Grey seals breed in a wide range of littoral environments and are thus suitable for investigating the mechanisms of behavioural adaptation under environmental influences. However, most studies of grey seal behaviour, energetics and mating strategies only focus on the typical breeding colony of this species: a large (more than 1000 seals) and dense land-based group. In this study, we measured the activity budget of reproductive individuals in such a typical colony and in three poorly known colonies, considered marginal by their geographic location, their small size (less than 500 individuals) or their environmental characteristics. The variation of activity among the studied colonies appears to be significant and could be shaped by the major environmental differences observed between these colonies (mainly topographical), supporting a previously raised hypothesis (Anderson and Harwood 1985). In one of the marginal colonies studied here, grey seals spend more than 40% of their breeding time in the water while in others, they stay on land for several weeks of reproduction. This type of difference might influence individuals energy balance and mating strategy. We suggest that the understanding of the grey seal behavioural plasticity is unlikely to be complete without integrating data on marginal populations.
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