Harbour seal grouping as an anti-predator strategy

Terrestrial grouping by western Atlantic harbour seals, Phoca vitulina concolor, was investigated to determine whether it has a functional significance and, if so, what the function might be. Two nofunctional hypotheses were tested: (1) groups arise from a shortage of haul-out sites or space on sites; and (2) terrestrial groups are extensions of groups formed in the water for reasons not related to hauling-out. Aerial surveys of haul-out site use by seals indicated that sites were not in short supply. Observations of the use of space on one haul-out site suggested that space on sites was not limiting. Arrivals and departures of seals at one study site showed that groups formed at the haul-out site and disbanded after the haul-out period. The hypothesis that grouping functions to increase the probability of predator detection was tested by measuring individual and group vigilance relative to group size, and simulating predator attacks in the field. Although individual vigilance decreased with increasing group size, group vigilance increased. Larger groups detected ‘attacks’ at greater distances and a solitary seal failed to detect an ‘attack’. It is proposed that an individual increases its probability of detecting a predator by joining a group.