor traffic</td><td>0.49</td><td>0.07</td><td>7.25</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Non-motor traffic</td><td>0.26</td><td>0.11</td><td>2.44</td><td>0.015</td><td></td><td></td></tr><tr><td>Cautious behavior ~ AS category + crossing zone</td><td></td><td></td><td></td><td></td><td>922 vs 926</td><td>0.021</td></tr><tr><td>(Intercept)</td><td>0.80</td><td>0.16</td><td>5.03</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature female</td><td>0.74</td><td>0.24</td><td>3.11</td><td>0.002</td><td></td><td></td></tr><tr><td>Mature male</td><td>1.37</td><td>0.25</td><td>5.42</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Crossing zone</td><td>-0.46</td><td>0.20</td><td>-2.33</td><td>0.020</td><td></td><td></td></tr><tr><td>Individual waiting time ~ AS category + crossing zone</td><td></td><td></td><td></td><td></td><td>1476 vs 1482</td><td>0.005</td></tr><tr><td>(Intercept)</td><td>2.72</td><td>0.12</td><td>21.81</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature female</td><td>0.05</td><td>0.18</td><td>0.27</td><td>0.79</td><td></td><td></td></tr><tr><td>Mature male</td><td>0.58</td><td>0.19</td><td>3.07</td><td>0.009</td><td></td><td></td></tr><tr><td>Crossing zone</td><td>-0.47</td><td>0.17</td><td>-2.82</td><td>0.005</td><td></td><td></td></tr><tr><td colspan="7">Hypothesis 3: Social influences on crossing behavior</td></tr><tr><td>Crossing gait ~ AS category + (sub)group size</td><td></td><td></td><td></td><td></td><td>985 vs 987</td><td>0.04</td></tr><tr><td>(Intercept)</td><td>2.16</td><td>0.43</td><td>5.08</td><td>&lt; 0.0001</td><td></td><td></td></tr><tr><td>Mature female</td><td>-1.35</td><td>0.53</td><td>-2.52</td><td>0.012</td><td></td><td></td></tr><tr><td>Mature male</td><td>-3.21</td><td>0.55</td><td>-5.88</td><td>&lt; 0.0001</td><td></td><td></td></tr></tbody></table>

(Continues)
<figure><img src="image_6.png" /><figcaption>**FIGURE 5** | Video screenshots showing mature male chimpanzees exhibiting protective behaviors during road crossings. (a) JK (rightmost individual) showing guarding behavior, i.e. standing in a quadrupedal and alert posture on the road for more than 3 s without moving (Hockings 2011), while others cross. Field assistant TS is standing nearby while the chimpanzees cross. (b) MR making a reassurance gesture, i.e. gesture produced in reaction to recipient's distress or solicited from another (here, embracing), to GD who was limping from a temporary injury. (c) MR (leftmost individual) and MO (third individual from the left) waiting for immature MK (rightmost individual). (d) SL gesturing to MR (at the roadside on the left) to cross while JK sits on the road, watched by waiting motorcyclists.</figcaption></figure>

model comparison: $\chi^2 = 5.32$, df = 1; $p = 0.021$). Chimpanzees waited for significantly less time when crossing in the riskier zone B and were less cautious (Figure 6). There was no effect of crossing zone on gait and protective behaviors (null model comparison: $\chi^2 = 0.89$, df = 1; $p = 0.34$; $\chi^2 = 0.19$, df = 1; $p = 0.66$; respectively).

We tested the influence of interventions by field assistants on cautiousness of the chimpanzees and found no influence (null model comparison: $\chi^2 = 1.71$, df = 1; $p = 0.19$).

**3.6 | Hypothesis 3—Social Influences on Crossing Behavior**

The mean crossing group size was $8.6 \pm 4.7$ (range 1–18) individuals, excluding infants. On average, chimpanzees crossed in $1.5 \pm 0.8$ (range 1–5) subgroups, composed of $5.1 \pm 3.8$ (range 1–18) individuals. Overall, 32.6% (42/129) of road crossing events comprised 2 or more subgroups, while in the remaining crossing events (87/129), the chimpanzees all crossed within 1 min of each other. Often the chimpanzees crossed in a loose manner with no linear progression discernible. Therefore, we could not always assign a precise progression rank to all the chimpanzees, except for the first and last individuals to step on or off the road. Notably, 34 of the 190 crossing (sub)groups included immatures crossing without their mother.

We found no effect of (sub)group size and proportion of mature males during crossings on cautiousness (null model comparison: $\chi^2 = 0.56$, df = 1; $p = 0.45$; $\chi^2 = 0.67$, df = 1; $p = 0.41$; respectively), and no effect of the proportion of mature males on crossing gaits (null model comparison: $\chi^2 = 1.25$, df = 1; $p = 0.26$). However, we found a significant but slight effect of (sub)group size on crossing gait (null model comparison: $\chi^2 = 4.25$, df = 1; $p = 0.04$): in larger groups the likelihood of

individuals crossing at a fast pace decreased. There was no effect of the proportion of mature males on waiting times (null model comparison: $\chi^2 = 0.41$, df = 1; $p = 0.52$), but there was a significant effect of (sub)group size on waiting times (null model comparison: $\chi^2 = 16.32$, df = 1; $p < 0.0001$): individual waiting times increased in larger groups (Figure 7).

**4 | Discussion**

In this study, which uses a larger dataset of behavioral observations than previous studies of road crossing in chimpanzees, we highlighted patterns of behavior that vary according to age and sex, level of risk, and group composition.

**4.1 | Does Road-Crossing Behavior Vary According to Sex and Age Class?**

Consistent with prediction (1.1), mature males were more likely to adopt forward and rearward positions than females and immatures. Younger chimpanzees predominantly occupied safer positions in the middle of crossing progressions. In support of prediction (1.2), males in general performed protective behaviors more frequently than others, such as waiting for specifics behind them, and adopting guarding postures while others crossed (Figure 5; Table S3–Video S4). Like previous studies at Bossou and Sebitoli (Hockings, Anderson, and Matsuzawa 2006; Cibot et al. 2015), these results confirm that chimpanzees can behave cooperatively to protect group members during “risky” situations. However, as Cibot et al. (2015) have already described, mothers did not always wait for their immature offspring or agree to carry them during crossings (Table S3, Video S5, Video S6; but see also Video S7 for the opposite behavior, i.e., maternal awareness of the vulnerability
<figure><img src="image_7.png" /><figcaption>**FIGURE 6** | Relationships between the crossing zone and individual waiting times [log-transformed, in seconds; (a)], and cautious behaviors (b) for each age-sex category. The probability of crossing at a fast or slow gait is shown in relation to the traffic intensity during crossings (c).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**FIGURE 7** | Line chart showing the effect of the crossing (sub)group size on individual waiting times (log-transformed, in seconds) for each age-sex category.</figcaption></figure>

of dependent offspring). In 34 instances, immatures crossed in a (sub)group without their mother, including juveniles aged 4 and 5 years.

In support of prediction (1.3), mature males were recorded crossing the road more often than females and immatures. Like other small chimpanzee communities (Lehmann and Boesch 2004), the Bulindi chimpanzees are socially cohesive with parties containing 46% of individuals (excluding infants) on

average (Satsias et al. 2022). However, parties in anthropogenic areas outside the forest, including croplands and the road, contain proportionally fewer females compared to parties inside the forest (Satsias et al. 2022). We therefore assume our sample of videos is representative of the composition of crossing groups and that field assistants were not more likely to record crossings involving mature males while missing crossings of females and immatures. Moreover, unlike females and immatures, males sometimes cross the road back and forth in short succession.
<figure><img src="image_9.png" /><figcaption>**FIGURE 8** | Screenshots showing mature male chimpanzees behaving “recklessly” during road crossings. MR (a) and MO (b) sitting in the middle of the road. (c) JK, standing alone in the middle of the road despite the rapid arrival of a motorbike, and (d) retreating at the last moment.</figcaption></figure>

This sex difference reflects the bolder behavior of male chim-
panzees generally (Bertolani and Boesch 2008; Gilby et al. 2017;
Haux et al. 2023) and specifically in human-dominated en-
vironments like Bulindi (Hockings 2007; McLennan and
Hill 2010; Satsias et al. 2022). These results also highlight how
roads might reduce chimpanzee mobility (Garriga et al. 2019),
particularly of females and immatures which may be reluctant
to cross busy roads. Besides, given that chimpanzee societies are
male-philopatric (Goodall 1986; Nakamura et al. 2015; Boesch
et al. 2019), greater road avoidance by females could lead to
decreased female dispersal (Cibot et al. 2015) and increased
inbreeding within chimpanzee populations (McCarthy et al.
2020). Long-term studies at multiple sites crossed by wide and
busy asphalt roads will be needed to assess the impact of these
human infrastructures on female dispersal.

Contrary to prediction (1.3), mature males at Bulindi waited
longer before crossing and were more cautious than females
and immatures. Nevertheless, males did not generally exhibit
signs of anxiety when surveying for traffic or pedestrians (such
as bipedalism [Pierce 2009; Krief et al. 2014] or self-scratching
[Aureli and de Waal 1997; Hockings et al. 2007]). We suggest
this difference reflects the greater willingness of males to be
visible to road users, and thus more likely to be recorded
checking traffic. In some primates, such as samango monkeys
(*Cercopithecus albogularis*), age-sex category has no effect on
the number of glances (i.e. scans of the environment) made
while crossing a canopy overpass (Linden et al. 2020), whereas
in others, such as olive baboons (*Papio anubis*), adult males
were described as checking traffic before crossing a road
(Rowell 1969), similar to male chimpanzees at Bulindi. We
further note that the traffic checking behavior we observed
might sometimes simply reflect curiosity of road users rather
than ‘cautiousness’.

Unlike immature chimpanzees, which crossed faster than
others as expected (prediction 1.3), mature males tended to

cross at a leisurely pace, sometimes standing or sitting on the
road without presenting signs of apprehension. At times they
behaved “recklessly” by remaining visible at the roadside
despite passing traffic (although we did not analyze such
behavior specifically) or by moving away from vehicles at the
last moment (Figure 8). Such behavior was not described in
the previous studies of road crossing in chimpanzees. The
difference in behavior with their closest relatives, the bonobos
(Pan paniscus) described by Druelle et al. (2020), is particu-
larly striking: when crossing a less risky dirt road, all bonobo
individuals, including males, ran quickly or even leaped from
tree to tree in response to a single passing cyclist. In the
present study, mature female chimpanzees were intermediate
in terms of crossing gait and cautiousness, and they were not
more vigilant when carrying a dependent, contrary to our
predictions and to what has been described previously in risky
situations like crop feeding (Hockings 2007; Wallace and
Hill 2012). These results suggest that Bulindi chimpanzees
might not consider road-crossing a high-risk situation in
many instances, except when traffic intensity is especially
high. Although immatures generally crossed faster than
mature individuals and occupied safer positions, they were
less cautious. Immature chimpanzees could therefore be at
highest probability of colliding with a vehicle (Supporting
Information: Table S3–Video S3).

Finally, consistent with prediction (1.4), we observed that
females in estrus (i.e. maximally swollen) tended to cross
more slowly, similar to males. Likewise, females were more
likely to cross in the more dangerous zone B when they
were swollen, even partially, compared to non-swollen. These
results align with previous observations (e.g. Matsumoto-
Oda 1999) that females tend to range with mature males
during estrus. Indeed, Satsias et al. (2022) reported that
maximally swollen females in Bulindi exhibited increased
gregariousness, even in “riskier” anthropogenic parts of the
habitat.
## 4.2 | Do Crossing Modalities Vary According to Degree of Risk?

Contrary to prediction (2.1), we found no evidence that traffic intensity influenced when chimpanzees crossed the road. While traffic intensity during crossings showed moderate variation at different times of the day (Figure 3), the chimpanzees crossed most frequently at times that correspond to their main hours of activity, i.e., early in the morning and late in the afternoon, when they do most foraging (e.g. Goodall 1986; Carlson, Rothman, and Mitani 2013). Indeed, chimpanzees at Bulindi are rarely active in the early afternoon before ca. 4 pm (unpublished data). We acknowledge that our measure of traffic intensity has limitations because (1) it was assessed over short durations during crossings (video recordings with mean duration of 2.1 min; range 14 s-16 min), (2) we likely missed some pedestrians or vehicles that were behind the camera and inaudible, and (3) recordings normally started once 1 or more chimpanzees had approached the roadside, whereas at times chimpanzees might have assessed traffic over longer periods before approaching the road. In zone A, they sometimes spent time in dense vegetation near the roadside before any individual crossed. In such instances, the chimpanzees might have been waiting for traffic intensity to reduce before deciding to cross; alternatively, they could have been resting and socializing out of view of passing road users. Unfortunately, we could not assess these possibilities since the chimpanzees were rarely visible at these times.

In agreement with prediction (2.2), chimpanzees—especially females and immatures—crossed most often in the safer zone A, which had improved visibility, lower vehicle speeds due to humps, and higher vegetation coverage on both sides compared to zone B. Hence, our results support the idea that chimpanzees choose where to cross to limit the risk of collisions or unexpected encounters with pedestrians. Nevertheless, this difference could also be explained by the distribution of habitat cover in their home range. The chimpanzees predominantly foraged around zone B because of the presence of cultivated fruit trees such as jackfruit, mango, and guava by homesteads (personal observations). Therefore, it is unlikely that food motivation explains why chimpanzees crossed more frequently in zone A. However, chimpanzees could access zone A with relatively little exposure to humans due to the vegetation cover south of the road (riverine forest, papyrus swamp and fallow fields; Figures 1c and 2). In contrast, to reach food sources north of the road in zone B, chimpanzees had to traverse open farmland near homes (covering a minimum distance of 500 meters from riverine forest to the southeast) and cross the road where there is little cover, or cross the road at zone A, where they were less conspicuous and could take advantage of vegetation cover on the northern side of the road as they travel to zone B. While a study of their daily travel and foraging routes would shed further light on this issue, the chimpanzees appeared to adopt the latter strategy most often, especially females. As noted above, Bulindi's females generally venture less often into “anthropogenic” parts of the habitat (including open farmland and village areas such as the surroundings of zone B) compared to males (Satsias et al. 2022). Thus, the sex difference in the relative frequency that males and females crossed the road in the safer versus more dangerous zone may

relate more to females' perception of risk in exposed areas, rather than to their risk perception of the road per se.

Contrary to prediction (2.3), chimpanzees waited less time before crossing in the riskier zone B. This is probably attributable to the limited cover available in zone B, making chimpanzees reluctant to spend time in an exposed position near the road. Additionally, they exhibited less caution (e.g. less traffic checking) in zone B. While humans typically wait longer in areas with busier traffic and faster vehicles, they are also more likely to attempt riskier crossings (Hamed 2001; Simpson, Johnston, and Richardson 2003). Surprisingly, however, although vehicles could travel faster on this section of the road, the chimpanzees did not cross at a faster gait. Overall, Bulindi chimpanzees seem to perceive risk differently from those at Bossou, who waited longer before crossing a wider and presumably more dangerous road, compared to a smaller one, and generally crossed it more quickly (Hockings 2011).

Nevertheless, the chimpanzees did cross faster when traffic intensity (especially motor vehicle traffic) increased, presumably to reduce the risk of collision. However, we found no variation in cautiousness or waiting time as a function of motor and non-motor traffic intensity during crossings. At times chimpanzees seemed to wait because of nearby pedestrians or approaching vehicles (and sometimes showed avoidance behavior), while at other times they crossed in close proximity to people and vehicles (Figure 9; Table S3 and Videos S1–S3). Overall, the chimpanzees in Bulindi seem less prudent in their road crossing behavior compared to those in Bossou and Sebitoli, despite heavy traffic. At Bulindi, the chimpanzees have a long experience of crossing this road (McLennan and Asiimwe 2016) and may have developed tolerance (i.e. reduced reactivity; Čapkun-Huot et al. 2024) to the risk it presents even though the danger has increased following the recent upgrades (tarmacking and widening; faster vehicles).

## 4.3 | Does Individual Vigilance Vary According to Group Size and Composition?

As described in olive baboons in Ethiopia (Aldrich-Blake et al. 1971) and in Sebitoli chimpanzees (Cibot et al. 2015), individuals in Bulindi frequently split into small subgroups within a larger crossing group. At Sebitoli, it was suggested that crossing in small subgroups was a strategy to enhance group protection on a road with high traffic intensity and short distances between vehicles. At Bulindi, the chimpanzees were more cohesive during crossings than the considerably larger Sebitoli community of around 80 individuals (i.e. subgroups were fewer and larger on average in Bulindi: 1.5 subgroups per crossing composed of 5.1 individuals versus 2.7 subgroups per crossing comprising 2.1 individuals in Sebitoli). While we are unable to compare traffic intensity at Bulindi and Sebitoli, this difference could reflect the fact that the small community at Bulindi is socially cohesive generally (Satsias et al. 2022), like small chimpanzee communities elsewhere (Lehmann and Boesch 2004).

In agreement with prediction (3.1) and similar to the Bossou chimpanzees (Hockings 2011), individuals at Bulindi waited longer in large crossing groups than in small ones, probably
<figure><img src="image_10.png" /><figcaption>**FIGURE 9** | Video screenshots showing instances of close proximity between chimpanzees and road users during road crossings. (a) Mature males (JK, AR, MO) and MN's family crossing near onlookers. (b) JK narrowly avoiding a car, with nearby onlookers (electricity workers) and motorcyclists. (c) From left to right: JK, RO (immature male), and MO crossing very close to a pedestrian, with an oncoming truck. (d) Mature males (MO, AR, GD) crossing at a speed hump (boundary between zones A and B) in proximity to a car that has stopped in the road; the driver is approaching the chimpanzees to film them.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**VIDEO S1** | Chimpanzees crossing the road in close proximity to pedestrian onlookers.</figcaption></figure>

because they waited for others to cross before doing so themselves. The latency between the crossing of the first chimpanzee and a subsequent individual may reflect the latter's confidence that the leader chose a safe moment to cross; however, we were unable to investigate this possibility in the present study. In line with prediction (3.2), chimpanzees tended

to cross more quickly in small groups than in large ones, contrary to those at Sebitoli (Cibot et al. 2015). Against expectation (3.3), however, the chimpanzees were no more cautious in small versus large groups, suggesting that they did not feel more vulnerable on the road in small numbers. Considering the Bubindi chimpanzees are habituated to researchers (McLennan
<figure><img src="image_12.png" /><figcaption>**VIDEO S2** | Chimpanzees crossing the road with busy traffic and close onlookers (electricity workers and motorcyclists), with JK narrowly avoiding a car (00:22).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**VIDEO S3** | Chimpanzees crossing the road with busy traffic, with immature male MK narrowly avoiding a car thanks to the driver braking upon noticing the preceding chimpanzees (0:36).</figcaption></figure>

et al. 2019), we hypothesize that the nearby presence of familiar field assistants meant the chimpanzees felt safer during the observed crossings, and therefore showed less caution than they might otherwise do when field assistants are

absent. Once again, this overall lack of fear could also be explained by their long experience of crossing this road, even if it was previously untarmacked with less vehicle traffic (McLennan and Asiimwe 2016).
<figure><img src="image_14.png" /></figure>

**VIDEO S4** | Chimpanzees crossing the road with pedestrians (visible and/or audible) and vehicle traffic, with MR displaying protective behavior towards GD who was limping from a temporary injury (00:56).

<figure><img src="image_15.png" /></figure>

**VIDEO S5** | Chimpanzees crossing the road with mature female MN refusing to carry her 4-year-old dependent offspring LC (00:05). Field assistant TS is standing at the roadside.

**4.4 Conservation and Management Implications**

We suggest there are probably regional differences in chimpanzees' responses to roads. Garriga et al. (2019) reported

general avoidance of roads by chimpanzees in West Africa (Sierra Leone), where human population density is much lower than in parts of East Africa, i.e. around 50 persons/km², versus > 150 persons/km² in this study. In contrast, chimpanzees in
Bulindi and elsewhere regionally (McLennan 2008; McLennan et al. 2021) have frequent contact with people, vehicles and roads, and mature males especially do not seem to avoid roads that cross their territories. Consequently, our findings, which

suggest that Bulindi chimpanzees are responding to roads dif-
ferently to what is described at other sites where anthropogenic
impact is lower, testifies to the behavioral flexibility of chim-
panzees (Hockings et al. 2015; McLennan, Spagnoletti, and

<figure><img src="image_16.png" /><figcaption>**VIDEO S6** | Chimpanzees crossing the road with LL already carrying her 2-year-old infant GF and refusing to also carry her 5-year-old dependent offspring WD (00:37).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**VIDEO S7** | LL crossed the road carrying her 2-year-old infant GF and showing protective behavior towards her 5-year-old dependent offspring WD (encouraging and waiting for her while crossing).</figcaption></figure>
Hockings 2017). Studies of chimpanzee road crossings in dif-
ferent habitats, and using larger datasets, are welcome to pro-
vide additional descriptions of the interactions between road
users and chimpanzees and the factors influencing them, and to
aid our understanding of risk perception and progression orders
in chimpanzees.

Despite the social and behavioral adjustments shown by the
Bulindi chimpanzees, the danger of these road crossings remains
very high given the volume of traffic and speed of vehicles on the
upgraded road (up to 120 km/h on some sections; personal
observations). Additionally, the road could potentially limit
female dispersal, as noted above. In western Uganda, chimpan-
zee habitats are subject to ongoing road and infrastructural
development, driven in part by the demands of the oil industry
(Maweje 2018; Ogwang and Vanclay 2019; McLennan
et al. 2021). It is therefore crucial to implement measures to
mitigate the impact of current and future road construction and
renovation on Uganda's chimpanzees and other wildlife.

We recommend that the road authorities install speed bumps along the stretch of road crossed by chimpanzees in Bulindi, and elsewhere where roads are being developed in chimpanzee habitats. Existing humps should be improved to enhance their effectiveness, for example by elevating them (see Antić et al. 2013) or by implementing double bumps (see Kiran, Kumar, and Abhinay 2020). Strict and regular police speed controls on roads passing through chimpanzee habitats could also help mitigate impacts on these endangered great apes (McLennan and Asiimwe 2016; Krief et al. 2020). These measures would also benefit the many pedestrians and cyclists who use these roads. In addition, studying the behavioral reactions of road users to chimpanzee crossings would be valuable for assessing the effectiveness of mitigation measures. In areas where humans and chimpanzees coexist, such as in western Uganda, we also recommend conducting awareness campaigns about chimpanzee road-crossing behavior, including the age-sex differences highlighted in our study, to help road users respond safely during encounters. This would allow local residents to better understand chimpanzee behavior on roads, and adjust their own behavior to limit the risk of collision or overly close encounters with the animals (Tellier et al. in prep). Finally, broadcasting a radio awareness campaign to encourage people to drive slower in areas frequented by chimpanzees, and to give them space to cross when encountered, would reach a wider population, including non-local workers commuting along these roads and drivers of tourist vehicles—both of which are less familiar with chimpanzee road crossing behavior than most local persons.

5 | Conclusion

This study offers valuable insights into the road-crossing
behavior of wild chimpanzees, highlighting variations based
on age, sex, and group dynamics. While chimpanzees in Bulindi
seem to perceive road crossings as dangerous and exhibit
behavioral strategies to reduce risks of collision or close en-
counters with humans—as described in previous studies
(Hockings, Anderson, and Matsuzawa 2006; Hockings 2011;

Cibot et al. 2015)—overall they were less vigilant than we ex-
pected: their use of the road was related to their main hours of
activity rather than low traffic intensity; they did not wait for
longer before crossing or show more caution when traffic
intensity was highest; juveniles sometimes crossed without
their mothers; they often crossed in a “nonchalant” manner;
and they were not more vigilant when crossing in small groups.
These findings indicate that chimpanzees at Bulindi have de-
veloped behavioral tolerance toward the road (presumably
through habituation-like processes; Blumstein 2016), even
though it was widened and paved shortly before the study
began. Prior to its upgrade, the chimpanzees crossed this road
for generations (McLennan and Asiimwe 2016). At the time of
this study, they crossed the road several times a week on
average (unpublished data). Their growing experience of
crossing the upgraded road may mean they pay increasingly less
attention to the risk it presents, potentially increasing the
likelihood of a collision. In that respect, their behavioral toler-
ance of the road could ultimately prove maladaptive if it
reduces individuals’ fitness (see Čapkun-Huot et al. 2024). The
chimpanzees’ overall lack of fear of the road is probably related
to their long history of exposure to people and long familiarity
with vehicles on the road (McLennan and Hill 2010; McLennan
et al. 2019; Satsias et al. 2022), despite the fact the road has
become busier and—from our perspective—more dangerous for
them after tarmacking. Thus, it is crucial to raise awareness
among local people and implement sustainable measures to
mitigate collision risks, pollution, and zoonosis transmission on
this road. Studying the behavior of other chimpanzee popula-
tions facing similar road crossings will help refine conservation
strategies and inform best practices for road construction and
renovation. Future research should also focus on longer-term
effects and broader geographic studies to better understand and
mitigate the impacts of human infrastructure on chimpanzee
populations (Videos S1–S7).

Author Contributions

**Marie Tellier:** conceptualization (equal), data curation (equal), formal analysis (equal), methodology (equal), writing—original draft (equal), writing—review and editing (equal). **François Druelle:** data curation (equal), formal analysis (equal), writing—review and editing (equal). **Marie Cibot:** conceptualization (equal), methodology (equal), supervision (equal), writing—review and editing (equal). **Johnmary Baruzaliire:** investigation (equal), resources (equal). **Tom Sabiiti:** investigation (equal), resources (equal). **Matthew R. McLennan:** conceptualization (equal), investigation (equal), methodology (equal), project administration (equal), resources (equal), supervision (equal), writing—review and editing (equal).

Acknowledgments

We are grateful to Uganda Wildlife Authority and Uganda National Council for Science and Technology for permission to conduct research in Bulindi. Fieldwork was made possible with the support of many local residents and leaders, to whom we are grateful. We thank the staff of the Bulindi Chimpanzee and Community Project for additional assistance. In particular, we acknowledge the extensive contributions of the late Jacqueline Rohen to this work. Monitoring of the chimpanzees and related conservation activities in Bulindi during this study was supported by funding from Born Free Foundation, Friends of Chimps, Jane Goodall Institute-Switzerland and Blair Drummond Safari Park. We are grateful to the reviewers whose valuable feedback helped us to improve the manuscript.
Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

A full data set and the R code can be found at the Open Science Framework at https://osf.io/jvm6n/?view_only=d690dca35caf4e76a270 708f3ced9193.

References

Aldrich-Blake, F. P. G., T. K. Bunn, R. I. M. Dunbar, and P. M. Headley. 1971. "Observations on Baboons, *Papio anubis*, in an Arid Region in Ethiopia." *Folia Primatologica* 15: 1-35. https://doi.org/10.1159/ 000155365.

Andrasi, B., J. A. G. Jaeger, S. Heinicke, K. Metcalfe, and K. J. Hockings. 2021. "Quantifying the Road-Effect Zone for a Critically Endangered Primate." *Conservation Letters* 14: e12839. https://doi.org/10.1111/conl. 12839.

Antić, B., D. Pešić, M. Vujanić, and K. Lipovac. 2013. "The Influence of Speed Bumps Heights to the Decrease of the Vehicle Speed-Belgrade Experience." *Safety Science* 57: 303-312. https://doi.org/10.1016/j.ssci. 2013.03.008.

Arcus Foundation. 2018. *State of the Apes: Infrastructure Development and Ape Conservation*. Cambridge: Cambridge University Press. https:// doi.org/10.1017/9781108436427.

Aureli, F., and F. B. M. de Waal. 1997. "Inhibition of Social Behavior in Chimpanzees Under High-Density Conditions." *American Journal of Primatology* 41: 213-228. https://doi.org/10.1002/(SICI)1098-2345(1997) 41:3%3C213::AID-AJP4%3E3.0.CO;2%23.

Bates, D., M. Mächler, B. Bolker, and S. Walker. 2015. "Fitting Linear Mixed-Effects Models Using Lme4." *Journal of Statistical Software* 67, no. 1: 1-48. https://doi.org/10.18637/jss.v067.i01.

Bennett, V. J. 2017. "Effects of Road Density and Pattern on the Conservation of Species and Biodiversity." *Current Landscape Ecology Reports* 2: 1-11. https://doi.org/10.1007/s40823-017-0020-6.

Bersacola, E., C. M. Hill, and K. J. Hockings. 2021. "Chimpanzees Balance Resources and Risk in an Anthropogenic Landscape of Fear." *Scientific Reports* 11: 4569. https://doi.org/10.1038/s41598- 021-83852-3.

Bertolani, P., and C. Boesch. 2008. "Habituation of Wild Chimpanzees (*Pan troglodytes*) of the South Group at Taï Forest, Côte d'Ivoire: Empirical Measure of Progress." *Folia Primatologica* 79, no. 3: 162-171. https://doi.org/10.1159/000111720.

Blumstein, D. T. 2016. "Habituation and Sensitization: New Thoughts About Old Ideas." *Animal Behaviour* 120: 255-262. https://doi.org/10. 1016/j.anbehav.2016.05.012.

Boesch, C., Wittig, R., Crockford, C., Vigilant, L., Deschner, T., and Leendertz, F., eds. 2019. *The Chimpanzees of the Taï Forest: 40 Years of Research*. Cambridge: Cambridge University Press. https://doi.org/10. 1017/9781108674218.

Čapkun-Huot, C., D. T. Blumstein, D. Garant, D. Sol, and D. Réale. 2024. "Toward a Unified Framework for Studying Behavioural Tolerance." *Trends in Ecology & Evolution* 39, no. 5: 446-455. https://doi.org/ 10.1016/j.tree.2023.12.006.

Carlson, B. A., J. M. Rothman, and J. C. Mitani. 2013. "Diurnal Variation in Nutrients and Chimpanzee Foraging Behavior." *American Journal of Primatology* 75, no. 4: 342-349. https://doi.org/10.1002/ajp. 22112.

Cibot, M., S. Bortolamiol, A. Seguya, and S. Krief. 2015. "Chimpanzees Facing a Dangerous Situation: A High-Traffic Asphalted Road in the Sebitoli Area of Kibale National Park, Uganda." *American Journal of Primatology* 77: 890-900. https://doi.org/10.1002/ajp.22417.

Cibot, M., S. Le Roux, J. Rohen, and M. R. McLennan. 2019. "Death of a Trapped Chimpanzee: Survival and Conservation of Great Apes in Unprotected Agricultural Areas of Uganda." *African Primates* 13: 47-56.

Couturier, C., S. Bortolamiol, S. Ortmann, J. P. Okimat, E. Asalu, and S. Krief. 2022. "All-You-Can-Eat: Influence of Proximity to Maize Gardens on the Wild Diet and the Forest Activities of the Sebitoli Chimpanzee Community in Kibale National Park." *Animals* 12: 806. https://doi.org/10.3390/ani12070806.

van Dijk, K., M. Cibot, and M. R. McLennan. 2021. "Chimpanzees (*Pan troglodytes*) Adapt Their Nesting Behavior After Large-Scale Forest Clearance and Community Decline." *American Journal of Primatology* 83: e23323. https://doi.org/10.1002/ajp.23323.

Druelle, F., P. Aerts, J. C. B. Ngawolo, and V. Narat. 2020. "Impressive Arboreal Gap-Crossing Behaviors in Wild Bonobos, *Pan paniscus*." *International Journal of Primatology* 41: 129-140. https://doi.org/10. 1007/s10764-020-00140-z.

Dulac, J. 2013. *Global land Transport Infrastructure Requirements to 2050*. Paris: International Energy Agency.

Fayissa, B., C. Nsiah, and B. Tadasse. 2008. "Impact of Tourism on Economic Growth and Development in Africa." *Tourism Economics* 14: 807-818. https://doi.org/10.5367/000000008786440229.

Fryns, C., G. Badihi, A. S. Crunchant, et al. 2021. "Interactions Between Chimpanzees (*Pan troglodytes schweinfurthii*) and Cattle (*Bos taurus*) in the Issa Valley, Western Tanzania." *African Primates* 15: 19-30.

Garriga, R. M., I. Marco, E. Casas-Díaz, et al. 2019. "Factors Influencing Wild Chimpanzee (*Pan troglodytes verus*) Relative Abundance in an Agriculture-Swamp Matrix Outside Protected Areas." *PLoS One* 14: e0215545. https://doi.org/10.1371/journal.pone.0215545.

Gates, T. J., D. A. Noyce, A. R. Bill, and N. Van Ee. 2006. "Recommended Walking Speeds for Timing of Pedestrian Clearance Intervals Based on Characteristics of the Pedestrian Population." *Transportation Research Record: Journal of the Transportation Research Board* 1982, no. 1: 38-47. https://doi.org/10.1177/0361198106198200106.

Gilby, I. C., Z. P. Machanda, R. C. O'Malley, et al. 2017. "Predation by Female Chimpanzees: Toward an Understanding of Sex Differences in Meat Acquisition in the Last Common Ancestor of *Pan* and *Homo*." *Journal of Human Evolution* 110: 82-94. https://doi.org/10.1016/j. jhevol.2017.06.015.

Goodall, J. 1986. *The Chimpanzees of Gombe: Patterns of Behavior*. Cambridge, MA: The Belknap Press of Harvard University Press. ISBN: 0674116496.

Hamed, M. M. 2001. "Analysis of Pedestrians' Behavior at Pedestrian Crossings." *Safety Science* 38: 63-82. https://doi.org/10.1016/S0925- 7535(00)00058-8.

Haux, L. M., J. M. Engelmann, R. C. Arslan, R. Hertwig, and E. Herrmann. 2023. "Chimpanzee and Human Risk Preferences Show Key Similarities." *Psychological Science* 34, no. 3: 358-369. https://doi. org/10.1177/09567976221140326.

Hicks, T. C., P. Roessingh, and S. B. J. Menken. 2013. "Impact of Humans on Long-Distance Communication Behaviour of Eastern Chimpanzees (*Pan troglodytes schweinfurthii*) in the Northern Democratic Republic of the Congo." *Folia Primatologica* 84: 135-156. https://doi. org/10.1159/000350650.

Hockings, K. J. 2007. *Human-Chimpanzee Coexistence at Bossou, the Republic of Guinea: A Chimpanzee Perspective (Doctoral thesis)*. Stirling, UK: University of Stirling. http://hdl.handle.net/1893/189.

Hockings, K. J., J. R. Anderson, and T. Matsuzawa. 2006. "Road Crossing in Chimpanzees: A Risky Business." *Current Biology* 16: R668-R670. https://doi.org/10.1016/j.cub.2006.08.019.

Hockings, K. J., J. R. Anderson, and T. Matsuzawa. 2012. "Socio-ecological Adaptations by Chimpanzees, *Pan troglodytes verus*,
Inhabiting an Anthropogenically Impacted Habitat." *Animal Behaviour*
83: 801-810. https://doi.org/10.1016/j.anbehav.2012.01.002.

Hockings, K. J., T. Humle, J. R. Anderson, et al. 2007. "Chimpanzees Share Forbidden Fruit." *PLoS One* 2: e886. https://doi.org/10.1371/journal.pone.0000886.

Hockings, K. J., and M. R. McLennan. 2012. "From Forest to Farm: Systematic Review of Cultivar Feeding by Chimpanzees-Management Implications for Wildlife in Anthropogenic Landscapes." *PLoS One* 7, no. 4: e33391. https://doi.org/10.1371/journal.pone.0033391.

Hockings, K. J., M. R. McLennan, S. Carvalho, et al. 2015. "Apes in the Anthropocene: Flexibility and Survival." *Trends in Ecology & Evolution* 30: 215-222. https://doi.org/10.1016/j.tree.2015.02.002.

Hockings, K. J., and C. Sousa. 2013. "Human-Chimpanzee Sympathy and Interactions in Cantanhez National Park, Guinea-Bissau: Current Research and Future Directions." *Primate Conservation* 26: 57-65. https://doi.org/10.1896/052.026.0104.

Hockings, K. J. 2011. "Behavioral Flexibility and Division of Roles in Chimpanzee Road-Crossing." In *The Chimpanzees of Bossou and Nimba*, edited by T. Matsuzawa, T. Humle, and Y. Sugiyama, 221-229. *Primate Monographs*. Tokyo: Springer. https://doi.org/10.1007/978-4-431-53921-6_24.

Humle, T., F. Maisels, J. F. Oates, A. Plumptre, and E. A. Williamson 2016. *Pan troglodytes* (errata version published in 2018). *The IUCN Red List of Threatened Species 2016*: e.T15933A129038584. Accessed January 30, 2025. https://doi.org/10.2305/IUCN.UK.2016-2.RLTS.T15933A17964454en.

Jaeger, J. A. G., J. Bowman, J. Brennan, et al. 2005. "Predicting When Animal Populations Are at Risk From Roads: An Interactive Model of Road Avoidance Behavior." *Ecological Modelling* 185: 329-348. https://doi.org/10.1016/j.ecolmodel.2004.12.015.

Kiran, K. R., M. Kumar, and B. Abhinay. 2020. "Critical Analysis of Speed Hump and Speed Bump and Geometric Design of Curved Speed Hump." *Transportation Research Procedia* 48: 1211-1226. https://doi.org/10.1016/j.trpro.2020.08.144.

Krief, S., M. Cibot, S. Bortolamiol, A. Seguya, J. M. Krief, and S. Masi. 2014. "Wild Chimpanzees on the Edge: Nocturnal Activities in Croplands." *PLoS One* 9: e109925. https://doi.org/10.1371/journal.pone.0109925.

Krief, S., A. Iglesias-González, B. M. R. Appenzeller, et al. 2020. "Road Impact in a Protected Area With Rich Biodiversity: The Case of the Sebitoli Road in Kibale National Park, Uganda." *Environmental Science and Pollution Research* 27: 27914-27925. https://doi.org/10.1007/s11356-020-09098-0.

Kühl, H. S., T. Sop, E. A. Williamson, et al. 2017. "The Critically Endangered Western Chimpanzee Declines by 80%." *American Journal of Primatology* 79, no. 9: e22681. https://doi.org/10.1002/ajp.22681.

Lacroux, C., B. Robira, N. Kane-Maguire, N. Guma, and S. Krief. 2022. "Between Forest and Croplands: Nocturnal Behavior in Wild Chimpanzees of Sebitoli, Kibale National Park, Uganda." *PLoS One* 17, no. 5: e0268132. https://doi.org/10.1371/journal.pone.0268132.

Laporte, N. T., J. A. Stabach, R. Grosch, T. S. Lin, and S. J. Goetz. 2007. "Expansion of Industrial Logging in Central Africa." *Science* 316: 1451. https://doi.org/10.1126/science.1141057.

Laurance, W. F., G. R. Clements, S. Sloan, et al. 2014. "A Global Strategy for Road Building." *Nature* 514: 262. https://doi.org/10.1038/nature13717.

Laurance, W. F., B. M. Croes, L. Tchignoumba, et al. 2006. "Impacts of Roads and Hunting on Central African Rainforest Mammals." *Conservation Biology* 20: 1251-1261. https://doi.org/10.1111/j.1523-1739.2006.00420.x.

Lehmann, J., and C. Boesch. 2004. "To Fission or to Fusion: Effects of Community Size on Wild Chimpanzee (*Pan troglodytes verus*) Social

Organisation.” *Behavioral Ecology and Sociobiology* 56, no. 3: 207–216.
https://doi.org/10.1007/s00265-004-0781-x.

Linden, B., S. Foord, Q. J. B. Horta-Lacueva, and P. J. Taylor. 2020. "Bridging the Gap: How to Design Canopy Bridges for Arboreal Gue-nons to Mitigate Road Collisions." *Biological Conservation* 246: 108560.
https://doi.org/10.1016/j.biocon.2020.108560.

Matsumoto-Oda, A. 1999. "Mahale Chimpanzees: Grouping Patterns and Cycling Females." *American Journal of Primatology* 47, no. 3: 197-207. https://doi.org/10.1002/(SICI)1098-2345(1999)47:3%3C197::AID-AJP2%3E3.0.CO;2-3.

Mawejje, J. 2018. "The Oil Discovery in Uganda's Albertine Region: Local Expectations, Involvement, and Impacts." *Extractive Industries and Society* 6: 129-135. https://doi.org/10.1016/j.exis.2018.09.007.

McCarthy, M. S., J. D. Lester, M. Cibot, L. Vigilant, and M. R. McLennan. 2020. "Atypically High Reproductive Skew in a Small Wild Chimpanzee Community in a Human-Dominated Landscape." *Folia Primatologica* 91, no. 6: 688-696. https://doi.org/10.1159/000508609.

McCarthy, M. S., J. D. Lester, E. J. Howe, M. Arandjelovic, C. B. Stanford, and L. Vigilant. 2015. "Genetic Censusing Identifies an Unexpectedly Sizeable Population of an Endangered Large Mammal in a Fragmented Forest Landscape." *BMC Ecology* 15: 21. https://doi.org/10.1186/s12898-015-0052-x.

McCarthy, M. S., J. D. Lester, and C. B. Stanford. 2017. "Chimpanzees (*Pan troglodytes*) Flexibly Use Introduced Species for Nesting and Bark Feeding in a Human-Dominated Habitat." *International Journal of Primatology* 38: 321-337. https://doi.org/10.1007/s10764-016-9916-y.

McLennan, M. R. 2008. "Beleaguered Chimpanzees in the Agricultural District of Hoima, Western Uganda." *Primate Conservation* 23: 45-54.
https://doi.org/10.1896/052.023.0105.

McLennan, M. R. 2013. "Diet and Feeding Ecology of Chimpanzees (*Pan troglodytes*) in Bulindi, Uganda: Foraging Strategies at the Forest-Farm Interface." *International Journal of Primatology* 34: 585-614.
https://doi.org/10.1007/s10764-013-9683-y.

McLennan, M. R., and C. Asiimwe. 2016. "Cars Kill Chimpanzees: Case Report of a Wild Chimpanzee Killed on a Road at Bulindi, Uganda." *Primates* 57: 377-388. https://doi.org/10.1007/s10329-016-0528-0.

McLennan, M. R., and C. M. Hill. 2010. "Chimpanzee Responses to Researchers in a Disturbed Forest-Farm Mosaic at Bulindi, Western Uganda." *American Journal of Primatology* 72: 907-918. https://doi.org/10.1002/ajp.20839.

McLennan, M. R., and C. M. Hill. 2012. "Troublesome Neighbours: Changing Attitudes Towards Chimpanzees (*Pan troglodytes*) in a Human-Dominated Landscape in Uganda." *Journal for Nature Conservation* 20: 219-227. https://doi.org/10.1016/j.jnc.2012.03.002.

McLennan, M. R., B. Hintz, V. Kiiza, J. Rohen, G. A. Lorenti, and K. J. Hockings. 2021. "Surviving at the Extreme: Chimpanzee Ranging is Not Restricted in a Deforested Human-Dominated Landscape in Uganda." *African Journal of Ecology* 59, no. 1: 17-28. https://doi.org/10.1111/aje.12803.

McLennan, M. R., C. P. Howell, M. Bardi, and M. Heistermann. 2019. "Are Human-Dominated Landscapes Stressful for Wild Chimpanzees (*Pan troglodytes*)?" *Biological Conservation* 233: 73-82. https://doi.org/10.1016/j.biocon.2019.02.028.

McLennan, M. R., G. A. Lorenti, T. Sabiiti, and M. Bardi. 2020. "Forest Fragments Become Farmland: Dietary Response of Wild Chimpanzees (*Pan troglodytes*) to Fast-Changing Anthropogenic Landscapes." *American Journal of Primatology* 82: e23090. https://doi.org/10.1002/ajp.23090.

McLennan, M. R., N. Spagnoletti, and K. J. Hockings. 2017. "The Implications of Primate Behavioral Flexibility for Sustainable Human-
Primate Coexistence in Anthropogenic Habitats.” *International Journal of Primatology* 38: 105–121. https://doi.org/10.1007/s10764-017-9962-0.

Mwavu, E., and E. Witkowski. 2008. "Land Use and Cover Changes (1988–2002) Around Budongo Forest Reserve, NW Uganda: Implications for Forest and Woodland Sustainability." *Land Degradation & Development* 19: 606–622. https://doi.org/10.1002/ldr.869.

Nakamura, M., Hosaka, K., Itoh, N., and Zamma, K., eds. 2015. *Mahale Chimpanzees: 50 Years of Research*. Cambridge: Cambridge University Press. https://doi.org/10.1086/698097.

Namusisi, S., M. Mahero, D. Travis, K. Pelican, C. Robertson, and L. Mugisha. 2021. "A Descriptive Study of Zoonotic Disease Risk at the Human-Wildlife Interface in a Biodiversity Hot Spot in South Western Uganda." *PLoS Neglected Tropical Diseases* 15: e0008633. https://doi.org/10.1371/journal.pntd.0008633.

Ogwang, T., and F. Vanclay. 2019. "Social Impacts of Land Acquisition for Oil and Gas Development in Uganda." *Land* 8, no. 7: 109. https://doi.org/10.3390/land8070109.

Onelcin, P., and Y. Alver. 2017. "The Crossing Speed and Safety Margin of Pedestrians at Signalized Intersections." *Transportation Research Procedia* 22: 3–12. https://doi.org/10.1016/j.trpro.2017.03.002.

Onyango, N. 2009. "On the Linear Mixed Effects Regression (LMER) R Function for Nested Animal Breeding Data." *Case Studies in Business, Industry and Government Statistics* 4: 44–58.

Pepper, J. W., J. C. Mitani, and D. P. Watts. 1999. "General Gregariousness and Specific Social Preferences Among Wild Chimpanzees." *International Journal of Primatology* 20: 613–632. https://doi.org/10.1023/A:1020760616641.

Perumal, L., M. G. New, M. Jonas, and W. Liu. 2021. "The Impact of Roads on Sub-Saharan African Ecosystems: A Systematic Review." *Environmental Research Letters* 16: e113001. https://doi.org/10.1088/1748-9326/ac2ad9.

Peters, D., L. Kim, R. Zaman, G. Haas, J. Cheng, and S. Ahmed. 2015. "Pedestrian Crossing Behavior at Signalized Intersections in New York City." *Transportation Research Record: Journal of the Transportation Research Board* 2519: 179–188. https://doi.org/10.3141/2519-20.

Pierce, A. H. 2009. "An Encounter Between a Leopard and a Group of Chimpanzees at Gombe National Park." *Pan Africa News* 16: 22–24. https://doi.org/10.5134/143505.

Plumptre, A. J., R. Rose, G. Nangendo, et al. 2010. *Eastern Chimpanzee (Pan troglodytes schweinfurthii): Status Survey and Conservation Action Plan 2010–2020*. Gland, Switzerland: IUCN/SSC Primate Specialist Group.

R. Core Team. 2022. *R: A Language and Environment for Statistical Computing*. Vienna (Austria): R Foundation for Statistical Computing. https://www.R-project.org/.

Reynolds, V., J. Wallis, and R. Kyamanywa. 2003. "Fragments, sugar, and chimpanzees in Masindi District, Western Uganda." In *Primates in Fragments*, edited by L. K. Marsh, 309–320. Boston, MA: Springer. https://doi.org/10.1007/978-1-4757-3770-7_20.

Rhine, R. J., and R. Tilson. 1987. "Reactions to Fear as a Proximate Factor in the Sociospatial Organization of Baboon Progressions." *American Journal of Primatology* 13: 119–128. https://doi.org/10.1002/ajp.1350130203.

Rowell, T. 1969. "Long-Term Changes in a Population of Ugandan Baboons." *Folia Primatologica* 11: 241–254. https://doi.org/10.1159/000155273.

Satsias, Z. M., M. J. Silk, K. J. Hockings, M. Cibot, J. Rohen, and M. R. McLennan. 2022. "Sex-Specific Responses to Anthropogenic Risk Shape Wild Chimpanzee Social Networks in a Human-Impacted Landscape." *Animal Behaviour* 186: 29–40. https://doi.org/10.1016/j.anbehav.2022.01.016.

Schmitt, D., M. Cartmill, T. M. Griffin, J. B. Hanna, and P. Lemelin. 2006. "Adaptive Value of Ambling Gaits in Primates and Other Mammals." *Journal of Experimental Biology* 209: 2042–2049. https://doi.org/10.1242/jeb.02235.

Simpson, G., L. Johnston, and M. Richardson. 2003. "An Investigation of Road Crossing in a Virtual Environment." *Accident Analysis & Prevention* 35, no. 5: 787–796. https://doi.org/10.1016/S0001-4575(02)00081-7.

Sugiyama, Y. 2004. "Demographic Parameters and Life History of Chimpanzees at Bossou, Guinea." *American Journal of Physical Anthropology* 124: 154–165. https://doi.org/10.1002/ajpa.10345.

Uganda Bureau of Statistics. 2016. *The National Population and Housing Census 2014 - Main Report*. Kampala, Uganda: Uganda Bureau of Statistics. https://www.ubos.org/wp-content/uploads/publications/03_20182014_National_Census_Main_Report.pdf.

Uganda National Roads Authority. 2023. *Annual performance report—FY 2022/23*. Kampala, Uganda: Uganda National Roads Authority. https://www.unra.go.ug/resources/publications/annual-performance-reports.

Vanthomme, H., J. Kolowski, L. Korte, and A. Alonso. 2013. "Distribution of a Community of Mammals in Relation to Roads and Other Human Disturbances in Gabon, Central Africa." *Conservation Biology* 27: 281–291. https://doi.org/10.1111/cobi.12017.

Wallace, G. E., and C. M. Hill. 2012. "Crop Damage by Primates: Quantifying the Key Parameters of Crop-Raiding Events." *PLoS One* 7: e46636. https://doi.org/10.1371/journal.pone.0046636.

Weng, L., A. K. Boedhihartono, P. H. G. M. Dirks, J. Dixon, M. I. Lubis, and J. A. Sayer. 2013. "Mineral Industries, Growth Corridors and Agricultural Development in Africa." *Global Food Security* 2: 195–202. https://doi.org/10.1016/j.gfs.2013.07.003.

Wheeler, B., and M. Torchiano. 2016. "Permutation Tests for Linear Models." *Comprehensive R Archive Network*. Accessed January 30, 2025. https://cran.r-project.org/web/packages/lmPerm/index.html.

Wilson, M., M. Hauser, and R. Wrangham. 2007. "Chimpanzees (*Pan troglodytes*) Modify Grouping and Vocal Behaviour in Response to Location-Specific Risk." *Behaviour* 144: 1621–1653. https://doi.org/10.1163/156853907782512137.

## Supporting Information

Additional supporting information can be found online in the Supporting Information section.