Of the importance to reconsider individual variability in infant studies: Family traits do impact turn-taking perception in 6-month-olds.

Turn-taking is a universal pattern of human conversations characterized by a fast exchange of turns between speakers and an avoidance of overlaps. Language is embedded in this conversational skill acquired well before it during infancy, through everyday interactions with caregivers. The earliness of this skill and its link with language allows us to test whether social environment shapes early language development. We therefore study turn-taking perception of 6-month-old infants by measuring their gazes during video presentation of three different conversational situations where the turn is explicitly given, normally taken or taken with an overlap. We studied 51 infants to cover several family and infant characteristics: infants' sex, presence of siblings, and family socioeconomic status (SES). We found that infants looked more at the second speaker when she overlapped the first speaker than in the other situations, but not all infants were equally sensitive. Indeed, infants from high-SES families reacted differently to the three situations, while infants from the two lower SES categories did not. Also, only singletons reacted differently by looking more at the second speaker after the overlapping and turn-giving situations, and not after the turn-taking situation. Our results emphasize the importance of early social experiences on language development.

Social factors drive vocal exchanges in red-capped mangabeys (Cercocebus torquatus).

Of all the calls made by non-human primates, the function of short-distance contact calls has largely remained to be determined. These calls are the most frequent in the repertoire and are most often exchanged between individuals in a non-random way. To our knowledge, no study has ever examined how vocal exchanges are structured in red-capped mangabeys (Cercocebus torquatus), a semi-terrestrial monkey living in the African forest with a complex semi-tolerant/semi-despotic social system. Our goal was to assess the organization of contact call exchanges in this species and their relationship with individual and social factors such as age, affinity, and hierarchy. Therefore, we observed several captive groups of red-capped mangabeys and collected data on vocal behavior, as well as grooming, agonistic behavior, and spatial proximity. We defined a vocal exchange in this species as a series of contact calls made by two or more individuals within a maximum inter-caller interval of 2 s. At the individual level, the higher the individual's hierarchical rank, the less they initiated exchanges. Furthermore, the most socially integrated individuals had a longer average response time than the less integrated ones. At the dyadic level, preferred exchange partners were individuals often observed near one other or individuals most distant in age. Also, the further apart two individuals were in the dominance hierarchy, the shorter the response time. Our results support both the social bonding hypothesis and a modulating key role of the dominance hierarchy on the social use of contact calls, which is in line with the social style of this species.

Hierarchical networks of food exchange in the black garden ant Lasius niger.

In most eusocial insects, the division of labor results in relatively few individuals foraging for the entire colony. Thus, the survival of the colony depends on its efficiency in meeting the nutritional needs of all its members. Here, we characterize the network topology of a eusocial insect to understand the role and centrality of each caste in this network during the process of food dissemination. We constructed trophallaxis networks from 34 food-exchange experiments in black garden ants (Lasius niger). We tested the influence of brood and colony size on (i) global indices at the network level (i.e., efficiency, resilience, centralization, and modularity) and (ii) individual values (i.e., degree, strength, betweenness, and the clustering coefficient). Network resilience, the ratio between global efficiency and centralization, was stable with colony size but increased in the presence of broods, presumably in response to the nutritional needs of larvae. Individual metrics highlighted the major role of foragers in food dissemination. In addition, a hierarchical clustering analysis suggested that some domestics acted as intermediaries between foragers and other domestics. Networks appeared to be hierarchical rather than random or centralized exclusively around foragers. Finally, our results suggested that networks emerging from social insect interactions can improve group performance and thus colony fitness.