Social influences on circadian behavioural rhythms in vertebrates

Although photoperiodic cycles are known to be the most powerful entraining agent of circadian rhythms in homeotherms, social factors can also play a role in synchronizing activity rhythms. Here we review social influences on circadian behavioural rhythms in vertebrates, focusing on the types of possible effects and the types of stimuli eliciting these effects. Social factors can strongly influence rhythms by inducing full entrainment, mutual synchronization or desynchronization. Such effects have been observed in social species living in a constrained environment, between mothers and offspring in altricial mammals, between sexual partners or competitors, and between social partners linked by hierarchical relationships. However, the presence of social partners sometimes induces weaker influences, such as partial entrainment or relative coordination with masking effects. A weak effect of social cues on rhythms between social and sexual partners has also been reported. The stimuli involved in these social influences were olfactory stimuli or social stress in rodents and auditory stimuli in birds and mammals. Social cues sometimes have no significant effects on rhythms, not only in solitary species, but also in social species. We discuss the technical problems with such research, in particular the difficulty of recording simultaneously the activities of several individuals. Finally, we point out that knowledge of a species' behaviour and life history may significantly improve chronobiological research. Social characteristics and different levels of organization (group, relationship or individual levels), in particular, should be considered when studying social influences on behavioural rhythms.