Male Chimpanzees Research Articles

Influence of personality, age, sex, and estrous state on chimpanzee problem-solving success

Despite the importance of individual problem solvers for group- and individual-level fitness, the correlates of individual problem-solving success are still an open topic of investigation. In addition to demographic factors, such as age or sex, certain personality dimensions have also been revealed as reliable correlates of problem-solving by animals. Such correlates, however, have been little-studied in chimpanzees. To empirically test the influence of age, sex, estrous state, and different personality factors on chimpanzee problem-solving, we individually tested 36 captive chimpanzees with two novel foraging puzzles. We included both female (N=24) and male (N=12) adult chimpanzees (aged 14-47 years) in our sample. We also controlled for the females' estrous state-a potential influence on cognitive reasoning-by testing cycling females both when their sexual swelling was maximally tumescent (associated with the luteinizing hormone surge of a female's estrous cycle) and again when it was detumescent. Although we found no correlation between the chimpanzees' success with either puzzle and their age or sex, the chimpanzees' personality ratings did correlate with responses to the novel foraging puzzles. Specifically, male chimpanzees that were rated highly on the factors Methodical, Openness (to experience), and Dominance spent longer interacting with the puzzles. There was also a positive relationship between the latency of females to begin interacting with the two tasks and their rating on the factor Reactivity/Undependability. No other significant correlations were found, but we report tentative evidence for increased problem-solving success by the females when they had detumescent estrous swellings.

Mutual grooming among adult male chimpanzees: the immediate investment hypothesis

Unidirectional grooming is a low-cost behaviour for which the groomer is repaid via kin selection or reciprocity. Return benefits can come in the form of increased probability of being groomed or social benefits such as coalitionary support. By contrast, the reasons for mutual grooming, which occurs when two individuals simultaneously groom each other, are not understood. In this study, we test three hypotheses regarding the function of mutual grooming among wild male chimpanzees, using 16 years of data. The social bonding hypothesis posits that mutual grooming promotes a return benefit by serving to strengthen and maintain social bonds, whereas the immediate investment hypothesis states that it functions as a signal to indicate willingness to invest in (continue) the grooming bout. The switching hypothesis states that mutual grooming results from overlap created when the direction of the grooming interaction is switched. The social bonding hypothesis was not supported: measures of association were not correlated with the probability of mutual grooming. We also found no support for the switching hypothesis, as mutual grooming was equally likely to occur without a switch in the direction of grooming as when a switch occurred. The immediate investment hypothesis was supported by our finding that bouts with mutual grooming (1) were longer, (2) contained a more equitable distribution of unidirectional grooming and (3) had more unidirectional grooming switches than bouts without mutual grooming. We conclude that male chimpanzees use mutual grooming to obtain short-term benefits in the form of prolonging a grooming bout, and suggest that mutual grooming thus represents a form of overlapping parcelling.

Brief Communication: Orphaned male Chimpanzees die young even after weaning

If a social-living animal has a long life span, permitting different generations to co-exist within a social group, as is the case in many primate species, it can be beneficial for a parent to continue to support its weaned offspring to increase the latter's survival and/or reproductive success. Chimpanzees have an even longer period of dependence on their mothers' milk than do humans, and consequently, offspring younger than 4.5-5 years old cannot survive if the mother dies. Most direct maternal investments, such as maternal transportation of infants and sharing of night shelters (beds or nests), end with nutritional weaning. Thus, it had been assumed that a mother's death was no longer critical to the survival of weaned offspring, in contrast to human children, who continue to depend on parental care long after weaning. However, in theory at least, maternal investment in a chimpanzee son after weaning could be beneficial because in chimpanzees' male-philopatric society, mother and son co-exist for a long time after the offspring's weaning. Using long-term demographic data for a wild chimpanzee population in the Mahale Mountains, Tanzania, we show the first empirical evidence that orphaned chimpanzee sons die younger than expected even if they lose their mothers after weaning. This suggests that long-lasting, but indirect, maternal investment in sons continues several years after weaning and is vital to the survival of the sons. The maternal influence on males in the male-philopatric societies of hominids may be greater than previously believed.

Chimpanzee food calls are directed at specific individuals

If primates were capable of vocalizing to inform a receiver about an external entity, it would represent an important element of continuity with human language. We tested experimentally whether chimpanzee rough grunts, which function to refer to food, are produced selectively, indicating voluntary control, and whether they are directed at specific individuals. These are prerequisites for a system capable of actively informing others about external events. We conducted a field playback experiment in which we presented silently feeding male chimpanzees, Pan troglodytes , with arrival pant hoots of a familiar group member. We found that subjects were significantly more likely to respond with food calls to the simulated arrival of an individual with whom the caller had a high rather than low level of friendship and where there was a large rather than small positive dominance rank difference between the individuals (i.e. caller was lower ranking). We concluded that chimpanzee food calls are not simply reflexive responses to food, but can be selectively directed at socially important individuals. Our findings are thus inconsistent with traditional views of primate vocalizations as inflexibly and indiscriminately produced. Instead, our results indicate that great apes can produce semantically meaningful calls in a highly selective, recipient-directed manner. Further research is needed to test whether chimpanzees use this flexible system to inform ignorant individuals about food, but the prerequisites to support this type of communication seem to be present. • Field playback experiment shows chimpanzees direct food calls to specific recipients. • Male chimpanzees selectively produce calls for the arrival of high-ranking friends. • Functionally referential chimpanzee food calls are not reflexive responses to food. • Chimpanzees produce referential calls in a voluntary recipient-directed manner. • Challenges view that primate vocal production is inflexible and indiscriminate.

Male–Female Association Patterns Among Free-ranging Chimpanzees (Pan troglodytes schweinfurthii)

Although male–female relationships can offer a number of advantages such as protection or social support, they are poorly studied among primates compared to same-sex relationships. We used 12 yr of data from the Kanyawara chimpanzee community to compare three independent measures of association (party association, 5m association, and grooming) among all adult dyads. Party association exhibited by male–female dyads was of intermediate strength between strong male–male and weak female–female association. Male–female dyads were less likely to be within 5m of one another and to groom as male–male dyads, but equally likely to be within 5m and more likely to groom as female–female dyads. Variation in male–female association strength was not related to male rank but was affected by female ranging patterns and female reproductive states. Females with core areas in the center of the home range were more likely to be in parties with males but did not show higher spatial proximity or grooming indices compared to females ranging in the periphery. Party association and 5m indices were higher for dyads of males and estrous females compared to those with anestrous females. These results indicate that male–female dyads are likely to associate with one another more often than female–female dyads because of overlapping ranging patterns and short-term changes in female reproductive state. We conclude that male and female chimpanzees do not exhibit proximity and grooming patterns indicative of strong affiliative bonds. This study also highlights the importance of using multiple independent measures of bond strength in studies of primate social dynamics.

Urethral inflammation in male chimpanzees caused by ureaplasmas and Chlamydia trachomatis

Specimens from men with acute non-gonococcal urethritis were tested to determine their microbial content and then given intra-urethrally to male chimpanzees. Two animals received ureaplasmas only and one became infected. The second did so when given a different strain. Both developed a polymorphonuclear leukocyte (PMNL) response. Two chimpanzees received a mixture of ureaplasmas and Chlamydia trachomatis and there was a suggestion that the ureaplasmas delayed or suppressed the chlamydial response. The latter, that is urethral infection with a pronounced PMNL response, was most clearly seen in a chimpanzee given C. trachomatis only. No inflammation was detected in two chimpanzees acting as controls. Three of five chimpanzees given ureaplasmas genitally, and one that had them endogenously, had them transiently in the oropharynx about 2 weeks later. The occurrence of ureaplasmas in the conjunctiva of three chimpanzees inoculated at this site was also transient and without inflammation. The possibility that Mycoplasma genitalium might have been in the inocula and caused urethral inflammation was discounted largely because no animal had antibody to this mycoplasma.

The fate of defeated alpha male chimpanzees in relation to their social networks

The fate of defeated alpha male chimpanzees in relation to their social networks

Perseverance and food sharing among closely affiliated female chimpanzees

Chimpanzees (Pan troglodytes) have been frequently observed to share food with one another, with numerous hypotheses proposed to explain why. These often focus on reciprocity exchanges for social benefits (e.g., food for grooming, food for sex, affiliation, kinship, and dominance rank) as well as sharing based on begging and deterring harassment. Although previous studies have shown that each of these hypotheses has a viable basis, they have only examined situations in which males have preferential access to food whereby females are required to obtain the food from males. For example, studies on male chimpanzee food sharing take advantage of successful crop-raids and/or acquisitions of meat from hunting, situations that only leave females access to food controlled by male food possessors. This begs the question how and with whom might a female chimpanzee in sole possession of a high-quality food item choose to share? In two large captive groups of chimpanzees, we examined each of the hypotheses with female food possessors of a high-quality food item and compared these data to a previous study examining food transfers from male chimpanzees. Our results show that alpha females shared significantly more with closely affiliated females displaying perseverance, while kinship and dominance rank had no effect. This positive interaction between long-term affiliation and perseverance shows that individuals with whom the female possessor was significantly affiliated received more food while persevering more than those with neutral or avoidant relationships towards her. Furthermore, females with avoidant relationships persevered far less than others, suggesting that this strategy is not equally available to all individuals. In comparison to the mixed-sex trials, females chose to co-feed with other females more than was observed when the alpha male was sharing food. This research indicates that male and female chimpanzees (as possessors of a desired food item) share food in ways influenced by different factors and strategies.

Social instability raises the stakes during social grooming among wild male chimpanzees

Explaining cooperative behaviour is a fundamental issue for evolutionary biology. The challenge for any cooperative strategy is to minimize the risks of nonreciprocation (cheating) in interactions with immediate costs and delayed benefits. One of a variety of proposed strategies, the raise-the-stakes (RTS) strategy, posits that individuals establish cooperation by increasing investment across interactions from an initial interaction. This model has received little quantitative support, however, probably because individuals of many social species engage in repeated interactions from a young age. In some situations, however, such as following conflicts, after prolonged absences or during social instability, established relationships may become unreliable predictors of future behaviour, creating an environment for RTS. We investigated grooming interactions among wild male chimpanzees, Pan troglodytes , testing RTS in these specific contexts. We found evidence to support the view that male chimpanzees employed RTS during social instability, but not under the other conditions. However, we also found that the duration of episodes (discrete parcels) of grooming was negatively related to aggression risk and in consequence suggest that the patterning of grooming interactions indicative of RTS was less to do with preventing cheating, and more to do with avoiding the elevated risks of intramale aggression during the period of social instability. We interpret the apparent support for RTS in our data as a by-product of the way chimpanzees cope with fluctuating (here, elevated then diminishing) risks of aggression. We suggest that social instability raises the stakes for grooming by creating a more hazardous marketplace in which to trade.

Ecology rather than psychology explains co-occurrence of predation and border patrols in male chimpanzees

The intense arousal and excitement shown by adult male chimpanzees, Pan troglodytes, during territorial attacks on other chimpanzees and predation upon monkeys suggest that similar psychological mechanisms may be involved. Specifically, it has been proposed that hunting behaviour in chimpanzees evolved from intraspecies aggression. Over 32 years, chimpanzees at Gombe National Park, Tanzania were significantly more likely to engage in a territorial border patrol on days when they hunted red colobus monkeys (Procolobus spp.), and vice versa, even after statistically controlling for male chimpanzee party size. We test the hypothesis that this correlation arises because hunting and patrolling are components of a specieslevel aggressive behavioural syndrome; specifically that predation arose as a by-product of territorial aggression in this species. However, hunting was equally likely to occur after a patrol and/or an intergroup interaction as it was before, and the occurrence of an intergroup interaction in which the chimpanzees approached strangers did not increase subsequent hunting probability. We also reject the hypothesis that hunting and patrolling reflect an individual-level behavioural syndrome. We identified two 'impact hunters' whose presence increased hunting probability. Similarly, there were also three 'impact patrollers', who increased the likelihood that a visit to the periphery of the community range resulted in a patrol. While this discovery has important implications for our understanding of the proximate causes of cooperation, it does not explain the temporal correlation between patrolling and hunting, since no males had such an impact in both contexts. Instead, the data suggest that the correlation arose because patrols typically involved males travelling long distances, which increased the probability of encountering prey. Additionally, parties that travelled to the periphery were more likely to encounter colobus in woodland, where hunts are more likely to occur and to succeed. Therefore, we conclude that ecological, rather than psychological, factors promote the co-occurrence of hunting and territorial aggression in this species.

ENVIRONMENTAL MANAGEMENT PROCEDURES FOLLOWING FATAL MELIOIDOSIS IN A CAPTIVE CHIMPANZEE (PAN TROGLODYTES)

A 40-yr-old male captive chimpanzee (Pan troglodytes) presented with depression and anorexia for 7 days. The tentative diagnosis, following a physical examination under anesthesia, was pneumonia with sepsis. Despite antibiotic treatment and supportive care the chimpanzee died a week following presentation. Gross pathology confirmed severe purulent pneumonia and diffuse hepatosplenic abscesses. Detected in serum at the time of the initial examination, the melioidosis serum antibody titer was elevated (> 1:512). Soil samples were collected from three sites in the exhibit at three depths of 5, 15, and 30 cm. By direct and enrichment culture, positive cultures for Burkholderia pseudomallei were found at 5 and 15 cm in one site. The other two sites were positive by enrichment culture at the depth of 5 cm. To prevent disease in the remaining seven troop members, they were relocated to permit a soil treatment with calcium oxide. The exhibit remained empty for approximately 1 yr before the chimpanzees were returned. During that period, the soil in the exhibit area was again cultured as before and all samples were negative for B. pseudomallei. Following the soil treatment in the exhibit, all chimpanzees have remained free of clinical signs consistent with melioidosis.

Pant hoot chorusing and social bonds in male chimpanzees

Vocal interactions, such as call exchanges or chorusing, are common behaviours in many animal species and their function has often been attributed to social bonding. However, few studies have investigated the effectiveness of vocalizations as bonding signals in comparison to other affiliative behaviours. We tested the hypothesis that male chimpanzee, Pan troglodytes schweinfurthii, pant hoot chorusing, a common behaviour in these primates, is a reliable but also flexible signal of affiliative relationships. The results of our study, conducted on the Kanyawara community of chimpanzees in Uganda, show that males were more likely to join in with the pant hoot of preferred long-term social partners to form a chorus. This supports the hypothesis that this behaviour is a good indicator of strong or long-term social bonds between male chimpanzees. However, our results also show that pant hoot chorusing reliably reflects short-term affiliations between males. For instance, male dyads were more likely to be involved in affiliative behaviours, such as reciprocated grooming, joint nonvocal displays and coalitions, on days when they chorused together, compared to days when they did not. This pattern applied to both preferred and neutral social partners. Moreover, on a short-term basis chorusing was a better indicator of other affiliative behaviours than grooming. We suggest that in male chimpanzees pant hoot choruses are efficient signals of short-term affiliative relationships. We conclude that potentially low-cost bonding behaviours such as coordinated vocal displays might be especially adaptive in highly fluid fission–fusion societies where grouping patterns are often unpredictable.

Behavioral recovery from tetraparesis in a captive chimpanzee

An adult male chimpanzee living in a captive social group at the Primate Research Institute of Kyoto University developed acute tetraparesis. He was paralyzed and received intensive care and veterinary treatment as previously reported in Miyabe-Nishiwaki et al. (J Med Primatol 39:336-346, 2010). The behavioral recovery of the chimpanzee was longitudinally monitored using an index of upright posture between 0 and 41 months after the onset of tetraparesis. Four phases were identified during the course of behavioral recovery. During Phase 0 (0-13 months), the chimpanzee remained lying on his back during the absence of human caretakers. An increase in upright posture occurred in Phase I (14-17 months), then remained at a stable level of around 50-70 % in Phase II (18-29 months). During Phases I and II, the subject's small treatment cage represented a spatial limitation. Thus, behavioral recovery was mainly mediated by arm muscle strengthening caused by raising the body trunk with the aid of materials attached to the cage walls as environmental enrichment. When the chimpanzee was moved to a larger rehabilitation room in Phase III (30-41 months), the percentage of upright posture constantly exceeded 80 %, except in the 40th month when he injured his ankle and was inactive for several days. The enlargement of the living space had a positive effect on behavioral recovery by increasing the types of locomotion exhibited by the subject, including the use of legs during walking. Rehabilitation works were applied in face-to-face situations which enabled the use of rehabilitation methods used in humans. The process of behavioral recovery reported in this study provides a basic data set for planning future rehabilitation programs and for comparisons with further cases of physical disability in non-human primates.

Stable isotope evidence of meat eating and hunting specialization in adult male chimpanzees

Observations of hunting and meat eating in our closest living relatives, chimpanzees (Pan troglodytes), suggest that among primates, regular inclusion of meat in the diet is not a characteristic unique to Homo. Wild chimpanzees are known to consume vertebrate meat, but its actual dietary contribution is, depending on the study population, often either unknown or minimal. Constraints on continual direct observation throughout the entire hunting season mean that behavioral observations are limited in their ability to accurately quantify meat consumption. Here we present direct stable isotope evidence supporting behavioral observations of frequent meat eating among wild adult male chimpanzees (Pan troglodytes verus) in Taï National Park, Côte d'Ivoire. Meat eating among some of the male chimpanzees is significant enough to result in a marked isotope signal detectable on a short-term basis in their hair keratin and long-term in their bone collagen. Although both adult males and females and juveniles derive their dietary protein largely from daily fruit and seasonal nut consumption, our data indicate that some adult males also derive a large amount of dietary protein from hunted meat. Our results reinforce behavioral observations of male-dominated hunting and meat eating in adult Taï chimpanzees, suggesting that sex differences in food acquisition and consumption may have persisted throughout hominin evolution, rather than being a recent development in the human lineage.

Male–female socio-spatial relationships and reproduction in wild chimpanzees

Recent research on primates and other taxa has shown that the relationships individuals form with members of the same sex affect their reproductive success. Evidence showing that intersexual relationships also influence reproduction, however, is more equivocal. Here, we show that male chimpanzees living in an exceptionally large community display long-term tendencies to associate with particular females. These association patterns are likely to arise because individuals of both sexes selectively range in specific areas of the communal territory, with males inheriting the ranging patterns of their mothers. These differentiated male–female socio-spatial relationships involved males of widely varying ranks, and their effect on reproduction is as strong as that of male dominance rank, which in turn is as strong a predictor of reproductive success at Ngogo as in other smaller chimpanzee communities. These results show that male–female socio-spatial relationships can play a large role in chimpanzee male reproductive strategies, although they probably neither weaken nor strengthen the relationship between male dominance rank and reproductive success. Our results linking male–female socio-spatial relationships to reproduction in chimpanzees suggest that the gap between the social and mating systems of humans and their closest living relatives may not be as large as previously thought.

The Social Function of Food‐Associated Calls in Male Chimpanzees

There is an ongoing debate about the adaptive function of chimpanzee's food-associated calls. Here, we tested a new hypothesis that food-associated calls in male chimpanzees function to signal that the calling individual is likely to initiate or prolong feeding. We propose that the signal functions to coordinate activities between individuals and that its ultimate function is to retain the nearby individuals in proximity. To test this hypothesis, we collected data on social and ecological correlates of food-associates calls in male chimpanzees. The results of this study, which was conducted on the Kanyawara community in the Kibale National Park, Uganda, show that males tended to feed for significantly longer after giving food-associated calls upon initiating feeding than after remaining silent. The type of audience had a significant effect on food calling, with males producing food-associated calls more often when males rather than females and preferred rather than neutral male social partners were in close proximity. However, the total party or male party size did not correlate with food calling behaviors, suggesting that the signal "targets" those in close proximity. Finally, a male feeding partner was more likely to remain with the focal until the end of a feeding bout after the focal gave a food-associated call at the beginning of the feeding bout than when he was silent. These results support our hypothesis and suggest that one of the functions of food calling in chimpanzees might be signaling that the caller is likely to initiate and prolong a feeding bout. This information might be used by receivers to make the decision whether or not to stay with the calling individual on a feeding patch or leave him (fission). The study suggests therefore that ultimately the function of food calling might be to coordinate feeding behaviors between males.

Fatal myocardial fibrosis in an aged chimpanzee (Pan troglodytes)

A 36-year-old male chimpanzee (Pan troglodytes) assigned to a life-long sign language communication project presented for sudden death. No other clinical or clinical pathological abnormalities were noted and given the signalment, death due to cardiac failure was suspected. Necropsy findings revealed moderate cardiomegaly and other chronic age-related findings including focal renal tubular cystic dilation and gingival hyperplasia. Histologic evaluation of the heart revealed interstitial fibrosing cardiomyopathy characterized by severe interstitial myocardial fibrosis replacing and separating myofibers within all chambers of the heart, especially the left ventricle, interventricular septum and subvalvular areas. This case report represents an additional case of sudden death associated with interstitial myocardial fibrosis in a chimpanzee. This process has been previously cited as the most common cause of sudden death in aged chimpanzees.

A Zoo-housed Chimpanzee’s (Pan troglodytes) Responses to Potentially Arousing Stimuli

In Experiment 1, we wished to determine whether a singly-housed adult male captive chimpanzee could discriminate the behavioral categories of sex and aggression. He was reinforced for selecting sexual rather than aggressive images on a touch-screen computer in a two-choice discrimination paradigm. He showed no discrimination after 24 sessions with non-human photos, but immediately selected human sexual images at above-chance levels. To explore whether this differential discrimination was due to a preference for human sexual images, he was presented with images of humans versus non-humans under non-differential reinforcement in Experiment 2. He preferred human photos if the images depicted sex, but not if the images depicted aggression. To further explore these preferences in Experiment 3 the chimpanzee was presented with images of genitalia of non-humans versus humans, genitalia versus eyes, and finally female versus male genitalia of both non-humans and humans, using non-differential reinforcement. The chimpanzee preferred human to non-human genitalia, and eyes to genitalia, but did not prefer female to male genitalia. This chimpanzee’s unusual social environment may have interfered with species-typical social preferences.

The roles of food quality and sex in chimpanzee sharing behavior (Pan troglodytes)

Both wild and captive chimpanzees (Pan troglodytes) share food with non-relatives. Researchers have proposed several hypotheses to explain this behavior, including ‘food for sex’, ‘food for grooming or agonistic support’, and ‘sharing under pressure’. We examined food sharing in two captive groups of socially-housed chimpanzees. In contrast to previous captive studies, which only examined transfers of low-quality foods, we conducted seven trials with high-quality food and seven with low-quality food for each group to directly compare transfers of different food qualities. We recorded how male chimpanzees shared food, including active transfers, passive transfers, and co-feeding. We also noted all instances of copulations, female estrous states, benign attempts to access food (termed ‘perseverance’), and aggressive attempts (termed ‘harassment’) to examine whether any of these factors influenced food sharing. Male food possessors shared at the same rate in both food quality conditions, but seemingly for different reasons, indicating that food quality may affect the exchange of social benefits in chimpanzees. In the low-quality condition, there was an interaction with rank and perseverance: while low- and middle-ranking females received more food the more they persevered, high-ranking females received more food without perseverance and gained relatively little benefit from persevering. In the high-quality condition, there was an interaction between copulations and perseverance: females who copulated with the male food possessor received more food during that trial with less perseverance. Non-copulating females received more transfers the more they persevered. This result was only observed in the short-term — copulations over the previous year were not correlated with food transfers. Further, the copulations observed here were unusual for these chimpanzees in that they were not confined to peak fertility, suggesting a non-conceptive function for copulations in chimpanzees. Copulations in this study may have functioned to reduce tension and increase short-term tolerance, allowing females better access to food.

Application of Computational Fluid Dynamics to Simulate a Steady Airflow in All Regions of Chimpanzee's Nasal Cavity

Knowing the detailed responses associated with the environment of our closest living relative allows us to comprehend more data about apes which may be useful to conclude hypotheses regarding the behavior of early hominins. The difference between humans and apes toward the adaptability to the living environment is caused by the difference among both bodies’ biological functions such as the nasal cavity biological function. Indeed, the temperature and the humidity of the inhaled air are adjusted by the heat and the water exchange in the nasal cavity wall's surface. In order to understand the nasal cavity functions for the adjustment of the temperature and the humidity of the inhaled air, it is necessary to know the flow distribution during breathing process. To do so, a noninvasive measurement is hard to be held. Therefore, this study simulates a laminar steady airflow, in a realistic adult male chimpanzee's nasal cavity shape reconstructed from Computed Tomography (CT) images, using CFD. So far, an airflow study related to chimpanzee's nasal cavity has not been previously reported. We present for the first time a computer simulation of a realistic chimpanzee's nasal cavity flow. This study uses a model with heat and humidity transport. The governing equations include a continuum equation and the equations describing momentum, energy, and water transport. The findings are in agreement with the expected results directly inferred from the biological function of the nasal cavity itself.