Barbara Tiddi

Social networks in primates: smart and tolerant species have more efficient networks

Network optimality has been described in genes, proteins and human communicative networks. In the latter, optimality leads to the efficient transmission of information with a minimum number of connections. Whilst studies show that differences in centrality exist in animal networks with central individuals having higher fitness, network efficiency has never been studied in animal groups. Here we studied 78 groups of primates (24 species). We found that group size and neocortex ratio were correlated with network efficiency. Centralisation (whether several individuals are central in the group) and modularity (how a group is clustered) had opposing effects on network efficiency, showing that tolerant species have more efficient networks. Such network properties affecting individual fitness could be shaped by natural selection. Our results are in accordance with the social brain and cultural intelligence hypotheses, which suggest that the importance of network efficiency and information flow through social learning relates to cognitive abilities.

Grooming up the hierarchy: the exchange of grooming and rank-related benefits in a new world primate.

Seyfarths model assumes that female primates derive rank-related benefits from higher-ranking females in exchange for grooming. As a consequence, the model predicts females prefer high-ranking females as grooming partners and compete for the opportunity to groom them. Therefore, allogrooming is expected to be directed up the dominance hierarchy and to occur more often between females with adjacent ranks. Although data from Old World primates generally support the model, studies on the relation between grooming and dominance rank in the New World genus Cebus have found conflicting results, showing considerable variability across groups and species. In this study, we investigated the pattern of grooming in wild tufted capuchin females (Cebus apella nigritus) in Iguazú National Park, Argentina by testing both the assumption (i.e., that females gain rank-related return benefits from grooming) and predictions (i.e., that females direct grooming up the dominance hierarchy and the majority of grooming occurs between females with adjacent ranks) of Seyfarths model. Study subjects were 9 adult females belonging to a single group. Results showed that grooming was given in return for tolerance during naturally occurring feeding, a benefit that higher-ranking females can more easily grant. Female grooming was directed up the hierarchy and was given more often to partners with similar rank. These findings provide supporting evidence for both the assumption and predictions of Seyfarths model and represent, more generally, the first evidence of reciprocal behavioural interchanges driven by rank-related benefits in New World female primates.

Grooming for infant handling in tufted capuchin monkeys: a reappraisal of the primate infant market

Newborn infants can affect female social dynamics, and provide ideal conditions to test the biological market theory and its assumptions. In infant markets, infants are the desired commodity, mothers control access to them, and other females (potential handlers) trade grooming for infant handling. The supply/demand ratio corresponds to the number of available infants per potential handler. Variation in the number of infants causes changes in the supply/demand ratio that can alter the market equilibrium. We investigated whether grooming was interchanged for handling in wild tufted capuchin females, Cebus apella nigritus. Behavioural observations were conducted on 10 mothers in three groups. Potential handlers were strongly attracted to infants and grooming the mothers (specifically, its occurrence rather than its duration) increased their probability of handling infants. However, the number of infants in the group did not affect the amount of grooming needed for access to infants. At least three nonmutually exclusive hypotheses can explain differences from previous findings. First, grooming in capuchin monkeys may represent a signal of benign intent with no market value. Second, dominance relations among handlers may have prevented competition by outbidding among handlers and thus hindered free trade. Third, the increased grooming required by mothers when infants were scarcer may reflect the need to calm more stressed mothers that were the subject of more frequent harassment; since in our study mothers did not appear to be more stressed when fewer infants were available, no effect of the number of infants on grooming by handlers emerged.

Social relationships between adult females and the alpha male in wild tufted capuchin monkeys.

Primates are notable for the widespread presence of long‐term female–male associations which go beyond the mating context. However, little attention has been given to the factors that affect within‐species variation in female–male relationships, especially among New World primates. Although detailed accounts of heterosexual relationships in Cebus species are scarce, a few studies have suggested the occurrence of strong associations between adult females and high‐ranking males. This study explores affiliative relationships between females and the alpha male during the nonbreeding season in wild tufted capuchin monkeys (Cebus apella nigritus). Affiliative relationships were explored through female–male patterns of spatial proximity and grooming. By adopting a social network approach, we analyzed: (1) whether the alpha male is the preferred male partner for females and, (2) whether variation (if any) in female–alpha male affiliation can be explained through both female individual characteristics and social network metrics. Our results showed that alpha males were the favorite male partner for adult females in the proximity networks, but this did not hold true in the grooming networks. In addition, female–alpha male interaction patterns showed considerable variation, with only some females being strongly associated with the alpha male. Our results suggest that such a variation can be explained by female dominance rank, level of centrality (the quantity and intensity of spatial connection with other females) and prestige (the quantity of grooming received by other females) in female–female social networks. Taken together, these findings highlight two aspects of female–alpha male relationships in tufted capuchin monkeys: the alpha male represents the most socially integrated male in the group, and females with high dominance ranks and high centrality in both proximity and grooming networks show stronger relationships with the alpha male. Am. J. Primatol. 73:812–820, 2011.