Functional analysis of chimpanzee (Pan troglodytes) private signing

The chimpanzee's use of American Sign Language (ASL) to communicate with humans and with each other has been empirically demonstrated in several reports, but this is the first research to experimentally examine their use of sign language in a nonsocial fashion: private signing. This experiment examined the private signing behavior of five signing chimpanzees, using a remote videotaping technique with no human present. It was found that all five chimpanzees signed to themselves for a total of 368 instances. These instances of private signing were classified into nine different functional categories as has been done in the analysis of private speech and signing in hearing and deaf human children. Similar to humans, a few of the categories accounted for the majority of the instances of private signing. These findings empirically demonstrate a behavior similar to private speech and signing in humans.     

Evidence of cave use by savanna chimpanzees (<Emphasis Type="Italic">Pan troglodytes verus</Emphasis>) at Fongoli,Senegal: implications for thermoregulatory behavior

Much attention has been paid to how humans both adapt and acclimate to heat stress, primarily due to the relevance of these issues to hominid evolution in open Plio-Pleistocene environments. However, little is known about the responses of human’s closest living relative, the chimpanzee (Pan troglodytes), to similar environmental stressors. In southeastern Senegal, one of the hottest and driest habitats that chimpanzees (P. t. verus) live in today, apes rely on behavioral mechanisms of dealing with thermal stress. Chimpanzees’ use of caves was based primarily on indirect evidence (feeding traces, feces, and hairs) gathered from one cave from January to December 2004, but data from observational records collected from May 2001 through March 2006 supplement these data. The hypothesis that chimpanzees’ use of caves is a response to heat was tested by collecting data on temperatures within the largest cave and in different habitats used by chimpanzees, such as gallery forest and woodland. Results indicate that chimpanzees primarily use caves as shelters during the hottest times of year and that caves are consistently and significantly cooler than open habitats. Insight into the way that chimpanzees in Senegal cope with extreme temperatures may help us to better understand the behavior of early hominids in such an environment.     

The role of cross-fostering in sign language studies of chimpanzees

In cross-fostering, the young of one species are reared by adults of another. In our cross-fostering laboratory, two-way communication by means of American Sign Language (ASL) brought the rearing conditions for chimpanzees much closer to those of human children than was possible in earlier studies. At the same time, ASL provided a means by which chimpanzees could express their intelligence in ways that permit closer comparisons with human children. Cross-fostered chimpanzees can communicate with human observers whose only source of information is the American Sign Language (ASL) signs of the chimpanzees. In order to show that the chimpanzees could name natural language categories — that the sign DOG could refer to any dog, FLOWER to any flower, SHOE to any shoe — each test trial was a first trial in that tests slides were presented only once. Analysis of errors showed that two aspects of the signs, gestural form and conceptual category, governed the distribution of errors. Like human adults and human children who sign, the chimpanzees modulated their signs in meaningful ways. Observations in field notes, video tape records, and systematic experiments illustrate how these modulations were related to the verbal and nonverbal, context and how they made signs more visible, more versatile, and more informative.     

Developing classification in action: II. Young chimpanzees (Pan troglodytes)

The development of spontaneous object manipulation in 5 chimpanzees (Pan troglodytes) from ages 15 to 54 months was investigated, focusing on formal properties of subjects’ acts and the objects they manipulated. Young chimpanzees’ manipulation progress from serial one-at-a-time acts on one object to parallel two-at-a-time acts on two or more objects. With age, simultaneous acts become increasingly transformational and identical or reciprocal to each other. Moreover, the class properties of objects manipulated simultaneously change. When presented with objects belonging to two different classes, subjects shift, with age, from manipulating different objects to manipulating identical or similar objects. In all these respects young chimpanzee’ development is similar to human infants’. In others it differs. Most especially, the onset age is later and the development is slower as well as less structurally complex.     

Visual preference in a human-reared agile gibbon (<Emphasis Type="Italic">Hylobates agilis</Emphasis>)

Visual preference was evaluated in a male agile gibbon. The subject was raised by humans immediately after birth, but lived with his biological family from one year of age. Visual preference was assessed using a free-choice task in which five or six photographs of different primate species, including humans, were presented on a touch-sensitive screen. The subject touched one of them. Food rewards were delivered irrespective of the subject’s responses. We prepared two types of stimulus sets. With set 1, the subject touched photographs of humans more frequently than those of other species, recalling previous findings in human-reared chimpanzees. With set 2, photographs of nine species of gibbons were presented. Chimpanzees touched photographs of white-handed gibbons more than those of other gibbon species. The gibbon subject initially touched photographs of agile gibbons more than white-handed gibbons, but after one and two years his choice patterns resembled the chimpanzees’. The results suggest that, as in chimpanzees, visual preferences of agile gibbons are not genetically programmed but develop through social experience during infancy.     

Copulation Calls in Female Chimpanzees (<Emphasis Type="Italic">Pan troglodytes schweinfurthii</Emphasis>) Convey Identity but Do Not Accurately Reflect Fertility

Copulation calls are a relatively common feature of female primate behavior thought to function in the advertisement of female receptivity and subsequent incitation of male–male competition. To date, the majority of work on copulation calling behavior has focused on various monkey species, with little empirical evidence from the great apes. Previous research on wild chimpanzees (Pan troglodytes schweinfurthii) has suggested that estrous females produce copulation calls to avoid monopolization by single males and to minimize competition from other females. We here extended these findings by investigating to what degree these social demands were reflected in the calls’ acoustic structure. We recorded and acoustically analyzed 71 copulation call bouts from 6 adult female chimpanzees in the Budongo Forest, Uganda. We did not find any acoustic differences in calls given by females in fertile and nonfertile periods, as assessed by their hormonal profiles. However, the calls’ acoustic structure did reliably encode identity cues of the calling female. We propose that, in chimpanzees, the use and morphology of copulation calls have jointly been shaped by the selective advantage of concealing fertility. Owing to the low visibility conditions associated with chimpanzees’ natural forest habitat and their dispersed social system, providing identity cues may be of particular biological relevance for these nonhuman primates.     

Group Release of Sanctuary Chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) in the Haut Niger National Park,Guinea, West Africa: Ranging Patterns and Lessons So Far

The release of wild or captive-bred mammals within their historical ranges typically aims to reestablish populations in areas where they have become extinct or extirpated, to reinforce natural populations, or to resolve human–wildlife conflicts. Such programs, which also typically in parallel help foster the protection of the release site, concern a wide range of endangered mammalian species, including our closest living relatives: chimpanzees. In June 2008, the Chimpanzee Conservation Center (CCC), which is located in the High Niger National Park (HNNP) in Guinea, released a group of 12 chimpanzees (Pan troglodytes verus) comprised of 6 females and 6 males (8–20 yr old). The selected release site lies 32 km from the sanctuary in the Mafou, a core area of HNNP where wild chimpanzees are also known to occur. The purpose of this release was therefore to reinforce the natural chimpanzee population within the Mafou core area and to promote the protection of the HNNP. Nearly 2 yr postrelease, 9 chimpanzees still remain free-living. Two thirds of the release chimpanzees were equipped with VHF-GPS store-on-board tracking collars. We used data from retrieved collars to explore the release chimpanzees’ habitat use, individual day range, and core area use (50% and 80%) during the first year of the release. Males traveled significantly further than females. Although minimum day range did not differ between the sexes or vary seasonally, some release males were active for longer during the day than the females. Males also ranged over larger areas and used a wider network of core areas than the females. Habitat use was similar to that recorded in wild chimpanzees in the HNNP. As of September 2010, 2 males and 3 females form a group at the release site. Two of these females gave birth to healthy offspring respectively 16 and 20 mo postrelease. Another female successfully immigrated into a wild chimpanzee community. We suggest that the success of this chimpanzee release can be attributed to the CCC’s lengthy rehabilitation process and the savanna-mosaic habitat of the HNNP. This release demonstrates that under special socioecological circumstances, the release of wild-born adult chimpanzees of both sexes is a viable strategy, which can also function as an effective conservation tool.     

Behavioral Ecology of Sympatric Chimpanzees and Gorillas in Bwindi Impenetrable National Park,Uganda: Diet

Via a field study of chimpanzees (Pan troglodytes schweinfurthii) and gorillas (Gorilla gorilla beringei) in Bwindi Impenetrable National Park, Uganda, we found that their diets are seasonally similar, but diverge during lean seasons. Bwindi chimpanzees fed heavily on fruits of Ficus sp., which were largely ignored by the gorillas. Bwindi gorilla diet was overall more folivorous than chimpanzee diet, but was markedly more frugivorous than that of gorillas in the nearby Virunga Volcanoes. During 4 mo of the year Bwindi gorilla diet included more food species than that of the chimpanzees. Three factors in particular—seasonal consumption of fibrous foods by gorillas, interspecific differences in preferred fruit species, and meat consumption by chimpanzees—contributed to dietary divergence between the two species. When feeding on fruits, gorillas ate Myrianthus holstii more frequently than chimpanzees did, while chimpanzees included more figs in their annual diet. Chimpanzee diet included meat of duikers and monkeys; gorilla frequently consumed decaying wood.     

Coordination of Chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) in a Stag Hunt Game

Group-living animals frequently face situations in which they must coordinate individual and sometimes conflicting goals. We assessed chimpanzees’ ability to coordinate in a Stag Hunt game. Dyads were confronted with a situation in which each individual was already foraging on a low-value food (hare) when a high-value food (stag) appeared that required collaboration for retrieval, with a solo attempt to get the stag resulting in a loss of both options. In one condition visibility between partners was open whereas in the other it was blocked by a barrier. Regardless of condition, dyads almost always (91%) coordinated to choose the higher valued collaborative option. Intentional communication or monitoring of the partner’s behavior before decision making—characteristic of much human coordination—were limited. Instead, all dyads adopted a leader–follower strategy in which one partner took the risk of going first, presumably predicting that this would induce the other to join in (sometimes communicating if she was slow to do so). These results show that humans’ closest primate relatives do not use complex communication to coordinate but most often use a less cognitively complex strategy that achieves the same end.     

Visual and auditory conditional position discrimination in chimpanzees (Pan troglodytes)

Chimpanzee cognition has been studied predominantly through the visual modality, and much less through the auditory modality. The aim of this study was to explore possible differences in chimpanzees’ processing of visual and auditory stimuli. We developed a new conditional position discrimination (CPD) task requiring the association between a stimulus (from either the auditory or the visual modality), and a spatial position (left or right). The stimuli consisted of the face and voice of two individuals well known to the subjects (one chimpanzee and one human). Six chimpanzees participated in both the visual and the auditory conditions. We found contrasting results between the two conditions: the subjects acquired the CPD more easily in the visual than in the auditory condition. This supports previous findings on the difficulties encountered by chimpanzees in learning tasks involving auditory stimuli. Our experiments also revealed individual differences: the chimpanzee with the most extensive experience in symbolic visual matching tasks showed good performance in both conditions. In contrast, the chimpanzee expert in an auditory-visual intermodal matching task showed no sign of learning in either condition. Future work should focus on finding the most appropriate procedure for exploring chimpanzees’ auditory-visual cognitive skills.     

What defines successful integration into a social group for hand-reared chimpanzee infants?

Hand-rearing of captive great ape infants is sometimes necessary but can have negative behavioral consequences. Modern hand-rearing protocols, including early integration into a diverse group of conspecifics, appear to reduce the negative consequences of hand-rearing, but the process of integration is not well studied. We investigated six potential metrics of success during the introduction of two hand-reared chimpanzee infants into a troop of nine other chimpanzees at the Maryland Zoo in Baltimore. Little aggression was observed and the infants continue to be maintained in the troop at publication. As we predicted, during the introduction the hand-reared infants showed consistent levels of stress-related behaviors, participated in affiliative interactions with all available partners, and acted, received, and mutually engaged in these interactions. Solitary behaviors by these infants were similar to a mother-reared infant in the same group. Each infant also formed a relationship with a specific female that involved nest-sharing, carrying, retrieval, and intervening to reduce risk to the infant; these relationships could be classified as allomothering because they involved maternal behavior but occupied significantly less of the infants’ time than a maternal relationship. Contrary to our prediction, the hand-reared infants therefore spent significantly less time in social behavior than a mother-reared infant of the same age. In addition, the hand-reared infants continued to show strong social preferences for each other as introductions progressed and to direct a low but consistent number of nonfeeding social behaviors to humans. The successful introduction of hand-reared infants appeared to involve adding conspecific social relationships to the infants’ social repertoire, but not eliminating social interactions directed at humans.     

Hierarchy and social status in Budongo chimpanzees

The status hierarchy is fundamental in the lives of male chimpanzees. This study describes the dominance interactions and social status among adult male chimpanzees of the Sonso community in the Budongo Forest Reserve, Uganda, during the period that they were first studied (1994 and 1995). Social dominance is typically measured using the behaviour of either the subordinate or the dominant individual, but a relationship is dependent on the behaviour of both parties and this study explicitly used both subordinate and dominant behaviours to investigate the status hierarchy. Among adult males of the Sonso community, agonistic interactions occurred at a low rate and pant-grunts were rare, but males could be ranked into separate hierarchies of agonistic dominance and pant-grunting (labelled respect) using ratios of behaviour performed/behaviour received. These hierarchies were combined to form a single hierarchy of social status that divided the males among five distinct status levels. The highest status level was held by an alliance between two males who replaced the previous alpha male during the first part of the study. Neither male in this alliance partnership pant-grunted to the other, although the reason for cooperative behaviour was unclear. Although the nominally beta male was treated as such by other adult males, he achieved surprisingly little mating success. Budongo Forest chimpanzees do not warrant the sometimes-expressed view that they are non-aggressive and peaceable and the broad pattern of their status interactions matches with that seen in other chimpanzee populations.     

Chimpanzees (Pan troglodytes) Flexibly Adjust Their Behaviour in Order to Maximize Payoffs,Not to Conform to Majorities

Chimpanzees have been shown to be adept learners, both individually and socially. Yet, sometimes their conservative nature seems to hamper the flexible adoption of superior alternatives, even to the extent that they persist in using entirely ineffective strategies. In this study, we investigated chimpanzees’ behavioural flexibility in two different conditions under which social animals have been predicted to abandon personal preferences and adopt alternative strategies: i) under influence of majority demonstrations (i.e. conformity), and ii) in the presence of superior reward contingencies (i.e. maximizing payoffs). Unlike previous nonhuman primate studies, this study disentangled the concept of conformity from the tendency to maintain one’s first-learned strategy. Studying captive (n=16) and semi-wild (n=12) chimpanzees in two complementary exchange paradigms, we found that chimpanzees did not abandon their behaviour in order to match the majority, but instead remained faithful to their first-learned strategy (Study 1a and 1b). However, the chimpanzees’ fidelity to their first-learned strategy was overridden by an experimental upgrade of the profitability of the alternative strategy (Study 2). We interpret our observations in terms of chimpanzees’ relative weighing of behavioural options as a function of situation-specific trade-offs. More specifically, contrary to previous findings, chimpanzees in our study abandoned their familiar behaviour to maximize payoffs, but not to conform to a majority.     

Early signs of language in cross-fostered chimpanzees

In cross-fostering the young of one species are reared by adults of another, as in the classical ethological studies of imprinting and song-learning. In our laboratory, infant chimpanzees were reared under human conditions that included two-way communication in American Sign Language (ASL) the gestural language of the deaf in North America. Here we describe the cross-fostering conditions of this laboratory and the contrast with operant conditioning. We also review the uses and the shapes of the signs in the vocabularies of five cross-fostered chimpanzees, tests demonstrating that the cross-fostered chimpanzees could use signs of ASL to communicate conceptual information to human observers whose only source of information was the signs of the chimpanzees, early development of inflections that resemble the early inflections of deaf human children, evidence based on errors and on inflections for duality of patterning, the continued use of sign language among the chimpanzees even when deprived of human input and the acquisition of signs by the infant Loulis from the cross-fostered chimpanzees.     

Comparing infant and juvenile behavior in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): a preliminary study

The dichotomy between the two Pan species, the bonobo (Pan paniscus) and chimpanzee (Pan troglodytes) has been strongly emphasized until very recently. Given that most studies were primarily based on adult individuals, we shifted the “continuity versus discontinuity” discussion to the infant and juvenile stage. Our aim was to test quantitatively, some conflicting statements made in literature considering species differences between immature bonobos and chimpanzees. On one hand it is suggested that infant bonobos show retardation in motor and social development when compared with chimpanzees. Additionally it is expected that the weaning process is more traumatic to chimpanzee than bonobo infants. But on the other hand the development of behaviors is expected to be very similar in both species. We observed eight mother–infant pairs of each species in several European zoos. Our preliminary research partially confirms that immature chimpanzees seem spatially more independent, spending more time at a larger distance from their mother than immature bonobos. However, the other data do not seem to support the hypothesis that bonobo infants show retardation of motor or social development. The development of solitary play, environmental exploration, social play, non-copulatory mounts and aggressive interactions do not differ between the species. Bonobo infants in general even groom other group members more than chimpanzee infants. We also found that older bonobo infants have more nipple contact than same aged chimpanzees and that the weaning process seems to end later for bonobos than for immature chimpanzee. Additionally, although immature bonobos show in general more signs of distress, our data suggest that the weaning period itself is more traumatic for chimpanzees.     

The subsistence technology of capuchins

Via Oswalt’s system of classification, I compare the tool-kits of wild and captive capuchins with those of Tanzanian chimpanzees and Tasmanian aborigines. The results indicate that capuchins have tool-kits that are smaller, and have lower ratios of artifacts to naturefacts, than those of Tanzanian chimpanzees and Tasmanian aborigines. Accordingly, Oswalt’s system can be used productively to assess the relative technological skills of monkeys versus those of apes and humans.     

Chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) learn to act with other individuals in a cooperative task

We presented two chimpanzees with a task in which they were required to pull each end of a rope simultaneously to drag blocks supporting food into reach. The chimpanzees did not succeed in initial tests. They did not immediately understand the necessity for cooperation, and they did not adjust their behavior to work with the partner. However, the frequency of success gradually increased as the number of sessions increased and the task was varied. They began to look at the partner frequently, wait if the partner was not holding the rope, and pull the rope in synchrony with the partner. However, they did not use interactive behaviors or eye contact to synchronize their behavior. One chimpanzee was then paired with a human partner in the same situation. After initial failures, the chimpanzee began to solicit the human partner for cooperation: looking up at his face, vocalizing, and taking the partner’s hand. When this chimpanzee was again paired with the chimpanzee partner, no soliciting behavior was observed. Thus, the chimpanzees could learn to coordinate their behavior through trial and error. Communicative behavior emerged during the task, but the communication differed according to the identity of the partner.     

Maintenance of biological rhythms during hibernation in Eastern woodchucks (<Emphasis Type="Italic">Marmota monax</Emphasis>)

We undertook a study to determine presence of circadian rhythms during woodchuck hibernation using continuously monitored body temperatures. Males had shorter torpor and longer euthermic periods than females. Circular statistics revealed a significant mean vector for males entering into torpor (10:21 h), but not for females. No significant mean vector was found for male or female arousal from torpor. A contingency test was applied to the torpor bout durations. All 7 males tested had significant τ’s between 24 and 26 h, while 6 of the 13 females tested had significant τ’s with a range of 22–27 h. These results implicate a free-running circadian clock during torpor bouts. Overall, the data support the existence of biological rhythms during hibernation in woodchucks, especially for males during arousals. Since entries into torpor appear to be synchronized for males, arousal periods may be used to resynchronize their circadian system. The persistence of biological rhythms during hibernation may help to insure successful mating in the spring after emergence.     

Antennation of nectar-receivers encodes colony needs and food-source profitability in the ant Camponotus mus

In social insects, interactions among individuals are important in colony organisation because they can be used in decision making. During trophallaxis in ants, antennal and foreleg contacts between both partners are established. It has been suggested that a modulatory communication channel could be involved in such contacts, but it remains undemonstrated. The aim of this work was to find variables plausible to be encoded in such contacts and quantify the consequent changes in the tactile stimulation the food-donor ant receives. We recorded nectar transference between pairs of workers in experimental arenas once one of them had returned from collecting sucrose solution (15 or 40%w/w), with different situations of colony’s sugar deprivation. The frequency of antennal strokes that the food-donor ant received on her head depended on both the colony’s sugar-deprivation and the concentration, the latter showed differences within 3–5 deprivation days. Antennal and foreleg movements of the food-receiver increase with increasing level of colony’s carbohydrate deprivation, as well as with increasing concentration of the transferred food. Not only does this study reopen an interesting question, but it gives evidence that variables related to the appetitive context are indeed encoded in the tactile stimulation during the trophallaxis as well. Consequently, they have the effective potentiality to play a communicational role in the organization of colony activities. Received 22 December 2005; revised 5 April 2006; accepted 12 April 2006.     

Chimpanzees Cope with Temporary Reduction of Escape Opportunities

Many studies have focused on the responses of primates to changes in population density, but little is known about the responses to more subtle changes in the physical environment. Studies on primate responses to high density supported the use of various coping strategies depending on the circumstances. We investigated which strategy was adopted by a large group of captive chimpanzees in response to a special case of space restriction: the temporary reduction of escape opportunities. Due to work on the outdoor enclosure, the chimpanzees at Chester Zoological Gardens were confined to their indoor quarters for 1 mo. As it was winter, the chimpanzees spent most of their time indoors also during the 7-week control period, but had the option to go outside. Therefore, there was no major difference in the actual density between the two conditions, but by being confined indoors the chimpanzees had reduced escape opportunities. Under this condition they would be expected not to alter their behavior dramatically but to adjust it in ways to avoid conflict and to reduce tension. The chimpanzees did not change their overall aggression rates during the period with a reduction of escape opportunities versus the control period. The patterns of allogrooming and submissive greetings also did not differ between the two periods. However, they appeared to refrain from joining ongoing aggressive interactions during the period with reduced escape opportunities, whereas no change in counteraggression occurred between the two periods. Our findings suggest that the chimpanzees may adopt a selective inhibition strategy when escape opportunities are limited. Further support for such selectivity comes from the analysis of different types of aggressor-target dyads. During the period with reduced escape opportunities, aggression rates decreased in dyads characterized by high aggression rates at baseline, i.e., high aggressor-target dyads, whereas the same individuals did not change their aggressive tendencies in low aggressor-target dyads. Ours is among the few studies showing a decrease, albeit selective, in aggressive behavior during a situation of space restriction. The ability to rapidly adopt an appropriate strategy and to inhibit aggression during such restriction seems to confirm findings of chimpanzees under high density conditions. The inhibition of aggressive tendency may be especially developed in chimpanzees and may be related to the natural challenges posed by the routine changes in subgroup membership characteristic of their fission-fusion social organization.