# Do Mixed-Species Groups Travel as One? An Investigation on Large African Herbivores Monitored Using Animal-Borne Video Collars

Romain Dejeante, Marion Valeix, Simon Chamaillé-Jammes

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Romain Dejeante, Marion Valeix, Simon Chamaillé-Jammes. Do Mixed-Species Groups Travel as One? An Investigation on Large African Herbivores Monitored Using Animal-Borne Video Collars. The American Naturalist, 2025, 205 (4), pp.451-458. 10.1086/734410. hal-05370863

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**Type of article:** Natural History Miscellany Notes

# Do mixed-species groups travel as one? An investigation on large African herbivores using animal-borne video collars

Romain Dejeante¹* ; Marion Valeix¹ ; Simon Chamaillé-Jammes¹,²

1. CEFE, Univ Montpellier, CNRS, EPHE, IRD, Montpellier, France

2. Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria, South Africa

**Corresponding author:** Romain Dejeante, CEFE-CNRS, 1919 route de Mende, 34293 Montpellier cedex 5, France. Phone: (+33)(0)467613218.

**Email list:**

romain.dejeante@cefe.cnrs.fr

marion.valeix@cefe.cnrs.fr

simon.chamaille@cefe.cnrs.fr
ABSTRACT

Although prey foraging in mixed-species groups benefit from a reduced risk of predation, whether heterospecific groupmates move together in the landscape, and more generally to what extent mixed-species groups remain cohesive over time and space, remains unknown. Here, we used GPS collars with video cameras to investigate the movements of plains zebras (*Equus quagga*) in mixed-species groups. Blue wildebeest (*Connochaetes taurinus*), impalas (*Aepyceros melampus*) and giraffes (*Giraffa camelopardalis*) commonly form mixed-species groups with zebras in savanna ecosystems. We found that zebras adjust their movement decisions solely to the presence of giraffes, being more likely to move in zebra-giraffe herds, and this was correlated to a higher cohesion of such groups. Additionally, zebras moving with giraffes spent longer time grazing, suggesting that zebras follow giraffes to forage in their proximity. Our results provide new insights on animal movements in mixed-species groups, contributing to a better consideration of mutualism in movement ecology.

**Keywords:** animal movement; heterospecifics; movement ecology; mutualism; landscape of fear; predator-prey interactions
# INTRODUCTION

Interactions with heterospecifics may affect animal movement decisions. An obvious example is how prey respond to an encounter with a predator (Courbin et al. 2016). However, although interspecific interactions are much more diverse than predator-prey interactions, ecologists mainly focus on the influence of the negative interspecific interactions (such as predation or competition) on animal movement ecology (Shaw et al. 2021). Therefore, little is known about how positive interspecific interactions may motivate or constrain movement decisions of individuals (Shaw et al. 2021).

Mixed-species groups exemplify positive interactions among prey, which may include early predator detection due to complementary sensory modalities of heterospecifics or a reduced risk of attack to individuals (predator dilution) when species share a common predator (Goodale et al. 2019, 2020). In savanna ecosystems, large herbivores commonly form mixed-species groups to level the landscape of fear (Pays et al. 2014; Beaudrot et al. 2020; Saltz et al. 2023). They are more likely to associate with heterospecifics when the risk of predation is high, an observation sometimes interpreted in accordance with the *stress gradient hypothesis* (Beaudrot et al. 2020; Kiffner et al. 2022). Additionally, resource availability and competition between heterospecifics also influence the formation of mixed-species groups. Large herbivores may associate with heterospecifics when resources are abundant and competition is limited, as during the wet season (Beaudrot et al. 2020; Kiffner et al. 2022). The decision to form mixed-species groups should balance the benefits of reduced predation risk against the costs of foraging with potential competitors.

Although the benefits of mixed-species groups have received a lot of attention, whether heterospecific groupmates move together in the landscape, and more generally to what extent
mixed-species groups remain cohesive over time and space remains largely unknown (Carlson et al. 2023). Our understanding of the link between positive interspecific interactions and animal movements is limited by the difficulty to simultaneously, and over time, track the movements of animals and the context in which they occur, in particular the presence or absence of other species. Although large deployment of GPS collars can sometimes allow the simultaneous investigation of spatial and social behaviours (Wilmers et al. 2015), many limitations remain: collars generally do not provide information on the fine-scale behaviour of the tracked individuals (e.g. foraging vs. vigilance) nor on the presence of untracked individuals. Therefore, complementary tools are required. In particular, animal-borne video cameras could fill this methodological gap, as they provide data to understand the social context and interactions of tracked animals (Yoda et al. 2011; Bombara et al. 2017; Kaczensky et al. 2019).

Here, we used GPS collars equipped with video cameras to investigate whether plains zebras (*Equus quagga*), which are regularly observed in mixed-species groups in African savannas (Schmitt et al. 2014, 2016; Stears et al. 2020; see Figure 1 for an illustration of a mixed-species group seen through the eyes of a collared zebra), do travel with heterospecifics in the landscape. Because zebras are social animals living in a group with conspecifics, we compared their movement and foraging behaviour when herding with conspecific-only or when herding in mixed-species groups composed of zebras and blue wildebeest (*Connochaetes taurinus*), impala (*Aepyceros melampus*), giraffe (*Giraffa camelopardalis*) or African buffalo (* Syncerus caffer*) groupmates.

Plain zebras, which have a central place in mixed-species networks (Kiffner et al. 2014), commonly form associations with wildebeest, buffalos, impalas or giraffes (Kiffner et al. 2014, 2022). The benefits they gain depend on the species they associate with. In particular, because of
their height and good eyesight (Mitchell et al. 2013), giraffes are particularly advantageous herding partners for zebras to detect predators (Schmitt et al. 2016). Some species can also dilute more the risk of predation. Indeed, as African lions (*Panthera leo*) preferentially prey upon buffalo, wildebeest and zebra rather than impala (Hayward and Kerley 2005), the latter can be considered as a "low-diluting partner" for zebras – that reduce less their individual-level vigilance when herding with impalas than with "high-diluting partners" as wildebeest (Schmitt et al. 2014). However, as grazers (grass-eater), zebras may compete more severally with wildebeest and buffalos – two grazers – than with impalas that are mixed-feeders (grass and non-grass-eaters) or with giraffes that are browsers (non-grass-eaters) (Arsenault and Owen-Smith 2008; Anderson et al. 2024). Here, we therefore investigated whether zebras form mixed-species groups over longer time – and potentially travel longer and further – with high-detection partners (i.e., giraffes), with high-diluting partners (i.e., buffalos or wildebeest) or with low-competing partners (i.e., impalas).

# METHODS

## Study site

The study was conducted in the ~ 900 km² Hluhluwe-iMfolozi Park, South Africa. In the park, rainfall and vegetation vary greatly with topography, resulting in a gradient between higher elevation (~750m), higher rainfall (~975 mm per year) and denser vegetation in the North to lower elevation (~60m), lower rainfall (~600 mm per year) and bushed grasslands in the South. A full guild of large mammalian carnivores is present in the park, including African lions (*Panthera leo*), leopards (*Panthera pardus*), spotted hyaenas (*Crocuta crocuta*), cheetahs (*Acinonyx jubatus*) and wild dogs (*Lycaon pictus*). Hluhluwe-iMfolozi Park has also a wide diversity of large mammalian herbivores, including giraffes, buffalo, zebras, wildebeest and impalas.
## Animal-born video cameras collars

We equipped 6 female zebras, each in a different harem, with side-view camera collars during two capture sessions in October 2022 (n=3) and October 2023 (n=3), i.e., at the beginning of the wet season. Each camera collar recorded 10-second videos every 5 minutes from 05:00 to 18:00 and collected one GPS-location every 15-minute during the day and night time over three weeks. To match each video clip to one zebra location, we interpolated zebra trajectories to 5-minute using a continuous-time correlated random walk model, as implemented in the R package * crawl* (Johnson et al. 2008). Each of the 17 085 video clips collected in total were observed and manually analysed by R.D. to record whether the tracked zebra was foraging or not and whether it was in a mixed-species group.

## Identification of mixed-species groups

In total, 2 298 out of the 17 085 video clips (13%) showed that the collared zebra was in close proximity (~50 meters) to heterospecifics. However, animals could be close to the monitored zebra and not be visible on the footage. Indeed, in 6 380 (37%) of the video clips, no other zebra was seen, although we know that individuals of a harem remain together and therefore other zebras were close by (but not seen). We therefore investigated over which interval duration we could assume a species' presence if not seen (see Appendix S1: Figures S1-S2), and found that we could consider that zebras remained with another species when the time between two video clips showing that species was shorter than one hour. Our results were robust to the use of different temporal thresholds (see Appendix S1: Figure S3 for the sensitivity analyses). We used the same temporal threshold for all species so that we can compare the duration of mixed-species groups for all species pairs.

## Influence of environmental conditions on the cohesion of mixed-species groups
Differences of environmental conditions are known to influence the formation of mixed-species
groups in savanna ecosystem: herbivores are more likely to be associated when resources are
abundant, as resource competition would constrain the formation of mixed-species groups
(Kiffner et al. 2014; Beaudrot et al. 2020). Here, collared zebras spent nearly all their time in
open areas, with no variation when herding with heterospecific or not. The averaged percentage
of habitat openness of zebras' locations was 96% (CI 95% = [95; 97]) when zebras herded with
impalas, 97% (CI 95% = [96; 98]) when zebras herded with buffalos, 97% (CI 95% = [95; 98])
when zebras herded with giraffes, 96% (CI 95% = [94; 97]) when zebras herded with wildebeest,
and the average habitat openness when zebras herded without heterospecifics was 95% (CI 95%
= [95; 95]). As a result, here, we could not address the influence of habitat structure on the
cohesion of mixed-species groups but it should also prevent us from any confounding effect due
to habitat structure.

Study of the fission dynamics of mixed-species groups

To examine the cohesion of mixed-species groups, we conducted a survival analysis of mixed-species groups, modelling the risk of fission of mixed-species groups over time. Since the risk of fission of mixed-species groups may depend on zebras' movements, we added an interaction term between zebras' step lengths and whether zebras were with giraffes, impalas, wildebeest or buffalos. We did so by fitting a Cox proportional hazard model using the R package *survival* (Therneau 2015). Since video clips provided information – and hence showed heterospecifics – during daytime only (from 05:00 to 18:00), we controlled for the fact that the observations could be censored, either if the monitored zebra was already with heterospecifics one hour after sunrise (15% of mixed-species groups) or if the monitored zebra was still with heterospecifics one hour before sunset (17% of mixed-species groups).
## Study of zebra movements in mixed-species groups

To investigate whether zebras cover long distances when in mixed-species groups, we fitted a Hidden Markov Model (HMM) to step length and turning angles of the movement trajectories to classify each step into a ‘moving’ (large step length and small turning angle) or a ‘encamped’ (small step length and large turning angle) state. We modelled step length using a gamma distribution and turning angle using a Von Mises distribution. To test whether zebras’ movement behaviours could be affected by the presence of heterospecifics, we modelled the transition probability between states as a function of the presence or absence of giraffes, impalas, wildebeest or buffalos. We controlled for diel changes in the probability of being in the ‘moving’ state, independently of heterospecifics, by including hour of the day as a non-linear predictor using splines. Zebra identity was added as a random intercept to account for individual heterogeneity in movement behaviour. This analysis was conducted using the R packages *moveHMM* (Michelot et al. 2016) to calculate step lengths and turning angles, and *hmmTMB* (Michelot 2023) to fit the hidden Markov model using non-linear predictor and random effects. In addition, we quantified the net displacement made by zebras during the periods when they were in mixed-species groups. We did this by calculating the maximum distance between the first location of a period they were seen with another species and all other locations of the same period.

## Study of zebra foraging behaviour in mixed-species groups

From each video clip, we classified whether the collared zebra was grazing or not (i.e., standing, scanning the environment, walking) based on the tilted horizon of the image (Figure 1). This approach has been used previously by Kaczensky et al. (2019) to estimate the foraging of khulans (*Equus hemionus*) monitored using video collars. We then investigated whether zebras’
probability of foraging increased when heterospecifics were present, by fitting a mixed logistic regression with the behaviour of the zebra as a binary response (foraging or not) and the presence of giraffes, impalas, wildebeest and buffalos as predictors. Additionally, because zebras' foraging behaviour in mixed-species groups may depend on their movement behaviours, we added interactions terms between the state identified from the HMM (i.e., encamped or moving state) and the presence / absence of giraffes, impalas, wildebeest or buffalos. As before, we controlled for diel changes in the probability of being foraging, independently of heterospecifics, by including hour of the day as a non-linear predictor using splines. Zebra identity was added as a random intercept to account for individual heterogeneity in foraging rates.

## RESULTS

### General information on mixed-species groups

We observed 373 independent events of zebras in mixed-species groups, mostly with impalas (n = 196) but also with buffalos (n = 80), wildebeest (n = 76) and giraffes (n = 21). On average, collared zebras spent about 27% of their time with another species (16% with impalas, 5% with buffalos, 4% with wildebeest and 3% with giraffes), and less than 2% of their time with more than one species (for example with wildebeest and impalas simultaneously). However, the percentage of time that zebras spent with heterospecifics greatly varied between the monitored harems (5%; 24%; 27%; 28%; 35%; 45%). The lowest value was for a harem in which an adult female gave birth during the tracking period, potentially leading this harem to adopt peculiar spatial and social behaviours. We did not find any clear diel variations (from 05:00 to 18:00) in the temporal pattern of zebras' co-occurrence with impalas, wildebeest, buffalos and giraffes (Appendix S2: Figure S1).

### Fission dynamics of mixed-species groups
On average, zebras remained 1.2 hour with impalas, 0.8 hour with wildebeest, 1 hour with buffalos and 2.2 hours with giraffes. However, the duration of mixed-species groups was associated with zebras' step lengths (Table 1; Appendix S2: Figure S2). Surprisingly, the risks of fission of zebra-wildebeest and zebra-giraffe herds were reduced when zebras moved along longer steps. For a mean step length of zebras, the risk of fission was 0.55 times lower [CI = 0.32; 0.97] when zebras herded with giraffes than with wildebeest and 0.68 times lower [CI = 0.53; 0.86] when zebras herded with impalas than with wildebeest.

## Zebra movements in mixed-species groups

The net displacement of zebras was the greatest during periods with giraffes [mean = 300 meters; 90% percentile = 915 meters], lower when with impalas [mean = 239 meters; 90% percentile = 631 meters], and even lower when with buffalos [mean = 141 meters; 90% percentile = 420 meters] or wildebeest [mean = 140 meters; 90% percentile = 311 meters]. Zebras were as likely to shift from moving to encamped states when herding with heterospecifics as when herding with conspecifics only (Appendix S2: Table S1; Appendix S2: Figure S3). Similarly, zebras were as likely to shift from encamped to moving states when herding with wildebeest or buffalos as when herding with conspecifics only (Appendix S2: Table S1; Appendix S2: Figure S3). However, zebras were more likely to shift from encamped to moving states when they herded with impalas and even more with giraffes, leading them to spend more time moving when herding with those species (Figure 2).

## Zebra foraging behaviour in mixed-species groups

Zebras in mixed species groups were more likely to be in a foraging state: they spent 55% [95% CI = 50; 59] of their time grazing when they herded with giraffes, 50% [95% CI = 46; 55] with wildebeest, 49% [95% CI = 45; 53] with buffalos, 49% [95% CI = 46; 52] with impalas and 45%
[95% CI = 43; 48] when zebras were without heterospecifics. Additionally, we found that zebras herding without heterospecific, or herding with buffalos, wildebeest or impalas were less likely to forage when moving, but not when they herded with giraffes (Figure 3; Appendix S2: Table S2).

DISCUSSION

Here, we investigated the movement of plains zebras in mixed-species groups, providing new insights on prey mutualism in savanna ecosystems and its dynamics. We showed that using animal-borne video collars is an adequate tool to explore the movement of mixed-species groups and offer promising perspectives on the links between animal movement and positive interspecific interactions, a current appealing question in movement ecology (Shaw et al. 2021).

Previous studies, as well as ours, documented the particular importance of giraffes in savanna ecosystems to level the landscape of fear, showing that zebras in their company reduce more their vigilance than when herding with other species (Schmitt et al. 2016) or that herbivores form mixed-species groups with giraffes specifically under a high risk of predation (Kiffner et al. 2022). Here, we additionally found that zebras switch more often towards a moving state when herding with giraffes and this was associated with a lower risk of fission of zebra-giraffe groups. Together, these results suggest that zebras actively maintain the cohesion of the mixed-species groups they form with giraffes, seeking to move in their company and potentially following them.

Unfortunately, because animal-borne video collars allowed us to detect the presence of giraffes but not to track their movement, we could not address which species was leader or follower.

Schmitt et al. (2016) reported observations of zebras herding with giraffes that “follow them and continue to feed in close proximity when the giraffe moved off”. Here, we provide quantified elements validating that zebras move more and spend longer time grazing when herding with giraffes. Possibly, the low abundance of giraffes in Hluhluwe-iMfolozi Park – which are 10 times
less abundant than impalas (le Roux et al. 2017) – may strengthen the behavioral adjustments of zebras to their presence, that may seek to stay as long as possible with a rare partner that increases their chances to detect a predator (Schmitt et al. 2016). It is indeed important to note that herding events with giraffes were rare, relatively to herding events with other species, a result likely driven by the abundance of each species in Hluhluwe-iMfolozi Park.

Although zebras spent more time grazing when herding with impalas, buffalos, and wildebeest than when herding with conspecific-only, they did not adjust their movement behaviour to the presence of other species than giraffes. Previous studies showed that zebras actively seek proximity with some heterospecific – especially wildebeest – leading to stronger association with them than expected by chance or based on habitat preferences (Kiffner et al. 2014, 2022). However, here, our results suggest that once mixed-species groups are formed, zebras do not travel with wildebeest, impalas or buffalos – or at least travel less than with giraffes. Similarly to the decision to form mixed-species groups (Beaudrot et al. 2020; Kiffner et al. 2022; Saltz et al. 2023), the decision to travel with heterospecific throughout the landscape should balance the benefits of reduced predation risk against the costs of foraging with potential competitors. Interestingly, we found that the cohesion of mixed-species groups that zebras formed with wildebeest and buffalos – three grazers – was lower than with giraffes and impalas – respectively browsers and mixed-feeders. Resource competition between grazers may constrain the cohesion of mixed-species groups by forcing one species to move away when resources are locally depleted (Anderson et al. 2024). However, although zebras, wildebeest and buffalos may compete for resources, dissimilarities in the plant species consumed still persist between these species (Pansu et al. 2022) and resource competition may therefore not totally explain differences of cohesion of mixed-species groups. On the contrary, because of differences in dietary niche or
in habitat selectivity, maintaining group cohesion is challenging and individuals should try to maintain cohesion on the long-run only if the benefits of doing so are high. Conflicts of interests in decision making indeed lower the cohesion of single-species groups (Conradt and Roper 2000; Sueur et al. 2011; Le Goff et al. 2024) and can be expected to challenge even more strongly the cohesion of mixed-species groups. For example, wildebeest are more selective grazers than zebras (Owen-Smith et al. 2015), preferentially using ‘grazing lawns’ that can be scarce in the landscape, and are therefore more constrained than zebras in where to move (Hopcraft et al. 2014; Martin and Owen-Smith 2016).

## CONCLUSION

Altogether, this work adds to the literature on the formation of mixed-species groups by originally assessing the cohesion and movement of these groups, and paves the way for future research on the fission-fusion dynamics and cohesion dynamics of mixed-species groups.

Additionally, an appealing, unanswered question would be to address whether the cohesion of mixed-species groups increases when the risk of predation is higher (spatially or temporally) i.e., when the benefits of moving with heterospecifics trade-off the cost of foraging with potential competitors.

## Conflict of interest statement

The authors declare no competing interests.

## Statement of authorship

SCJ initiated the zebra research project, acquired funding and coordinated data acquisition. SCJ and RD conceived the ideas and designed the statistical methodology. RD collected data and
conducted the statistical analyses. RD, SCJ and MV interpreted the results. RD led the writing of
the manuscript. RD, SCJ and MV revised and edited the manuscript.

## Acknowledgements

We are grateful to Ezemvelo for providing the authorization to conduct this research, and
particularly to D.J. Druce and S. Mbongwa for their continuous administrative and field support.
We acknowledge the field efforts of B. Gehr, Y. Richard, R. Leeming, J. Lawrence, M. Krings
and anyone involved in the zebra captures. This study was partly funded by the TERRA FORMA
project (ANR-21-ESRE-0014), the REPOS project funded by the i-Site MUSE (ANR-16-IDEX-
0006), the SOCIALIPOP project (ANR-22-CE02-0011-02) and the FutureFear project sponsored
by the Fondation BNP Paribas.

Archiving Data: Data and code are archived on a figshare depository publicly available (Dejeante et al. 2024)

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TABLE

<table><caption>Table 1. Coefficients (β), hazard ratios (exp[β]) and standard errors (SE) for the Cox model testing the influence of herd mate species and zebras' step lengths on the risk of fission of mixed-species groups. Positive coefficients show higher risk of fission of the mixed-species groups, relatively to the reference case, here zebra-wildebeest herds. The step length variable is scaled.</caption><thead><tr><th>Variables</th><th>β</th><th>exp (β)</th><th>SE</th><th>z-value</th><th>p-value</th></tr></thead><tbody><tr><td colspan="6"><strong>Risk of fission of mixed-species groups</strong></td></tr><tr><td>Zebra – Buffalo</td><td>-0.07</td><td>0.93</td><td>0.25</td><td>-0.27</td><td>0.78</td></tr><tr><td>Zebra – Impala</td><td>-0.39</td><td>0.68</td><td>0.12</td><td>-3.21</td><td>&lt; 0.01</td></tr><tr><td>Zebra – Giraffe</td><td>-0.59</td><td>0.55</td><td>0.28</td><td>-2.08</td><td>&lt; 0.05</td></tr><tr><td>Step length</td><td>-0.15</td><td>0.86</td><td>0.06</td><td>-2.52</td><td>&lt; 0.05</td></tr><tr><td>Zebra – Buffalo : Step length</td><td>0.18</td><td>1.20</td><td>0.08</td><td>2.23</td><td>&lt; 0.05</td></tr><tr><td>Zebra – Impala : Step length</td><td>0.20</td><td>1.22</td><td>0.07</td><td>2.81</td><td>&lt; 0.01</td></tr><tr><td>Zebra – Giraffe : Step length</td><td>-0.72</td><td>0.49</td><td>0.66</td><td>-1.08</td><td>0.28</td></tr></tbody></table>
<figure><img src="image_2.png" /></figure>

Figure 1. Example of a collared zebra herding with giraffes (pointed by dark arrows) as captured by side-view animal-borne video cameras collecting 10-second videos every 5 minutes.
<figure><img src="image_3.png" /><figcaption>Figure 2. Influence of the presence of other species on the probability that zebras are in the ‘encamped’ or ‘moving’ states. The ‘Zebra-only’ case refers to zebras moving without heterospecifics. Points show the mean parameter estimates and whiskers show 95% confidence intervals.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

Figure 3. Influence of the presence of other species, and of the movement state of zebras, on the probability to observe zebras grazing. Grazing events were identified based on the tilted horizon of the image recorded by camera collars. The ‘Zebra-only’ case refers to zebras moving without heterospecifics. Points show the mean parameter estimates and whiskers show 95% confidence intervals.
# Appendix S1. Finding the interval over which assuming a species' presence if not seen is correct

Because animals could be close to the monitored zebra and not be visible on the footage (side-view video), we needed an approach to define whether zebras remained in mixed-species herds when we recorded the presence of another species but that species had not been seen on video clips for a certain time. To do so, we decided to use a temporal threshold below which we could assume a species' presence if not seen (under the condition that the species was seen a certain time ago). To use a relevant temporal threshold, we used three approaches:

First, R.D. conducted field observations to record whether other species were around the collared zebra. Observations lasted as long as collared zebras were visible in the field. When zebras were known (from field observations) to herd with heterospecifics over a certain period (for example herding with wildebeest during 2h), we compared how many 10-second video clips were needed to detect the presence of wildebeest around the collared zebras (from collar observations) (Figure S1). Doing so, we found that after 1h of video clip observations, at least one video clip showed the heterospecific i.e., conversely, after 1h of video clip observations, if no videos showed heterospecific it is likely that no other species was around the collared zebra.

Second, because zebra harems are cohesive, we expect to observe one zebra-only herd event per day, which should last the whole day (~13 hours). We therefore iteratively tried different temporal thresholds (from 5 minutes to 3 hours) and then compared the number of zebra-only herd events detected and their durations according to these thresholds. Detecting more than one zebra-only herd event would indicate that the corresponding temporal threshold is too short and does not allow us to interpolate the presence of zebras not seen on video clips – but known to be in the surrounding. Figure S2 shows that using a temporal threshold higher than 1
hour is needed; otherwise, we would frequently consider temporal periods when zebras are assumed to move alone, whereas the collared zebra is still in a harem.

Third, we used the different temporal thresholds (from 5 minutes to 3 hours) to compare the number of independent encounters with heterospecifics identified from video clips. Figure S3 shows that using a temporal threshold higher than 1 hour is needed; otherwise, the number of encounters is not at a plateau: encounters with heterospecifics are not independent.

<figure><img src="image_5.png" /></figure>

Figure S1. Cumulative probability to detect the presence of a mixed-species herd from video clips when the presence of heterospecific was known from field observation of collared zebras.
<figure><img src="image_6.png" /></figure>

Figure S2. (A) Mean duration and (B) mean number of zebra-only group-sightings detected per day from 10-second video clips according to the time of interpolation used to assess that a zebra is still present around the collared zebra even when not visible on video clips. Because zebra harems are cohesive, we expect to observe only one zebra-only herd event per day, throughout the day (~13 hours).
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

Figure S3. Numbers of zebra-wildebeest group-sightings, zebra-giraffe group-sightings, zebra-impala group-sightings and zebra-buffalo group-sightings according to the time of interpolation used to assess that a species is still present around the collared zebra even when not visible on video clips.
## Appendix S2. Additional tables and figures

<table><caption>Table S1. Statistics of the model testing the influence of being in a mixed-species herd on the probability that zebras shifted from moving to encamped or, conversely, from encamped to moving states.</caption><thead><tr><th>Variables</th><th>β</th><th>IC low</th><th>IC up</th></tr></thead><tbody><tr><td>Moving to Encamped: Zebra-single</td><td>-1.33</td><td>-1.46</td><td>-1.21</td></tr><tr><td>Moving to Encamped: Zebra – Impala</td><td>0.14</td><td>-0.06</td><td>0.34</td></tr><tr><td>Moving to Encamped: Zebra – Giraffe</td><td>0.18</td><td>-0.20</td><td>0.56</td></tr><tr><td>Moving to Encamped: Zebra – Wildebeest</td><td>0.03</td><td>-0.33</td><td>0.40</td></tr><tr><td>Moving to Encamped: Zebra – Buffalo</td><td>-0.20</td><td>-0.55</td><td>0.16</td></tr><tr><td>Encamped to Moving: Zebra-single</td><td>-2.21</td><td>-2.48</td><td>-1.94</td></tr><tr><td>Encamped to Moving: Zebra – Impala</td><td>0.30</td><td>0.09</td><td>0.50</td></tr><tr><td>Encamped to Moving: Zebra – Giraffe</td><td>0.61</td><td>0.20</td><td>1.01</td></tr><tr><td>Encamped to Moving: Zebra – Wildebeest</td><td>-0.10</td><td>-0.48</td><td>0.27</td></tr><tr><td>Encamped to Moving: Zebra – Buffalo</td><td>-0.19</td><td>-0.54</td><td>0.16</td></tr></tbody></table>
<table><caption>Table S2. Statistics of the model testing the influence of being in a mixed-species herd and of the movement state of zebras on the probability to observe zebras grazing.</caption><thead><tr><th>Variables</th><th>β</th><th>SE</th><th>z-value</th><th>p-value</th></tr></thead><tbody><tr><td>Zebra-single</td><td>0.47</td><td>0.17</td><td>2.76</td><td>&lt; 0.01</td></tr><tr><td>Zebra – Impala</td><td>0.15</td><td>0.06</td><td>2.63</td><td>&lt; 0.01</td></tr><tr><td>Zebra – Giraffe</td><td>0.24</td><td>0.12</td><td>1.92</td><td>0.06</td></tr><tr><td>Zebra – Wildebeest</td><td>0.30</td><td>0.11</td><td>2.81</td><td>&lt; 0.01</td></tr><tr><td>Zebra – Buffalo</td><td>0.26</td><td>0.09</td><td>2.81</td><td>&lt; 0.01</td></tr><tr><td>Moving</td><td>-0.13</td><td>0.04</td><td>-3.24</td><td>&lt; 0.01</td></tr><tr><td>Zebra – Impala: Moving</td><td>0.05</td><td>0.10</td><td>0.50</td><td>0.62</td></tr><tr><td>Zebra – Giraffe: Moving</td><td>0.35</td><td>0.19</td><td>1.84</td><td>0.07</td></tr><tr><td>Zebra – Wildebeest : Moving</td><td>-0.24</td><td>0.17</td><td>-1.36</td><td>0.17</td></tr><tr><td>Zebra – Buffalo: Moving</td><td>-0.30</td><td>0.15</td><td>-1.95</td><td>0.05</td></tr></tbody></table>
<figure><img src="image_11.png" /><figcaption>Figure S1. Distribution of mixed-species herds observations over the day. Each bar shows the number of video clips showing the corresponding species according to the hour of the day. In total, we obtained 315 video clips in which wildebeest could be seen, 1264 video clips with impalas, 472 video clips with buffalos and 247 video clips with giraffes.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

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

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

Figure S2. Temporal changes in the probability of cohesion of (a) zebra-wildebeest herds (b) zebra-impala herds (c) zebra-giraffe herds (d) zebra-buffalo herds. Fission events of zebra-giraffe herds were less likely when zebras moved along long distances (dark blue) than short distances (light blue) (p-value < 0.001; Table S). Time start at 0 is the onset of fusion of mixed-species herds. Ribbon extremities show 95% confidence interval, whereas lines show the mean probability of cohesion of mixed-species herds. Small vertical lines show actual fission events observed.
<figure><img src="image_20.png" /></figure>

Figure S3. Transition probability (1) from the moving to the encamped state and (2) from the encamped to the moving state as a function of the species zebras herd with. The ‘Zebra-only’ case refers to zebras moving without heterospecifics. Points show the mean parameter estimate and whiskers show 95% confidence intervals.