Social contact synchronizes free-running activity rhythms of diurnal palm squirrels.

Social contact with conspecifics entrains rhythms of a number of species, although convincing demonstrations of the phenomenon in diurnal mammals are limited. The present study examined the question of whether social contact mutually synchronizes free-running locomotor activity rhythms of the diurnal Indian palm squirrel, Funambulus pennanti. Twelve male squirrels were housed individually, without visual contact, in two separate laboratories (six in each laboratory). The squirrels were initially held under opposing light-dark (LD) schedules (with an 11 h phase difference), and were then placed under constant bright light (LL). Squirrels from separate laboratories were paired together, and each pair was placed into a fresh cage on the day of the pairing. After 48 days of social contact, the squirrel pairs were separated, and returned to their original positions in the two laboratories in fresh cages. Free-running phase and period were assessed prior to and after the social contact for each squirrel. The phase difference in the free-running rhythms of pairs of squirrels was significantly decreased following social contact. Actogram records revealed strong evidence of social synchronization of free-running rhythms in four of the six pairs. For the remaining two pairs, the data were ambiguous. This study confirmed the findings in other species, that social cues are a potent zeitgeber for F. pennanti.