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Chapter One: General Introduction
the thousands. Group sizes can also vary within population depending on the
behavioural state of the dolphins (e.g., Würsig & Würsig 1980). These variations
presumably represent adaptation to different environmental pressures, depending on
life history parameters of, and habitat use by, the species. In general, the social
structure of female mammals is thought to be more influenced by predation and prey
resources, while male social structure is more influenced by access to mates
(Wrangham & Rubenstein 1986).
In the delphinid family, other than the few populations that have been investigated in
detail, the sizes of stable social units and the social boundaries of the communities
are unknown. Furthermore, these populations that have been studied over long time
scales are those that form relatively stable and small social groups within a short
distance of the coast and whose movements do not regularly take them out of their
study areas. Therefore, one must be aware that the social patterns of these
populations may not reflect the organisation of offshore populations of the same
species (LeDuc 2002).
An accurate model of a social system is essential to improve understanding of the
ecological pressures affecting sociality (Myers 1983). Here, social system refers to
the combination of three distinct elements: social organisation, social structure and
mating system (Kappeler & van Schaik 2002). Social organisation describes the size,
sexual composition and spatio-temporal cohesion of a community. Social structure
refers to the pattern of social interactions and the resulting relationships among the
members of a community. Mating system describes social interactions related to
mating (social component) and the reproductive consequences of these interactions
(genetic component). The study of sociality in dolphins is still in its early stages
compared, for example, to research on primates (Smuts et al. 1987, Kappeler & van
Schaik 2002). However, despite limited information, previous studies have revealed a
high level of diversity and complexity in the social systems of dolphins. Interestingly,
diversity exists between species as well as within species. Many populations of
bottlenose dolphins around the world have been described as ‘fission-fusion’
societies (Connor et al. 2000b), where fission-fusion refers to social systems in which
individuals of a same population live in subgroups that frequently merge and
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