The Biological Market Theory (BMT) posits that cooperation between non-human animals can be seen as a mutually beneficial exchange of commodities similarly to what observed in human economic markets. Positive social interactions are commodities in non-human animals, and mutual exchanges fulfilling the criteria of the BMT have been shown in several species. However, the study of biological markets suffers from methodological limitations that are mainly linked to the difficulty of clearly identifying the currencies and their exchanges in the short-term. Here, we test whether bonobo females are more attractive during their maximum swelling phase, whether they exchange grooming and Genito-Genital Rubbing (GGR) on a daily level of analysis, and whether these daily exchanges fulfil the BMT criteria. Females engaged more in GGR when their sexual swelling was in the maximum phase. Moreover, they exchanged grooming and sex according to the daily “market fluctuations” associated with swelling status. Females in the minimum phase (low-value) increased their probability to engage in GGR with females in the maximum phase (high-value) by grooming them preferentially. In line with the supply/demand law, the female grooming strategy varied depending on the daily number of swollen females present: the higher the number of swollen females, the lower the individual grooming preference. As a whole, our study confirms BMT as a valid model to explain daily commodity exchanges as a function of the temporary value of traders, and underlines the importance of a day-by-day approach to unveil the presence of a biological market when the value of traders frequently changes.

Different Old World primates show conspicuous anogenital swelling, with the Maximum Swelling Phase (MSP) signaling the ovulatory phase. MSP synchronization between females has been linked to social dynamics. In bonobos, characterized by female dominance, MSP is not a fully reliable signal of fertility because it may cover anovulatory periods. We investigated whether bonobo females synchronized their MSP and whether this phenomenon was modulated by social factors. Data were collected at LaValléedesSinges (France). In the period 2009–2022, swelling cycles data were collected daily on bonobo females (N = 9). In the period 2018–2022, ethological data (aggression/affiliation/socio-sexual behaviors) were also collected. We found that: (i) females synchronized their MSP and most likely experienced MSP onset following the MSP onset in other females; (ii) synchronization increased as the years spent together by females increased; (iii) synchronization preferentially occurred between females that affiliated less; (iv) synchronization on the MSP was linked to increased female-female socio-sexual contacts, which probably favored MSP synchronization maintenance. Hence, in bonobos MSP can be modulated by social factors and its synchronization, possibly underlying autonomic contagion, might have been positively selected during evolution in relation to the benefits females obtain in terms of intra-group cohesion.

Among African great apes, play is virtually absent between adult lowland gorillas (Gorilla gorilla gorilla). Here, we report an extremely rare case of adult–adult play observed in the lowland gorilla group housed at La Vallée de Singes (France). We recorded three playful interactions between the silverback and an adult lactating female. Given the diverse causal and functional nature of play, different factors may join in promoting this behaviour. In our group, contrary to what has been shown by previous studies in wild and captive gorillas, adult females spent more time in spatial proximity with the silverback than with other females. Hence, the probability of social interaction (including play) between silverback and adult females was enhanced. Moreover, the motivation of the lactating female to play might be an effect of oxytocin, a hormone that reaches high concentration levels during lactation and that promotes social affiliation. The fact that play invitations were always performed by the female might support this hypothesis. Certainly, we cannot exclude the possibility that the play event is a group idiosyncrasy or an artefact of captivity, even though the subjects never showed abnormal behaviour. Structurally, play sessions showed a suitable degree of pattern variability and switching frequency from one pattern to another. The proportion of offensive patterns was higher in the female during play and in the male during aggression, which conforms to the role reversal play phenomenon. In conclusion, this report confirms that the absence of evidence is not the evidence of absence. It is likely that under particular physiological or socio-ecological conditions, adult–adult play may be manifested as an “unconventional” part of gorilla social behaviour.

Contagious yawning (CY)—linked to physiological synchronization and possibly emotional contagion—occurs when one individual's yawn induces yawning in others. CY was investigated over different time windows (minutes from the triggering stimulus) via naturalistic or experimental studies (using real and video yawns, respectively) with contrasting results, especially in bonobos. We verified whether in bonobos result divergences may derive from different methods. We gathered yawning data on 13 bonobos at Twycross Zoo (UK) via a naturalistic (all‐occurrences observations) and experimental approach (by showing yawn/control video stimuli). Based on literature, we used 1‐ and 3‐min windows to detect CY. Due to fission‐fusion management, individuals could form permanent or non‐permanent associations (more/less familiar subjects under naturalistic setting). Video yawn stimuli may come from group mates/stranger models (more/less familiar subjects under the experimental setting). Stimulus type and time window affected CY modulating factors but not CY detection. Familiarity and age effect on CY showed opposite trends in 3‐min trials and 1‐min observations. CY was highest in oldest, non‐permanently (rather than permanently) associated subjects in the naturalistic setting, but in the youngest subjects and with ingroup (rather than outgroup) models in trials. The age effect differences on CY might be due to decontextualized yawns and immature subject curiosity toward videos. The reversed familiarity effect suggests CY's context‐dependent function in promoting social synchronization with socially distant group mates, as failing to coordinate as a group may lead to social disruption. Complementary methods are needed to fully understand motor replication phenomena.

Intersubjectivity, which refers to the capacity to create shared value or connection between individuals, is a complex and multi-faceted phenomenon involving both cognitive and affective components. Intersubjectivity has often been lauded as one of the defining features that separates the social minds and existence of humans and non-human animals. Despite the apparently profound importance of inter-subjectivity for the socio-cognitive functioning of our species, we know surprisingly little about its evolution, nor how its evolution relates to the evolution of other related phenomena, such as empathy. In this review, we embrace the “bottom-up” perspective to consider recent theoretical and empirical advances in the fields of non-human animal cognition and emotion and what they can tell us about how complex socio-emotional capacities evolve. In particular, we focus on great ape species. Given their close phylogenetic relationship to us, great apes (the non-human, extant hominids) offer a unique lens to identify which of our capacities may be evolutionarily derived or phylogenetically shared.