Possible use of heterospecific food-associated calls of macaques by sika deer for foraging efficiency

The function and evolution of child-directed communication

Humans communicate with small children in unusual and highly conspicuous ways (child-directed communication (CDC)), which enhance social bonding and facilitate language acquisition. CDC-like inputs are also reported for some vocally learning animals, suggesting similar functions in facilitating communicative competence. However, adult great apes, our closest living relatives, rarely signal to their infants, implicating communication surrounding the infant as the main input for infant great apes and early humans. Given cross-cultural variation in the amount and structure of CDC, we suggest that child-surrounding communication (CSC) provides essential compensatory input when CDC is less prevalent-a paramount topic for future studies.

Human listeners' perception of behavioural context and core affect dimensions in chimpanzee vocalizations

Vocalizations linked to emotional states are partly conserved among phylogenetically related species. This continuity may allow humans to accurately infer affective information from vocalizations produced by chimpanzees. In two pre-registered experiments, we examine human listeners' ability to infer behavioural contexts (e.g. discovering food) and core affect dimensions (arousal and valence) from 155 vocalizations produced by 66 chimpanzees in 10 different positive and negative contexts at high, medium or low arousal levels. In experiment 1, listeners (n = 310), categorized the vocalizations in a forced-choice task with 10 response options, and rated arousal and valence. In experiment 2, participants (n = 3120) matched vocalizations to production contexts using yes/no response options. The results show that listeners were accurate at matching vocalizations of most contexts in addition to inferring arousal and valence. Judgments were more accurate for negative as compared to positive vocalizations. An acoustic analysis demonstrated that, listeners made use of brightness and duration cues, and relied on noisiness in making context judgements, and pitch to infer core affect dimensions. Overall, the results suggest that human listeners can infer affective information from chimpanzee vocalizations beyond core affect, indicating phylogenetic continuity in the mapping of vocalizations to behavioural contexts.

Deer Mates: A Quantitative Study of Heterospecific Sexual Behaviors Performed by Japanese Macaques Toward Sika Deer

This is the first quantitative study of heterospecific sexual behavior between a non-human primate and a non-primate species. We observed multiple occurrences of free-ranging adolescent female Japanese macaques (Macaca fuscata) performing mounts and sexual solicitations toward sika deer (Cervus nippon) at Minoo, central Japan. Our comparative description of monkey-deer versus monkey-monkey interactions supported the "heterospecific sexual behavior" hypothesis: the mounts and demonstrative solicitations performed by adolescent female Japanese macaques toward sika deer were sexual in nature. In line with our previous research on the development of homospecific sexual behavior in immature female Japanese macaques, this study will allow us to test other hypotheses in the future, such as the "practice for homospecific sex," the "safe sex," the "homospecific sex deprivation," the "developmental by-product," and the "cultural heterospecific sex" hypotheses. Further research will be necessary to ascertain whether this group-specific sexual behavior was a short-lived fad or an incipient cultural phenomenon and may also contribute to better understanding the proximate and ultimate causes of reproductive interference.

Small but wise: Common marmosets (Callithrix jacchus) use acoustic signals as cues to avoid interactions with blonde capuchin monkeys (Sapajus flavius)

Vocalizations are often used by animals to communicate and mediate social interactions. Animals may benefit from eavesdropping on calls from other species to avoid predation and thus increase their chances of survival. Here we use both observational and experimental evidence to investigate eavesdropping and how acoustic signals may mediate interactions between two sympatric and endemic primate species (common marmosets and blonde capuchin monkeys) in a fragment of Atlantic Rainforest in Northeastern Brazil. We observed 22 natural vocal encounters between the study species, but no evident visual or physical contact over the study period. These two species seem to use the same area throughout the day, but at different times. We broadcasted alarm and long distance calls to and from both species as well as two control stimuli (i.e., forest background noise and a loud call from an Amazonian primate) in our playback experiments. Common marmosets showed anti-predator behavior (i.e., vigilance and flight) when exposed to blonde capuchin calls both naturally and experimentally. However, blonde capuchin monkeys showed no anti-predator behavior in response to common marmoset calls. Blonde capuchins uttered long distance calls and looked in the direction of the speaker following exposure to their own long distance call, whereas they fled when exposed to their own alarm calls. Both blonde capuchin monkeys and common marmosets showed fear behaviors in response to the loud call from a primate species unknown to them, and showed no apparent response to the forest background noise. Common marmoset responses to blonde capuchin calls suggests that the latter is a potential predator. Furthermore, common marmosets appear to be eavesdropping on calls from blonde capuchin monkeys to avoid potentially costly encounters with them.