Infanticide in chimpanzees: Taphonomic case studies from Gombe

A case of intercommunity lethal aggression by chimpanzees in an open and dry landscape, Issa Valley, western Tanzania

Intercommunity (lethal) aggression is a familiar component of the behavioural repertoire of many forest-dwelling chimpanzee (Pan troglodytes) communities. However, until now, the absence of intercommunity attacks - including killings - in communities that live in open, mosaic environments has supported hypotheses of reduced resource competition in drier habitats, and informed referential models of early hominin social dynamics in a similar habitat. In June 2020, we observed the first instance of intercommunity lethal aggression, a male-committed infanticide, by the Issa chimpanzee community, which live in a savannah-mosaic habitat in the Issa Valley, western Tanzania. The carcass was recovered by researchers after it was abandoned by the attackers. Here, we give a detailed account of the events leading up to and including the infanticide, and contextualise our observations with what has been described for other chimpanzee communities. Notably, in contrast to the majority of reported intercommunity infanticides, the infant male victim was castrated (and not cannibalised), making this the youngest reported castration. This observation of intercommunity aggression disproves its hypothesised absence in savannah-dwelling chimpanzees, which by extension, has implications for early hominin evolution. We suggest that the near absence of observations of intercommunity aggression in savannah chimpanzee communities is most likely due to the lack of long-term study communities, and in some cases geographic isolation. We hypothesise that food-rich areas within a habitat with otherwise widely distributed food sources may select for intense intercommunity aggression despite the low population density characteristic of savannah communities. Anecdotes such as this add to the comparative database available on intercommunity killings in chimpanzee society, improving our ability to draw inferences about their evolutionary significance.

Infant cannibalism in wild white-faced capuchin monkeys

Cannibalism has been observed in a variety of animal taxa; however, it is relatively uncommon in primates. Thus, we rely heavily on case reports of this behavior to advance our understanding of the contexts under which it occurs. Here, we report the first observation of cannibalism in a group of wild white-faced capuchin monkeys (Cebus imitator). The subject was a dead infant, estimated to be 10 days old, and the probable victim of infanticide. Consumption of the corpse was initiated by a 2-year-old male (second cousin of the infant), though it was eventually taken over and monopolized by the group's alpha female (grandaunt of the infant). Although most group members expressed interest in the corpse (sniffing, touching, and threatening it), no others made an attempt to consume it. Given that this is the only observation of cannibalism recorded in over 37 years of study on this population, we consider it to be a rare behavior in this species. This detailed record contributes new data, which, when combined with other reports within and across species and contexts, enables the evaluation of adaptive explanations of cannibalism.

Anthropomorphism in comparative affective science: Advocating a mindful approach

Anthropomorphism is the attribution of human-like capacities and traits to non-human entities. Anthropomorphism is ubiquitous in everyday life and in scientific domains, operating both implicitly and explicitly as a function of the human lens through which we view the world. A rich history of work in psychology, animal behavior, cognitive science, and philosophy has highlighted the negative and, to a lesser degree, the positive implications of anthropomorphism. In this article, we aim to provide a nuanced perspective of how anthropomorphism impacts the work of comparative affective science. Specifically, we discuss three domains of empirical inquiry in which lessons can be drawn about the benefits and pitfalls of anthropomorphism: responses to death, inequity aversion, and prosocial behavior. On balance, we advocate a mindful approach to anthropomorphizing in comparative affective science, and comparative science more generally.

A chomped chimp: New evidence of tooth marks on an adult chimpanzee (Pan troglodytes verus)

OBJECTIVES

To describe and interpret previously unreported marks on the dry cranium of an adult chimpanzee (Pan troglodytes verus) from Côte d'Ivoire at the Smithsonian's National Museum of Natural History (USNM 450071).

MATERIALS AND METHODS

All marks on the cranium were documented and assessed through physical examination of the specimen, photography, micro-computed tomography (micro-CT), and 3D laser scanning. Pits and punctures were measured with digital calipers for comparison with published carnivore tooth mark measurements.

RESULTS

The cranium shows perimortem or postmortem damage to the temporal, occipital, and maxillary regions that is not recent. Size and color variation in the marks suggest two damage events, possibly involving chewing by different animals, at least one of which was a large-bodied mammal. The 22 tooth pits and punctures (0.89-8.75 mm in maximum length and 0.88-6.63 mm in breadth) overlap in size with those inflicted by wild leopards, the most significant predators of common chimpanzees due to their largely overlapping ecological distributions.

CONCLUSIONS

Based on qualitative and quantitative evidence, we conclude that leopards are the most likely cause of the most prominent marks on the cranium. However, we cannot rule out the additional possibility of other chimpanzees, although there are no published studies of chimpanzee tooth marks for direct comparison. This study is the most extensive documentation to date of a modern adult chimpanzee skull exhibiting tooth marks by a large mammal, thus providing new evidence to help identify and interpret other events of predation and scavenging of large-bodied apes in the modern and fossil records.

Oral microbiome diversity in chimpanzees from Gombe National Park

Historic calcified dental plaque (dental calculus) can provide a unique perspective into the health status of past human populations but currently no studies have focused on the oral microbial ecosystem of other primates, including our closest relatives, within the hominids. Here we use ancient DNA extraction methods, shotgun library preparation, and next generation Illumina sequencing to examine oral microbiota from 19 dental calculus samples recovered from wild chimpanzees (Pan troglodytes schweinfurthii) who died in Gombe National Park, Tanzania. The resulting sequences were trimmed for quality, analyzed using MALT, MEGAN, and alignment scripts, and integrated with previously published dental calculus microbiome data. We report significant differences in oral microbiome phyla between chimpanzees and anatomically modern humans (AMH), with chimpanzees possessing a greater abundance of Bacteroidetes and Fusobacteria, and AMH showing higher Firmicutes and Proteobacteria. Our results suggest that by using an enterotype clustering method, results cluster largely based on host species. These clusters are driven by Porphyromonas and Fusobacterium genera in chimpanzees and Haemophilus and Streptococcus in AMH. Additionally, we compare a nearly complete Porphyromonas gingivalis genome to previously published genomes recovered from human gingiva to gain perspective on evolutionary relationships across host species. Finally, using shotgun sequence data we assessed indicators of diet from DNA in calculus and suggest exercising caution when making assertions related to host lifestyle. These results showcase core differences between host species and stress the importance of continued sequencing of nonhuman primate microbiomes in order to fully understand the complexity of their oral ecologies.

Intra-community infanticide in wild, eastern chimpanzees: a 24-year review

Infanticide is well documented in chimpanzees and various hypotheses have been proposed to explain this behavior. However, since infanticide by chimpanzees is relatively rare, it has thus far not been possible to thoroughly test these hypotheses. Here we present an analysis of the largest dataset of infanticides from a single community of chimpanzees, a full record of all intra-community infanticides and failed attempts at infanticide over a 24-year period for the Sonso community of chimpanzees in the Budongo Forest, Uganda. We use these data to test four hypotheses for this behavior: the sexual selection hypothesis, male mating competition, resource competition, and meat acquisition. Our dataset consisted of 33 attacks on 30 victims, 11 of which were ‘definite’ infanticides, four of which ‘almost certain’, and nine were ‘suspected’, while nine were ‘attempted’ infanticides. The majority of attacks where the perpetrators were known (23) had only male attackers and victims were disproportionately young (two-thirds of victims with known ages were under 1 week old). Our data best support the sexual selection hypothesis for infanticide. Cannibalism was infrequent and partial, suggesting meat acquisition was a by-product of infanticide, and there was no evidence to suggest that infanticide was part of a male strategy to eliminate future competitors. Infanticide by females was rare, but we suggest sexual selection, operating through intra-sexual competition, may also be responsible for infanticide by females.

Chimpanzees and death

Information about responses to death in nonhuman primates is important for evolutionary thanatology. This paper reviews the major causes of death in chimpanzees, and how these apes respond to cues related to dying and death. Topics covered include disease, human activities, predation, accidents and intra-species aggression and cannibalism. Chimpanzees also kill and sometimes eat other species. It is argued that, given their cognitive abilities, their experiences of death in conspecifics and other species are likely to equip chimpanzees with an understanding of death as cessation of function and irreversible. Whether they might understand that death is inevitable-including their own death, and biological causes of death is also discussed. As well as gathering more fundamental information about responses to dying and death, researchers should pay attention to possible cultural variations in how great apes deal with death.This article is part of the theme issue 'Evolutionary thanatology: impacts of the dead on the living in humans and other animals'.