View - ResearchSpace@Auckland - The University of Auckland

Chapter One: General Introduction
Similarly, two populations of spinner dolphins from distant islands in the Hawaiian
Archipelago (the Big Island and Midway Atoll) have been found to have different
social organisations (Norris et al. 1994, Karczmarski et al. 2005). In the remote atoll
of Midway, spinner dolphins seem to live in a smaller and more stable society, with
no obvious fission-fusion fusion. This is unlike the large groups and fission-fusion
observed in spinner dolphins at the Big Island (Karczmarski et al. 2005). The authors
suggest that these disparate characteristics were triggered by different environmental
pressures.
A common pattern in terrestrial mammals is a male-biased dispersal with female
philopatry (Greenwood 1980). Such a pattern also seems to occur in some dolphin
populations, as shown by molecular studies on bottlenose (Möller & Beheregaray
2004) and dusky dolphins (Cassens et al. 2005), and also by long-term demographic
studies (e.g., Wells 1986). However, female philopatry and male dispersal is not
always the rule in dolphins. Long-term studies on killer whales and molecular studies
on long-finned pilot whales have suggested that in some populations of these two
species, individuals live in matrilineal groups with no dispersal by either sex (Bigg et
al. 1990, Amos et al. 1993). Such an extreme scenario of philopatry has not yet been
described in any terrestrial mammals (Connor 2000). Note that killer whales offer
another example of intra-species diversity of social structure. In the eastern North
Pacific, communities of fish-eating “resident” killer whales (where no dispersal of
either sex occurs) are more stable than the communities of mammal-eating
“transient” killer whales, where individuals of both sexes might disperse from their
natal group (Baird & Whitehead 2000). The authors suggest that these dissimilar
social structures could be related to differences in foraging ecology.
Among the factors influencing group composition in social mammals, kinship is
thought to play a major role. While an individual can enhance its fitness directly by
maximising its own reproduction (Williams 1966), it can also do so indirectly by
maximising the reproduction of its relatives (Hamilton 1964a, b). Following the latter,
kin selection theory predicts that individuals should preferentially associate and
cooperate with kin whenever the inclusive benefits outweigh the costs (Hamilton
1964a, b). In the population of Tursiops sp. from Shark Bay, Krützen et al. (2003)
17