To go for a swim or not? Consequences of neonatal aquatic dispersal behaviour for growth in grey seal pups

Grey seal, Halichoerus grypus, pups in the breeding colony at Froan, Norway, have a bimodal pattern of early aquatic behaviour. About 40% of the pups spend their time ashore to save energy, which can be allocated to growth or deposition of energy-rich adipose tissue. The other 60% of the pups enter the sea during suckling and the early postweaning period, and disperse to other locations within the breeding colony. Pups may swim distances up to 12 km. Neonatal aquatic dispersal behaviour may lead to increased energy expenditure for thermoregulation and swimming, and thus lead to a low rate of body mass gain during suckling and a high rate of body mass loss after weaning. Thus, we examined relationships between natal aquatic dispersal behaviour and change in body mass (ΔBM) in suckling and weaned pups. Suckling pups that had dispersed >2000 m had a significantly lower ΔBM than suckling pups that dispersed <2000 m or that did not disperse. In weaned pups, there were no effects of aquatic dispersal behaviour on ΔBM. We suggest that the bimodal natal aquatic dispersal behaviour in grey seals at the study site reflects two different strategies for postweaning survival: to stay ashore and get fat, or to take a swim and acquire diving and feeding skills.