Dead infant carrying by chimpanzee mothers in the Budongo Forest

It has been suggested that non-human primates (hereafter primates) can respond to deceased conspecifics in ways that suggest they experience psychological states not unlike humans, which would indicate they exhibit some notion of death. Here, we report long-term demographic data from two East African chimpanzee groups. During a combined 40-year observation period we recorded 191 births of which around a third died in infancy, mostly within the first year. We documented the post-mortem behaviour of the mothers and found that Budongo chimpanzee mothers routinely carried deceased infants after their death, usually until the body started to decompose after a few days. However, we also observed three cases of extended carrying lasting for more than 2-weeks, one of which was followed by the unusual extended carrying of a substitute object and another which lasted three months. In each case, the corpses mummified. We discuss these data in view of functional hypotheses of dead infant carrying in primates and regarding the potential proximate mechanisms involved in this behaviour. We suggest that chimpanzees may exhibit psychological processes related to death of conspecifics similar to human grieving.

Do chimpanzee pant hoots have semantic content?

I read with great interest the exciting study by Leroux et al. [(2021) Anim Behav 179, 49–50] who investigated the nature of pant-hoot–food-call combinations in a community of wild chimpanzees (Pan troglodytes schweinfurthii) at the Budongo Conservation Field Station, Budongo Forest, Uganda. The authors propose, among others, that they reveal the first evidence that wild chimpanzees are able “to combine meaning-bearing units into larger structures” (i.e., that they are capable of semantic compositionality and, by extension, syntax). Their analysis represents an important addition to a growing body of research and discussions on communicational combinatoriality in wild primates and specifically apes, and, by extension, extinct hominins. Incidentally, I have recently published a paper in Animal Cognition in which I also suggested, based on a reanalysis of existing data, that wild chimpanzees can display semantic compositionality and syntax in their communication [Gabrić (2021) Anim Cogn, online ahead of print]. In the present commentary, I argue that Leroux et al.’s (2021) interpretation of the data may be ungrounded given that (1) unlike for food calls, there is currently very little if any indication in the scientific literature that pant-hoots have semantic content (i.e., are meaningful) and given that (2) Leroux et al. (2021) did not investigate their a priori assumption that the observed pant-hoots are in fact semantic. Since pant-hoots feature prominently in the chimpanzee vocal repertoire and the debate on their eventual semanticity is still wide open, this represents a fine opportunity to revisit this issue in the context of Leroux et al.’s (2021) study. Their paper further raises several other less significant questions.

Social bonding, gestural complexity and displacement behaviour of wild chimpanzee

Kinship and demography affect social affiliation in many different contexts such as co-feeding, resting, travel, grooming, visual attention and proximity. Chimpanzees may coordinate these social interactions by using gestural communication to make signaller’s goal transparent to the recipient and also by increasing commitment of the recipient through including rewarding property in communication. The rewards of gesturing can be measured through the rates of displacement behaviour made in response to these gestures by the recipient. We tested hypothesis that gestural communication affects social affiliation after controlling for kinship and demography in wild, adult chimpanzees living in Budongo Forest, Uganda. We found that affiliative but not antagonistic gestures positively predicted social affiliation. Contexts differed in their association with gestures according to complexity and association with displacement behaviour. More complex, less intense gestures predicted mutual grooming, travel, visual attention whereas less complex, more intense gestures predicted unidirectional grooming. Mirroring these patterns, reduced displacement activity occurred in response to gestures associated with unidirectional grooming but not other contexts. We highlight that these tactical decisions that wild chimpanzees make in their use of gestural communication may be driven by complexity of social environment that influences effectiveness with which signalers can influence the recipient.

Ecological and dietary differences between Ugandan chimpanzee communities with possible implications on tool use

ABSTRACTSome East African chimpanzee (Pan troglodytes schweinfurthii) communities, such as the Sonso chimpanzees, display an unusually limited range of tool-use, but it remains unclear whether this is due to ecological and/or cultural factors. Information on ecological conditions and the diet of the Sonso chimpanzees in relation to neighbouring communities is needed. Here, we studied three adjacent communities in Budongo Forest (Sonso, Waibira, and Kamira), and the presumed core area of an undescribed community (Mwera), in the neighbouring Bugoma Forest. Through line-transects, we investigated (i) whether there were differences in food diversity and abundance between the communities’ home ranges; (ii) whether the home ranges differed in abundance of sticks and insect nests; and (iii) whether Sonso and Mwera chimpanzees differed in their diet (using faecal samples). Across communities, Sonso had the richest food availability and the lowest insect nest abundance. However, food availability in Mwera, Bugoma, was richer than Budongo communities that neighbour the Sonso territory, suggesting that there may be variation within Budongo. Data from faecal samples replicated our direct observations of food availability suggesting that Sonso chimpanzees had a broader diet than Mwera chimpanzees. This difference in foods availability may partially explain the Sonso chimpanzees’ lack of stick-tool-use, and low levels of insectivory. The tool repertoire of the other communities is currently unknown; however, we make predictions based on our ecological data. More detailed knowledge of small-scale variation in ecology within and between forest habitats may be important to advancing our understanding of the drivers of tool-use.SIGNIFICANCE STATEMENT – HIGHLIGHTED STUDENT PAPERTo advance our knowledge of the role of ecological factors in the emergence of tool use in chimpanzees, a nuanced understanding of the ecological conditions different chimpanzee communities experience is needed. We studied four Ugandan chimpanzee communities in two forests. One of these communities, Sonso, in the Budongo Forest, is well-known for its restricted range of tool types, including a total absence of stick use. Food diversity and abundance were highest, and stick tool use opportunities (abundance of sticks and insect nests) were lowest for the core-habitat of the Sonso chimpanzees in contrast to the other communities. We argue that ecological factors play a role in their unusual pattern of tool use, and make predictions about the expected types of tool use in the other communities based on their ecology. Thus, our study provides information that may help advance our understanding of how tool use arises under varied socioecological circumstances.

Chimpanzee quiet hoo variants differ according to context

In comparative studies of evolution of communication, the function and use of animal quiet calls have typically been understudied, despite that these signals are presumably under selection like other vocalizations, such as alarm calls. Here, we examine vocalization diversification of chimpanzee quiet ‘hoos’ produced in three contexts—travel, rest and alert—and potential pressures promoting diversification. Previous playback and observational studies have suggested that the overarching function of chimpanzee hoos is to stay in contact with others, particularly bond partners. We conducted an acoustic analysis of hoos using audio recordings from wild chimpanzees ( Pan troglodytes schweinfurthii ) of Budongo Forest, Uganda. We identified three acoustically distinguishable, context-specific hoo variants. Each call variant requires specific responses from receivers to avoid breaking up the social unit. We propose that callers may achieve coordination by using acoustically distinguishable calls, advertising their own behavioural intentions. We conclude that natural selection has acted towards acoustically diversifying an inconspicuous, quiet vocalization, the chimpanzee hoo. This evolutionary process may have been favoured by the fact that signallers and recipients share the same goal, to maintain social cohesion, particularly among those who regularly cooperate, suggesting that call diversification has been favoured by the demands of cooperative activities.