# Rank and social context influence sleep in wild chimpanzees

Clara Hozer, Francis Ahabwe, Noémie Freymond, Mathilde Cirulnikow, Bosco

Chandia, Monday Mbotella, David Samson, Klaus Zuberbühler

► To cite this version:

Clara Hozer, Francis Ahabwe, Noémie Freymond, Mathilde Cirulnikow, Bosco Chandia, et al. Rank and social context influence sleep in wild chimpanzees. Current Biology, In press, 23, pp.1323 - 1339. 10.1016/j.cub.2025.11.057. hal-05438352

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# Current Biology

## Rank and social context influence sleep in wild chimpanzees

### Highlights

* As an evolved function, sleep is best studied comparatively under natural settings

* Chimpanzees sleep in nocturnal nests monitored with remote IR cameras

* Sleeping in a group improved sleep quality, although with large individual differences

* Male sleep deteriorated with rank, male company, and estrous female presence

### Authors

Clara Hozer, Francis Ahabwe,
Noémie Freymond, ...,
Monday Mbotella, David Samson,
Klaus Zuberbühler

### Correspondence

hozerclara@hotmail.fr

### In brief

Using an innovative infrared-recording method, Hozer et al. document wild chimpanzees' nocturnal behavior. Group nesting generally improves sleep, but male sleep declines with high rank and the company of other males and estrous females. The study shows how fitness-enhancing diurnal activities generate nighttime physiological costs in an understudied evolutionary trade-off.
<figure><img src="image_2.png" /></figure>

Article

Rank and social context influence sleep
in wild chimpanzees

Clara Hozer,<fnref n="1" />,<fnref n="2" />,<fnref n="5" />,* Francis Ahabwe,<fnref n="2" /> Noémie Freymond,<fnref n="1" />,<fnref n="2" /> Mathilde Cirulnikow,<fnref n="2" /> Bosco Chandia,<fnref n="2" /> Monday Mbotella,<fnref n="2" /> David Samson,<fnref n="3" /> and Klaus Zuberbühler<fnref n="1" />,<fnref n="2" />,<fnref n="4" />

SUMMARY

Sleep is subject to Darwinian fitness and thereby constrained by ecological and social factors. Nevertheless, most comparative research on sleep is conducted in laboratory settings, detached from environmental and social influences, which is problematic for evolutionary theories. We examined the natural sleeping patterns of wild chimpanzees in Budongo Forest, Uganda, using a remote-controlled, infrared camera system. We found that sleep in chimpanzees was significantly affected by social factors, including the sleeper's own rank and the composition of the nearby sleeping party. Nesting in groups increased sleep duration and decreased sleep fragmentation compared with sleeping alone, despite the fact that it delayed nesting times and advanced wake times. Rank had little impact on female sleep but a strong influence on male sleep, with high-ranking males generally experiencing shorter and more fragmented sleep compared with subordinate males. The presence of sexually active females also reduced sleep duration, by delaying nest building, advancing wake time, and increasing sleep fragmentation. Our data show that natural sleep patterns in chimpanzees are largely determined by social variables that continue to exert their influence into the night. We discuss the implications of studying sleep patterns of our closest relatives in ecologically and socially valid situations for future research on the evolution of sleep.

INTRODUCTION

Sleep is universal—documented throughout the animal kingdom
from worms to humans.<fnref n="1" /> In many animal species, sleep is known
to impact positively on fitness-related variables, including im-
mune function, cognitive performance, and decision-making.<fnref n="2" />⁻<fnref n="4" />
However, sleep is also in a trade-off relationship with other vital
activities, such as foraging, mating, defending territory, caring
for offspring, and defense against predation.<fnref n="5" /> Selection is thus
expected to optimize an individual's time budget in species-spe-
cific ways by balancing the costs and benefits along physiolog-
ical, socio-ecological, and life-history constraints.⁶

In group-living animals, various intrinsic and extrinsic factors
are known to affect sleep. First, an individual's social position
within its group has a demonstrated impact. In freely moving
mice, dominant males show reduced deep sleep and increased
sleep fragmentation compared with subordinates,⁷ whereas in
gelada baboons (Theropithecus gelada), sleep duration in fe-
males and the amount of relaxed sleep in males are negatively
correlated with rank.⁸ Second, sleeping in groups is thought to
be more beneficial, compared with solitary sleeping, by reducing
individual predation risk, optimizing thermoregulation, and
facilitating information transfer.⁹ For instance, female teals
(Anas crecca) decrease their vigilance and neotropical bats
tend to sleep longer with increasing numbers of conspecifics
nearby.¹⁰,¹¹ In chimpanzees (Pan troglodytes), sleeping in

groups reduces an individual's exposure to biting insects, suggesting decreased exposure to pathogens. ¹² However, sleeping in groups can also impact an individual's sleep due to the activities of awake neighbors or, in the case of social subordinates, exclusion from favorable sleeping sites. ⁹ In olive baboons (Papio anubis) and orangutans (Pongo abelii), for instance, individual sleep duration is negatively related to the number of other group members at the sleeping site. ¹³,¹⁴ The trade-off between sleep and other vital functions is most evident in the mating context. Here, the assumption is that sexually competing males gain mating opportunities by sleeping less, albeit at the cost of decreased health and cognitive functioning, with potential long-term effects. Examples are male pectoral sandpipers (Calidris melano-tos) reducing their sleep during the breeding season ¹⁵ or male fruit flies (Drosophila melanogaster) exposed to female phero-mones suppressing sleep to increase mating opportunities. ¹⁶,¹⁷

The previous studies have illustrated how sleep is likely under
considerable selection pressure, suggesting that laboratory and
clinical studies are only partially suited to advance understanding
of the evolution of sleep. ¹⁸,¹⁹ Key selection pressures, such as
variation in climate, predation risk, food availability, and social fac-
tors, are typically controlled for in laboratory studies, despite their
profound influence on sleep and impact on time budgets. For
example, animals in captivity naturally spend less time foraging
than in the wild, which likely leads to differences in resting
duration, as demonstrated, for example, in elephants (Elephas

<footnote n="1">¹Institute of Biology, University of Neuchâtel, Neuchatel 2000, Switzerland</footnote>

<footnote n="2">²Budongo Conservation Field Station, PO Box 362, Masindi, Uganda</footnote>

<footnote n="3">³Department of Anthropology, University of Toronto Mississauga, 3359 Mississauga Road, Mississauga, ON L5L 1C6, Canada</footnote>

<footnote n="4">⁴School of Psychology and Neuroscience, University of St. Andrews, St. Andrews KY16 9JP, UK</footnote>

<footnote n="5">⁵Lead contact</footnote>

<footnote>*Correspondence: hozerclara@hotmail.fr</footnote>

<footnote>https://doi.org/10.1016/j.cub.2025.11.057</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1. Influence of solitary vs. group nesting on sleep-related variables**</figcaption></figure>

(A) Sleep duration, (B) sleep efficiency, (C) mean duration of wake bouts, (D) wake time, (E) nesting time, and (F) nest height. Large dots represent model-predicted values for each nesting condition (alone vs. group), with error bars indicating 95% CIs. Small points, color-coded by individual, show raw data values. Results are reported separately for males and females if there was a significant interaction with sex.

See STAR Methods for definitions of sleep-related variables. See Tables S2 and S3 for detailed model results.

maximus) or ducks (Anatidae species).<sup>20,21</sup> Among primates, the sleep patterns of wild slow lorises (Nycticebus javanicus) were shaped by environmental factors and were significantly shorter compared with captive individuals.<sup>22</sup> Similarly, zoo chimpanzees have higher frequencies of nighttime activity—particularly in the early morning hours—and they engage more often in feeding and social behavior during the night than wild ones.<sup>23</sup> In sum, variability in sleep behavior due to ecological and social factors, and its trade-off with other vital functions, is core to understanding its evolution, and this requires systematic field research.<sup>13</sup>

Chimpanzees and bonobos enjoy a special status in comparative evolutionary studies, as our closest living primate relatives. So far, nocturnal activity in wild chimpanzees has only been studied using ground-based cameras or direct observations under low-light conditions.<sup>24-27</sup> Although this has furthered understanding, particularly regarding the role of ambient temperature, illumination, and other ecological factors,<sup>24-27</sup> little is known about the actual sleep patterns and the impact social variables may have on this. The aim of our study was to address this knowledge gap with infrared video recordings from within night nests, constructed and occupied by wild chimpanzees (Pan troglodytes schweinfurthii) in Budongo Forest, Uganda.

We were interested in how a chimpanzee's own social status in the group and the composition of the nesting party affected its sleep. In this primate, high-ranking males are known to be under

more social pressure than low-ranking ones,<sup>28</sup> possibly because they are more concerned about social events and their potential ramifications for their own status than are lower-ranking males, and this might have a negative impact on their sleep. Regarding the relation between nesting party and sleep, we predicted that nesting in larger groups would be more likely to lead to more fragmented sleep, due to a higher likelihood of disturbances, than nesting in smaller groups. We further predicted that some group compositions, such as the presence of estrous females, were also likely to negatively impact male sleep, by delaying nesting times, advancing wake times, and increasing sleep disruptions.

## RESULTS

We observed a total of *N* = 519 nest-building events. For *N* = 35 individuals (19 adult females, 9 adult males, 3 subadult females, 1 subadult male, and 3 juvenile males; Table S1), we managed to document the entire night period (*N* = 116 nights, example Figure S1). Individuals almost always remained in their nests for the entire night, with only three documented nest departures (2.6% of monitored nights).

## Impact of social surroundings

We found that group nesting led to significantly longer sleep (β ± SE = +0.34 ± 0.19 h, *p* < 0.05; Figure 1A; Table S2, sleep
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Influence of the sleeping party size on sleep-related variables</figcaption></figure>

(A) Sleep fragmentation, (B) probability of vigilance, and (C) probability of vocalizing. Large dots represent model-predicted values, with the shaded area indicating the 95% CI of the regression. In (A), small points, color-coded by individual, show raw data values. See STAR Methods for definition of sleep fragmentation.
See Tables S2 and S5 for detailed model results. See also Figure S3.

duration), due to better sleep efficiency (β ± SE = +0.04 ± 0.02, p < 0.05; **Figure 1B**; **Table S2**, sleep efficiency) and shorter wake bouts (β ± SE = -0.06 ± 0.02 h, p < 0.01; **Figure 1C**; **Table S2**, wake bouts' mean duration), despite earlier wake times (β ± SE = -0.21 ± 0.11 h, p < 0.05; **Figure 1D**; **Table S2**, wake time), compared with solitary nesting. For females, group nesting significantly resulted in delayed nesting time (β ± SE = -0.23 ± 0.07 h, p < 0.01; **Figure 1E**; **Table S2**, nesting time) and lower nest heights (β ± SE = -2.91 ± 0.95 m, p < 0.01; **Figure 1F**; **Table S3**). Generally, larger sleeping parties were associated with less sleep fragmentation (β ± SE = -0.02 ± 0.01 wake bouts/h for each additional party member, p < 0.01; **Figure 2A**; **Table S2**, sleep fragmentation), lower likelihood of vigilance (each additional party member decreased the odds of vigilance by 12%, odds ratio = 0.88, 95% confidence interval [CI] [0.8-0.97], β ± SE = -0.12 ± 0.05, p < 0.01; **Figure 2B**; **Table S5**, vigilance), but higher likelihood of vocalizing (each additional party member increased the odds of vocalizing by 24%, odds ratio = 1.24, 95% CI [1.07-1.43], β ± SE = +0.22 ± 0.07, p < 0.001; **Figure 2C**; **Table S5**, vocalizing) compared with smaller parties.

When looking at the effects of adult males in a sleeping party,
we found that increasing numbers of adult males were

associated with later nesting times (β ± SE = +0.05 ± 0.01 h for each additional adult male, *p* < 0.001; **Figure 3A**; **Table S2**, nesting time) and lower nesting heights (β ± SE = -0.48 ± 0.22 m for each additional adult male, *p* < 0.05; **Figure 3B**; **Table S3**). During nocturnal wake bouts, the number of adult males negatively predicted the likelihood of urinating (each additional male increased the odds of urinating by 28%, odds ratio = 1.28, 95% CI [1.06-1.56], β ± SE = +0.25 ± 0.10, *p* < 0.05; **Figure 3C**) but not any of the other nocturnal behaviors (**Table S5**, urinating). We found no effect of the alpha male's presence within the sleeping party on any of the parameters of interest (**Tables S2-S5**). Sleep posture and sleep restlessness were also not affected by any of the social variables (**Table S2**, sleep posture and restlessness).

The presence of one or more sexually active females within the sleeping party significantly delayed nesting time (β ± SE = +0.20 ± 0.08 h, *p* < 0.05; **Figure 4D**; **Table S2**, nesting time). It generally increased nest heights (β ± SE = +1.84 ± 0.92 m, *p* < 0.05; **Figure 4A**; **Table S3**) and sleep fragmentation (β ± SE = +0.24 ± 0.11 wake bout/h, *p* < 0.05; **Figure 4B**; **Table S2**, sleep fragmentation) but decreased sleep efficiency (β ± SE = -0.05 ± 0.02, *p* < 0.05; **Figure 4C**; **Table S2**, sleep efficiency) for everyone.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Influence of the number of males in the party on sleep-related variables</figcaption></figure>

(A) Nesting time, (B) nest height, and (C) probability of urinating. Large dots represent model-predicted values, with the shaded area indicating the 95% CI of the regression. Small points, color-coded by individual, show raw data values.
See Tables S2, S3, and S5 for detailed model results. See also Figure S3.
<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" /><figcaption>Figure 4. Influence of sexually active females in the sleeping party on sleep-related variables</figcaption></figure>

(A) Nest height, (B) sleep fragmentation, (C) sleep efficiency, and (D) nesting time. Large dots represent model-predicted values, with bars indicating 95% CIs. Small points, color-coded by individual, show raw data values. Results are reported separately for males and females if there was a significant interaction with sex.

See STAR Methods for definitions of sleep-related variables. See Tables S2 and S3 for detailed model results. The presence of an estrous female in the sleeping party also tended to decrease sleep duration in males and females and to advance wake time in males. See Figure S4 and Table S2, sleep duration and wake time.

0.06 h for each additional adult male, $p > 0.05$; Figure 6A; Table S2, sleep duration). A similar interaction was observed for vigilance during nocturnal awakenings (Table S5, vigilance): high-ranking individuals showed a greater likelihood of looking around as the number of adult males increased (each additional adult males increased the odds of vigilance by 19% in high-ranking individuals, odds ratio = 1.19, 95% CI [1.02–1.38], $\beta \pm SE = +0.17 \pm 0.08$, $p < 0.05$; Figure 6B; Table S5, vigilance). Despite a reduction in duration when sleeping near other males, high-ranking individuals did not sleep alone more frequently.

Solitary nesting occurred in 12 of 82 cases in high-ranking males, 10 of 68 in mid-ranking males, and 41 of 156 in low-ranking males. Solitary nesting occurred in 27 of 92 cases in high-ranking females, 19 of 94 in mid-ranking females, and 16 of 95 in low-ranking females. Another behavioral effect was that high- and mid-ranking males and females were significantly less likely to urinate when nesting alone compared with when nesting in a group (nesting alone decreased the odds of urinating by 67% in high-rankings, odds ratio = 0.33, 95% CI [0.16–0.72], $\beta \pm SE = -1.10 \pm 0.39$, $p < 0.01$; and by 65% in mid-rankings, odds ratio = 0.35, 95% CI [0.17–0.74], $\beta \pm SE = -1.05 \pm 0.38$, $p < 0.05$, Figure 6C; Table S5, urinating). Finally, concerning the presence of sexually active females, we found that high-ranking males woke up significantly earlier ($\beta \pm SE = -0.80 h \pm 0.23$, $p < 0.001$; Figure 6D; Table S2, wake time), whereas high-ranking females were not affected ($\beta \pm SE = 0.41 h \pm 0.34$, $p > 0.05$; Figure S4C; Table S2, wake time).

We summarized the main results of all analyses in a separate table (Table 1).

DISCUSSION

We have provided systematic evidence of natural sleep patterns and nocturnal behavior in wild chimpanzees. First, we found an overall effect that group nesting improved individual sleep, by

Impact of social rank

When splitting males and females into rank categories, we found several relevant interactions (Tables S2–S5). For males, we found that high-ranking individuals slept less than low-ranking ones ($\beta \pm SE = -0.94 \pm 0.34 h$, $p < 0.05$; Figure 5B; Table S2, sleep duration; model 1) and that their sleep was more fragmented ($\beta \pm SE = +0.35 \pm 0.15$ wake bouts/h, $p < 0.05$; Figure 5C; Table S2, sleep fragmentation) and less efficient ($\beta \pm SE = -0.11 \pm 0.04$, $p < 0.05$; Figure 5D; Table S2, sleep efficiency). For females, we found no comparable effects (Table S2, sleep duration) but a rank effect on nesting time, with low-ranking females nesting significantly later than high- and mid-ranking females (low vs. mid: $\beta \pm SE = +0.25 \pm 0.08$ h, $p < 0.01$; low vs. high: $\beta \pm SE = +0.17 \pm 0.07$ h, $p < 0.05$, Figure 5A; Table S2, nesting time).

We then looked at the effect of other male presence and found a significant interaction between social rank and the number of adult males in the sleeping party on an individual’s sleep duration (Table S2, sleep duration, model 2). As the number of adult males increased, high-ranking males and females slept less ($\beta \pm SE = -0.20 \pm 0.05$ h for each additional adult male, $p < 0.01$; Figure 6A; Table S2, sleep duration, model 2), mid-ranking males and females slept longer ($\beta \pm SE = +0.15 \pm 0.05$ h for each additional adult male, $p < 0.01$; Figure 6A; Table S2, sleep duration), and low-ranking individuals were not affected ($\beta \pm SE = +0.05 \pm 0.01$ h for each additional adult male, $p < 0.01$; Figure 6A; Table S2, sleep duration).
<figure><img src="image_11.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>C</figcaption></figure>

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

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

prolonging sleep duration, enhancing sleep efficiency, and shortening wake bouts. This contrasts with findings in other primates, where larger groups were associated with reduced sleep<sup>13,14</sup> or more nocturnal disruptions.<sup>29</sup> Unlike monkeys, chimpanzees sleep in individual nests spaced apart, which likely buffers mutual disturbances.<sup>30</sup> The benefits of group sleeping may be psychological, by creating a sense of safety<sup>31</sup> or by reducing individual sentinel obligations.<sup>32</sup> In line with this, we found that nest heights decreased as the number of adult males increased, further suggesting that individuals felt safer in male-dense groups, possibly due to an evolutionarily ancient anti-predator adaptation.<sup>33,34</sup> Nest location within the group's home range was systematically included in the analyses but showed no significant effects, suggesting that individuals only build nests in areas that they judge as sufficiently safe or where they face little risk of territorial challenges at night. Future studies, including information on stress hormones and multi-nest monitoring of nocturnal behavior, are likely to provide additional insights regarding these topics.

**The impact of social status on sleep**

Although sleeping in groups had a generalized positive effect for adult males and, to a lesser degree, females, this was compromised by interaction between their own rank and the composition of the sleeping party. In particular, high social rank had a negative effect on sleep, with high-ranking males sleeping less, having more fragmented sleep, and lower sleep efficiency than

<caption>**Figure 5. Influence of one's own rank on sleep-related variables**</caption>

(A) Nesting time, (B) sleep duration, (C) sleep fragmentation, and (D) sleep efficiency. Large dots represent model-predicted values, with error bars indicating 95% CIs. Small points, color-coded by individual, show raw data values.

See STAR Methods for definitions of sleep-related variables. See Table S2 for detailed model results. See Figure S6 for the influence of one's own rank on time spent lying down during the day.

low-ranking males. High-ranking males are socially more challenged than low-ranking ones (e.g., de Waal<sup>35</sup> and Wittig et al.<sup>36</sup>), with psychological and hormonal consequences most likely extending into the night.<sup>37,38</sup> Stress from any source is a well-known disruptor of sleep, in species ranging from mice to humans,<sup>7,39</sup> and chimpanzees appear to be no exception. Interestingly, high-ranking males slept longer when nesting alone or in the presence of only a few other adult males, further suggesting that the proximity of (specific) other males impacts sleep. Interestingly, despite the apparently considerable costs of sleeping near other males, high-ranking males did not sleep alone more frequently than other males

did, suggesting that they were not actively avoiding sleeping near other males. In comparison, low-ranking males started to build their nests earlier when alone and, when nesting with others, synchronized their nesting times with them, suggesting that they were mere social bystanders and not primary decision-makers (Figure S5; Table S2, nesting time).

**Sleeping near other males**

We also looked at the effects that other males have on an individual's sleep and found a negative relationship between the number of males in the party and sleep duration, but this time also for high-ranking females and irrespective of overall party size. Parties with many males also delayed individuals' nesting time, possibly due to higher rates of social interactions, such as grooming or displays.<sup>40</sup> Also interesting was that high-ranking individuals showed higher levels of vigilance during nocturnal awake bouts, suggesting a rank-dependent social role. We can think of several hypotheses that could account for these effects. First, chimpanzees may prefer to nest in larger groups when spending the night in more dangerous areas (i.e., border areas), with increased vigilance in high-ranking individuals due to social obligations.<sup>8,35,41</sup> As mentioned, however, we did not find any geographic differences in nocturnal vigilance, suggesting that chimpanzees do not perceive some areas as more dangerous or generally avoid such areas for nesting.

It is equally possible that high-ranking individuals occupy more central positions within nesting parties, which may have
<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" /><figcaption>Figure 6. Interaction effects between one's own rank and social context on sleep-related variables</figcaption></figure>

(A) Effect of rank and number of adult males in the sleeping party on sleep duration.

(B) Effect of rank and number of adult males in the sleeping party on probability of vigilance.

(C) Effect of nesting condition (alone vs. group) and rank on probability of urinating.

(D) Effect of rank and presence of an estrous female on wake time in males. In (A) and (B), lines show fitted linear regression models, with shaded areas representing 95% CIs. Small points, color-coded by social rank, show raw data values. In (C) and (D), large dots represent model-predicted values, with error bars indicating 95% CIs, and small points, color-coded by individual, show raw data values.

See Tables S2 and S5 for detailed model results. See Figure S4 for the influence of rank and presence of an estrous female on wake time in females. See Figure S5 for the influence of the interaction between sex, rank, and solitary vs. group nesting on nesting time. See also Figure S3.

from 22 chimpanzee study sites, Tagg
et al. found no evidence for nocturnal
copulations in a sample of N = 119 independent recordings.²⁶ In savannah chimpanzees, the presence of sexually active females also led to increased nocturnal activity but not to any observed copula-

tions.²⁵ This suggests that the disruption is less about direct mat-
ing attempts and more likely linked to increased male-male
competition in anticipation of reproductive opportunities the
following day.

Do chimpanzees compensate for sleep deprivation?

How do dominant individuals cope with their apparent sleep
deficit? Of course, it is possible that the shorter sleep of high-
ranking individuals in fact represents a physiological optimum,
with subordinates sleeping more simply because they have little
else to do.⁴⁵ However, this is at odds with the finding that high-
ranking individuals slept longer when nesting alone. Assuming
that high-ranking individuals suffer from sleep deprivation, one
possibility for coping with this is some sort of compensation,
such as increased sleep depth during the night, as found in sand-
pipers,¹⁵ some mammals,⁴⁶ and, most recently, in wild orangu-
tans.¹⁴ Testing this hypothesis would require specific data on
sleep structure (i.e., distinguishing light, deep, and rapid eye
movement (REM) sleep), which is certainly a major challenge un-
der field conditions. However, we managed to record some indi-
rect measures of sleep structure, i.e., restlessness and sleep frag-
mentation, but found no clear rank differences, contradicting the
compensation hypothesis.⁴⁷,⁴⁸ Similarly, olive baboons that sleep
little during a given night do not compensate by sleeping more the
following night.¹³ A related possibility is diurnal compensation,
with high-ranking individuals taking more rest periods and lying
down more frequently compared with other individuals, but, again
we found no such evidence (Figure S6; Table S4).

a negative impact on sleep, as found in baboons and female
mouse lemurs.⁴²,⁴³ Finally, the presence of other males may sim-
ply be perceived as socially stressful, with a corresponding
impact on cortisol levels and circadian rhythms.²⁸ Again, future
research might want to track the spatial geometry of nesting
parties, as well as the hormonal profiles of key individuals.

The impact of sexually active females

Another—perhaps less surprising—finding was that the pres-
ence of sexually active females negatively affected male sleep.
Here, we found that the presence of such females increased
sleep fragmentation in both males and females, suggesting a
group-wide disturbance. More specifically, estrous female pres-
ence delayed nesting time in males, presumably because the
males attempted to build their own nests near the female’s.
We also found that high-ranking males woke up earlier than other
males when an estrous female was in the party. Chimpanzee
males compete for mating access by following and, if possible,
trying to lure estrous females away from the main group,
although they are remarkably tolerant of each other.⁴⁴ Presum-
ably, earlier wake times were male attempts to track the estrous
female once she left the nest. We also found some effects in
other females, likely as collateral consequences of the height-
ened nighttime activity of males.

Interestingly, however, we only observed two nocturnal mat-
ing events, both of which were initiated by an estrous female
entering the nest of a low-ranking male before dawn, around
05:00 EAT. Similarly, reviewing group-based camera trap data
<table><caption>Table 1. Social factors influencing sleep characteristics</caption><tr><td>number of adult males</td><td>+</td><td>nesting time</td></tr><tr><td>in females, low rank</td><td>+</td><td></td></tr><tr><td>in females, sleeping in a group</td><td>+</td><td></td></tr><tr><td>in males, presence of a sexually active female</td><td>+</td><td></td></tr><tr><td>sleeping in a group</td><td>-</td><td>wake time</td></tr><tr><td>in high-ranking males, presence of a sexually active female</td><td>-</td><td></td></tr><tr><td>sleeping in a group</td><td>+</td><td>sleep duration</td></tr><tr><td>in males, high rank</td><td>-</td><td></td></tr><tr><td>in high-ranking individuals, number of adult males</td><td>-</td><td></td></tr><tr><td>in mid-ranking individuals, number of adult males</td><td>+</td><td></td></tr><tr><td>sleeping party size</td><td>-</td><td>sleep fragmentation</td></tr><tr><td>presence of a sexually active female</td><td>+</td><td></td></tr><tr><td>in males, high rank</td><td>+</td><td></td></tr><tr><td>sleeping in a group</td><td>+</td><td>sleep efficiency</td></tr><tr><td>presence of a sexually active female</td><td>-</td><td></td></tr><tr><td>in males, high rank</td><td>-</td><td></td></tr><tr><td>sleeping in a group</td><td>-</td><td>wake bouts duration</td></tr><tr><td>in females, sleeping in a group</td><td>-</td><td>nest height</td></tr><tr><td>number of adult males</td><td>+</td><td></td></tr><tr><td>presence of a sexually active female</td><td>+</td><td></td></tr><tr><td>sleeping party size</td><td>-</td><td>vigilance</td></tr><tr><td>in high-ranking individuals, number of adult males</td><td>+</td><td></td></tr><tr><td>number of adult males</td><td>-</td><td>urinating</td></tr><tr><td>in high and mid-ranking individuals, nesting in a group</td><td>+</td><td></td></tr><tr><td>sleeping in a group</td><td>+</td><td>vocalizing</td></tr></table>

+": significant positive relation; "-": significant negative relation.

## Cognitive effects of sleep deprivation

A further intriguing hypothesis is that high-ranking individuals pay higher cognitive costs due to shorter and more fragmented sleep. In humans and other primates, sleep deprivation is known to impair decision-making, social interactions, and even reproductive success,⁴⁹⁻⁵² suggesting similar consequences in chimpanzees. Here, future research would need to track individual decision-making and social behavior of high- and low-ranking individuals following nights when sleep was monitored.

## Evolution of sleep in hominids

Our comparative findings shed some light on the evolutionary roots of human sleep. Modern humans generally sleep less than other primates,⁵³ which has freed up time for other

activities, notably social behavior.⁵⁴ In our study, reduction in sleep was driven by the high numbers of adult males, which delayed nesting time and advanced wake time. Some individuals—particularly high-ranking males—were capable of markedly reducing their sleep duration, an effect amplified in the presence of other adult males in the sleeping party. These findings suggest that chimpanzees and humans face the same trade-off between sleep and social interactions but that in humans the balance has tilted much more strongly in favor of social interactions.

Another point of evolutionary relevance is that, in humans, the species-wide reduction in sleep was not to the detriment of health or reproduction. Regarding advanced human cognition, it has been argued that natural selection has favored the capacity to sleep deeper, more efficiently, and with a greater proportion of REM sleep.⁵⁴ Here, comparative research is needed to test the degree to which chimpanzees possess such adaptations and their relation to fitness. Unlike what has been reported for orangutans,¹⁴ we found no evidence that chimpanzees compensated through the use of naps during the day, even if nocturnal sleep was fragmented.

Chimpanzees also show highly synchronized sleeping behavior at the group level, with relatively little individual differences in nesting and wake times. This contrasts with what has been found in humans, who can be grouped into distinct chronotypes.⁵⁵ Here, an argument has been made that this is an evolved feature, enabling early humans to take turns in sentinel duties.³² It may have been that early humans were under considerably higher predation pressure compared with what is normally seen in modern chimpanzees and that this may have favored the evolution of chronotypes in humans but not chimpanzees, who seem to prioritize social synchronization.

## Limitations of the study

This study inevitably comes with some limitations. First, around one-fifth of the nests were beyond the reach of our 16.5-m pole. Although we find it unlikely, the patterns we reported may not generalize to higher nests, which could be constructed in response to higher predator or parasite pressure,³³,³⁴,⁵⁶ thereby increasing the risk of falls or discomfort.⁵⁷ Second, as explained, because most individuals were already asleep when the camera system became operational, we often had to approximate exact sleep onset time from secondary visual and auditory cues. In future research, it may be possible to habituate chimpanzees to the camera system, removing the need to operate in complete darkness. Third, we identified sleep from behavioral indicators on camera footage, as currently there are no alternative ethically or practically feasible ways of using polysomnography or actigraphy with free-ranging apes.¹⁸,⁵⁸ Without further technology advancements, camera trapping remains the only viable direct method for assessing sleep in wild chimpanzees, offering a rare opportunity to explore how internal, ecological, and social factors shape sleep in a natural context.

## Conclusions

Our findings show that multiple social factors determine sleep in wild chimpanzees, demonstrating that studying sleep as a purely physiological process in the laboratory is inadequate. Instead, sleep is a highly flexible function, allowing individuals to adaptively respond to social and ecological factors that determine
lifetime reproductive success. Rather than trying to control these variables, our study has demonstrated that they should be considered as part of a complex trade-off between physiological and social functions, something that can only be studied in natural environments.

## RESOURCE AVAILABILITY

### Lead contact

Requests for further information and resources should be directed to and will be fulfilled by the lead contact, Clara Hozer (hozerclara@hotmail.fr).

### Materials availability

This study did not generate new, unique reagents.

### Data and code availability

* All data used in this study are publicly available at Figshare: https://doi.org/10.6084/m9.fishshare.30418156.

* All original code is publicly available at Figshare: https://doi.org/10.6084/m9.fishshare.30418156.

* Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

## ACKNOWLEDGMENTS

Financial support was provided by the Fyssen Foundation, the Leakey Foundation, the University of Neuchâtel, and the Société Francophone De Primatologie. We gratefully acknowledge Echoni Atayo, Raymon Rupiny, and Geoffrey Ukethi for their help with data collection and the Budongo Conservation Field Station staff for logistical support. We thank Gal Badhini and Cat Hobaiter for assisting with home range analysis and Radu Slobodeanu for statistical advice.

## AUTHOR CONTRIBUTIONS

Conceptualization, C.H. and K.Z.; formal analysis, C.H.; funding acquisition, C.H.; data collection, C.H., F.A., N.F., M.C., B.C., and M.M.; methodology, C.H. and K.Z.; project administration, C.H.; resources, K.Z.; supervision, C.H., K.Z., and D.S.; writing – original draft, C.H.; and writing – review and editing, all authors.

## DECLARATION OF INTERESTS

The authors declare no competing interests.

## STAR★METHODS

Detailed methods are provided in the online version of this paper and include the following:

* KEY RESOURCES TABLE

* EXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS

* Study site and subjects

* Ethical statement

* METHOD DETAILS

* Focal daily data collection

* Night sleep data collection

* Nocturnal behavior

* Ecological variables

* Social variables

* Nest location

* Daily activities

* QUANTIFICATION AND STATISTICAL ANALYSIS

* Assessing sleep patterns

* Nocturnal behavior analysis

* Variables selection and model checking

* Follow-up analyses

## SUPPLEMENTAL INFORMATION

Supplemental information can be found online at https://doi.org/10.1016/j.cub.2025.11.057.

Received: September 4, 2025

Revised: November 2, 2025

Accepted: November 20, 2025

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70. Neumann, C., Duboscq, J., Dubuc, C., Ginting, A., Irwan, A.M., Agil, M., Widdig, A., and Engelhardt, A. (2011). Assessing dominance hierarchies: validation and advantages of progressive evaluation with Elo-rating. Anim. Behav. 82, 911-921. https://doi.org/10.1016/j.anbehav.2011.07.016.

71. Badihi, G., Bodden, K., Zuberbühler, K., Samuni, L., and Hobaiter, C. (2022). Flexibility in the social structure of male chimpanzees (Pan troglodytes schweinfurthii) in the Budongo Forest, Uganda. R. Soc. Open Sci. 9, 220904. https://doi.org/10.1098/rsos.220904.

72. Sandel, A.A., Derby, R.N., Chesterman, N.S., McNamara, A., Dudas, M.M., and Rawat, I. (2022). Parallel lasers and digital photography to estimate limb size of chimpanzees (Pan troglodytes) at Ngogo, Kibale National Park, Uganda. Primates 63, 217-224. https://doi.org/10.1007/s10329-022-00982-6.

73. Furuichi, T. (1987). Sexual swelling, receptivity, and grouping of wild pygmy chimpanzees females at Wamba, Zaire: Primates 28(3) 1987: 309-318, Illustr. Primates 28, 309-318. https://doi.org/10.1007/BF02381014.
<figure><img src="image_19.png" /></figure>

## STAR★METHODS

### KEY RESOURCES TABLE

<table><thead><tr><th>REAGENT or RESOURCE</th><th>SOURCE</th><th>IDENTIFIER</th></tr></thead><tbody><tr><td colspan="3">Deposited data</td></tr><tr><td>Repository data</td><td>This paper</td><td>Figshare: https://doi.org/10.6084/m9.fishshare.30418156</td></tr><tr><td>Videos illustrating some of the observed nocturnal behaviors</td><td>This paper</td><td>Figshare: https://doi.org/10.6084/m9.fishshare.29980180</td></tr><tr><td colspan="3">Experimental models: Organisms/strains</td></tr><tr><td>Pan troglodytes<br>schweinfurthii</td><td>Budongo Conservation Field<br>Station, Uganda</td><td>https://www.budongo.org/</td></tr><tr><td colspan="3">Software and algorithms</td></tr><tr><td>R v4.2.1</td><td>R Foundation for Statistical<br>Computing, www.r-project.org</td><td>RRID: SCR_001905</td></tr><tr><td>adehabitatHR<sup>59</sup></td><td>https://cran.r-project.org/web/packages/adehabitatHR/index.html</td><td>N/A</td></tr><tr><td>blme from lme4<sup>60</sup></td><td>https://cran.r-project.org/web/packages/lme4/index.html</td><td>RRID: SCR_015654</td></tr><tr><td>car<sup>61</sup></td><td>https://CRAN.R-project.org/package=car</td><td>RRID: SCR_022137</td></tr><tr><td>MuMln<sup>62</sup></td><td>https://cran.r-project.org/web/packages/MuMln/index.html</td><td>N/A</td></tr><tr><td>emmeans<sup>63</sup></td><td>https://cran.r-project.org/package=emmeans</td><td>RRID: SCR_018734</td></tr><tr><td>ggeffects<sup>64</sup></td><td>https://cran.r-project.org/package=ggeffects</td><td>RRID: SCR_022496</td></tr><tr><td>Analysis code</td><td>This paper</td><td>https://doi.org/10.6084/m9.fishshare.30418156</td></tr></tbody></table>

### EXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS

#### Study site and subjects

Data collection took place from January to October 2023 and from January to August 2024 in the Sonso chimpanzee community of the Budongo Forest Reserve, Uganda (1°35'-1°55'N, 31°18'-31°42'E), at the time comprising 76 individuals with 10 adult males and 33 adult females. Chimpanzees live in multi-male, multi-female groups marked by fission-fusion dynamics with fluid subgroups ('parties').<sup>65</sup> Adults and subadults build their own nest in trees every evening, which they then occupy until dawn. Mothers share their nest with their younger offspring, whereas independent, older offspring build their own nests next to the mother's. Occasionally, chimpanzees also construct day nests, but they hardly ever reuse existing nests.<sup>66,67</sup> N=53 individuals (27 adult females, 10 adult males, 3 subadult females, 1 subadult males, 3 juvenile females and 9 juvenile males; Table S1) were monitored.

#### Ethical statement

Our study was strictly observational and non-invasive. Ethical review and permission to conduct this study was granted by the Uganda National Research Council and the Uganda Wildlife Authority under permit n°NS433ES.
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METHOD DETAILS

Focal daily data collection

We monitored individuals from dawn (around 7:00 East Africa Time (EAT)) to dusk (around 18:30 EAT), including the nest building period. We searched for individuals either at their previous night's nesting location or in areas where vocalizations were detected. Once a party was located, we selected a focal individual and followed it continuously until it completed its evening nest. If the focal individual was lost during the day, we attempted to relocate it for up to two hours, after which we selected another focal individual.

Night sleep data collection

Once the individual had nested, we monitored nighttime sleeping behavior using a customized, remotely controlled infrared (IR) cam-
era system (Figure S1).

We were extremely careful in eliminating any eventual disturbance by our presence during the nesting and subsequent rest period. For this reason, we mounted the camera system only in full darkness, starting around 30 min after sunset. A well-trained team then mounted the telescopic pole in near-total silence onto a tripod at a distance of about 5-10 m to the nest, while another team member adjusted the camera orientation remotely. This was possible because of the high ambient forest noise levels at dusk. In most cases, individuals were already asleep when recording started (i.e., motionless with its eyes closed > 5 min), so it was mostly impossible to assess the exact time of sleep onset. However, we monitored a range of indicative behaviors during dusk, such as vocalizations, large body movements or eye openings to estimate the time of sleep onset. Most individuals remained awake until after sunset (range 18:37 - 19:09 EAT), in which case we estimated sleep onset by calculating the average interval between the time of sunset and the time of the first recorded image. Since continuous recording was not possible with our equipment, we programmed the camera to operate in 5-min recording blocks during which it (a) recorded for the first 1 min continuously and (b) was triggered by any larger body movement during the remaining 4 min for another 1 min continuous recording unit. For each 5-min block, we applied the decision key described in Figure S2 to determine whether the subject was classified as asleep or awake. We then extracted the following variables to characterize the subject's sleep over the entire night:

- **Nesting duration**: Time period between entering and leaving the nest (hours)

- **Resting duration**: Time period between estimated sleep onset and morning wakening (hours)

- Sleep block: 5-min interval coded as asleep (see Figure S2)

- Awake block: 5-min interval coded as awake (see Figure S2)

- Wake bout: Period of being awake between two successive periods of being asleep (min)

- Sleep bout: Period of being asleep between two successive periods of being awake (min)

- Sleep duration: Cumulative time asleep during the entire night (hours)

- Sleep fragmentation: Number of wake bouts divided by sleep duration (number per hour)

- **Restlessness**: Relative frequency of 5 min blocks during which individual was scored as being asleep but was seen moving or triggered a recording due to moving

- Sleep efficiency: Sleep duration divided by resting duration (index)

- Sleep posture: Time spent sleeping on the side divided by time sleeping on the back (index). Sleeping on the front or sitting was rare (8% of recordings) and excluded.

- **Nest departure**: Individual leaves its nest during the night to construct a new nest (yes/no)

Nocturnal behavior

We recorded the subjects’ nocturnal behavior during each wake bout (N=628) according to the following common categories: (a) self-scratching (raking one’s hair or skin with fingernails, involving movement of the fingers, hand, or arm⁶⁸); (b) vigilance (maintaining a fixed gaze on the surrounding environment while in a head-up, stationary posture for at least 30 s⁶⁹); (c) urination or defecation; (d) self-grooming, and (e) vocalization (see Figure S3). Less common were nest building, feeding, branch shaking, drumming and copulating. Videos illustrating some of the observed behaviors are available at the following Figshare link: https://doi.org/10.6084/m9.f figshare.29980180.

Ecological variables

We measured temperature and humidity every 15 min, using a digital thermo-hygrometer (Elitech RC51). We then calculated the average daily temperature and humidity (07:00-19:00 EAT) and the average nocturnal temperature and humidity (19:00-7:00 EAT). Precipitation was recorded every hour using a rain gauge (Bresser ref. 7002530) installed in an open area at the Budongo Conservation Field Station. Sunrise and sunset times were obtained from suncalc.org, moon phase data from mooncalc.org. Nest height was measured with a commercial laser distance meter (Bosch); heights above 20 m were estimated.

Social variables

Age, sex and, for adult females, the number of infants and juveniles were extracted from the Budongo long-term data. Social rank was
calculated as an individual's Elo score, based on long-term data on pant-grunts (unidirectional submissive vocalization).⁷⁰ We then
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split males and females separately into three equally large groups according to their ordinal rank - high, middle, low. This enabled us to classify females more easily, for which Elo scores are notoriously difficult to calculate due to their less gregarious nature. Each night, we recorded if the individual was nesting alone or as part of a subgroup (i.e. a 'sleeping party'), considering a subgroup as equal or more than three individuals within 35 m (i.e., identical to how travelling parties are defined during daytime observations⁷¹). Field assistants conducted party composition scans every 15 min during daily focal follows. During each scan, both the party's location and the identities of all independent individuals present were recorded. Party composition data are part of the Budongo long-term dataset, for this study covering the period from January 2023 to August 2024. We determined the sleeping party size, the number of adult males per party (>16 years old ⁷²), the number of sexually active females per party (sexual swellings: 3-4) and whether the alpha male was present. Sexual activity in females was determined by the relative degree of wrinkling of her sexual swelling from 0 (no sexual activity) to 4 (maximum sexual activity).⁷³

Nest location

We hypothesized that the location of nests within the group's home range might affect sleep patterns, due to differences in the likelihood of encountering neighboring communities or familiarity with the terrain. Nest locations were determined in relation to the Budongo grid system (www.budongo.org), which consists of blocks encased by north-south and east-west trails. To georeference each block, we used UTM coordinates of the southwest corner of each block (Garmin BaseCamp version 4.7.5, Garmin Europe Ltd., Southampton, UK). When parties moved outside the grid system, their locations were recorded continuously using a GPS device (GPSMAP 64S). To estimate individual home ranges, we used the R package 'adehabitatHR'⁵⁹, calculating both 75% and 95% kernel utilization distributions based on the most frequently visited locations. Nest locations were recorded nightly using a GPS device (GPSMAP 64S). For each nest, we assessed whether it fell within the individual's 75% core home range. We then calculated the barycentre (centroid) of each individual's 95% home range and measured the distance from each nest to that point.

Daily activities

While following a focal individual during the day, we recorded the following behaviors': (a) travelling (walking or running on the ground or moving through the trees for more than one minute), (b) feeding, (c) sitting without feeding, and (d) lying down (whether on the ground, a branch, or in a day nest). We did not attempt to quantify sleep duration during the day, as individuals' eyes were not always visible—particularly when they were in nests or facing away from the observer—making it difficult to determine whether they were actually asleep. Because chimpanzees were not always located immediately at 7:00 a.m. and occasionally only found several hours later, we restricted our analysis to the period between 9:00 a.m. and the time of evening nesting. This allowed us to maximize the number of individuals included in the dataset. Only days during which the focal individual was continuously observed from 9:00 a.m. until nesting were included. In total, we collected 74 such complete observation days in N=53 individuals.

QUANTIFICATION AND STATISTICAL ANALYSIS

Assessing sleep patterns

For the following response variables—nesting time (N=519 nesting events), sleep duration (N=116 nights), sleep fragmentation (N=116 nights), sleep efficiency (N=116 nights), nest height (N=393 nests), wake time (N=116 nights), mean duration of wake bouts (N=116 nights), sleep posture (N=116 nights) and sleep restlessness (N=116 nights)—we fitted frequentist linear mixed models (LMMs) using the ‘blmer’ function from the ‘blme’ package, which extends the ‘lme4’ package.⁶⁰ As multiple nesting events per individual and multiple individuals nesting each evening were recorded, we included individual identity and date as random effects in the model investigating nesting time. For the remaining models, we typically monitored only one individual per night (with three exceptions where two individuals were recorded simultaneously). Fixed effects for the ecological variables were: mean daily temperature, mean night-time temperature, presence or absence of rain during the day, presence or absence of rain during the night, moon phase, sunset time, sunrise time. We entered the focal animal’s age, sex, rank, whether it nested alone or with a sleeping party, the sleeping party size, the number of adult males in the sleeping party, the presence of one or more sexually active females, the presence of the alpha male, if the nest was within the individual’s 75% home range, distance of the nest from the barycentre of the 95% home range and nest height. We also reran the same models for females only, to control for their sexual status and the presence of absence of an infant or a juvenile.

Nocturnal behavior analysis

To assess whether certain factors influenced the behaviors exhibited during nocturnal wake bouts, we compiled a new dataset in which each entry represented a single nocturnal wake bout (N=628), indicating whether specific behaviors—scratching, vigilance, urinating, self-grooming or vocalizing—were observed. To model the likelihood of each behavior occurring during a wake bout, we then used Generalized Linear Mixed Models (GLMMs) with a binomial error structure (Bernoulli family) as a function of the same predictor variables described earlier. Individual identity was again included as a random effect to account for repeated measures.
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## Variables selection and model checking

To identify potential multicollinearity problems, we calculated the Variance Inflation Factor (VIF) for all models using the 'vif' function of the 'car' package.⁶¹ To select the variables to be included in each model, we retained the models with the lowest AIC (Akaike Information Criterion) using the 'dredge' function of the 'MuMln' package to classify the different possible models.⁶² Assumptions of normally distributed and homogenous residuals were fulfilled, as per visual inspection of qq plots and residual plots against fitted values. To assess the significance of the fixed effects, we used likelihood ratio tests (LRT) using the 'anova' function of the 'car' package.⁶¹ Where necessary, we used post hoc tests with the 'emmeans' function from the 'emmeans' package,⁶³ with Holm correction, when the interaction was between two or three qualitative variables, and with the 'emtrends' function from the same package when the interaction was between a qualitative and a quantitative variables.⁶³

## Follow-up analyses

To assess a potential sleep compensation during the day, we used simple linear models (LMs) (and not mixed models, due to the limited number of repeated observations per individual) for each daily activity (travelling, feeding, sitting, lying down) (N=74 days). The fixed effects incorporated in the models were: mean daily temperature, mean night-time temperature, presence or absence of rain during the day, presence or absence of rain during the night, moon phase, sunset time, sunrise time, age, sex, social rank, if the individual nested alone or in group, sleeping party size, number of adult males in the sleeping party, presence or absence of one or more sexually active females in the sleeping party, presence of the alpha male in the sleeping party, if the nest was within the individual's 75% home-range, distance of the nest from the barycentre of the 95% home-range, nest height.

All analyses were performed on R v4.2.1 (R Development Core Team, 2001). We used a significance criterion of p < 0.05. For each significant effect, model-predicted values and 95% confidence intervals were calculated and plotted on the same software using the 'ggpredict' function of the 'ggeffects' package.⁶⁴