Social cohesion and foraging decrease with group size in fallow deer (Dama dama)

We studied the impact of group size on foraging behaviour and level of movement synchronisation amongfemale herdmates of a fallow deer population in Central Italy. Both proportion of foraging events and movementsynchronisation decreased with increasing group size. The proportion of foraging events was higher for animals on theedge of the group than for deer in the centre of the group; hence, there appears to be a trade-off between protectionagainst predators and foraging interference, both of which decrease from the centre to the periphery of the group. Thisis the first time this type of behaviour has been recorded for wild ungulates. As expected, we also found that the movement of peripheral animals was less synchronised than that of central animals. Consequently, peripheral animals may lose contact with their herdmates and split off the group.We conclude that social inequalities may lead to conflicting requirements among group members and instabilityof large groups. Movement synchronisation (as a function of group size) appears to interact with habitat opennessto produce variations of group size (which appear to be adaptive for individuals) as an emergent property of theseaggregations.