Sex Differences in Patterns of Association Among Indian Ocean Bottlenose Dolphins

Abstract

Patterns of association among bottlenose dolphins resident in Shark Bay, Western Australia were analyzed using party membership data. Parties contained an average of 4.8 individuals, but party size and composition were unstable. While these temporary parties often contained both males and females, long term consistent associations generally were between members of the same sex. The highest association coefficients, resulting from very frequent co-occurrence within parties were between males and between mothers and offspring. Males formed subgroups of two or three individuals who consistently associated with each other, and these were stable over periods of at least seven years in some cases. Male subgroups preferentially associated with particular other male subgroups. Females associated most consistently with other females, although not to the same extent as some males. Female associations were better described as a network rather than discrete subgroups. Male-female associations were generally inconsistent and depended in part on female reproductive state. Mothers and their offspring associated very consistently for at least 4 years.

Social networks reveal cultural behaviour in tool-using dolphins

“…Using all individuals in our main study area with > 11 sightings (n = 421), we examined sex differences for five basic social network metrics ( Table 1). As expected, based on male alliance structure 8,25,31 , permutation tests revealed that males had higher strength, eigenvector centrality and clustering coefficient than females. Given these robust sex differences and the sex difference in sponging, we separated males and females for some subsequent analyses.…”

“…We do not argue that foraging type is the primary determinant of association. Dolphin association is likely to be driven by multiple factors, including a suite of contexts such as behavioural state 42 , foraging type, the history of interactions, reproductive state, and enduring traits such as sex 25,26,31 , kinship 30,43 , age 44 and geography 30 . Homophily on a variety of traits has a critical role in human (sub)cultures 22 and this may be true for dolphin society as well.…”

“…Under certain conditions, selection is expected to favor social learning mechanisms and biases; the resulting behaviours are then favoured by socio-ecological and other processes, thereby reinforcing the evolution of social learning. In the dynamic, inherently flexible fission-fusion social system that Shark Bay bottlenose dolphins live in, individuals vary on a wide range of features: sociability, network structure, alliance formation, life history traits, habitat use and foraging tactics 10,11,14,21,25,26,29,31,36 . Social tolerance, at least between females 38 , and low travel costs 47 allow dolphins from different cliques and subcommunities to associate with each other, including those with distinct attributes, histories and lifestyles.…”

“…We refer to our hypothesis, that spongers preferentially affiliate with other spongers as the 'homophily hypothesis. ' In Shark Bay, bottlenose dolphin females form relatively loose kin-based networks 25,26 , males form tight long-term alliances, and there is no evidence of emigration 27 . Diverse foraging tactics characterize the population 14 and most are female biased 10,28 .…”

“…Males cannot afford to compromise their fitness by specializing in foraging tactics that would restrict their ability to form and maintain alliances, or range widely to find females. To date, at least three attributes, matrilineal kinship, sex and geographic proximity, influence dolphin association 25,26,30,31 , but affiliation based on foraging type has never been tested.…”