Physiological resonance between mates through calls as possible evidence of empathic processes in songbirds.

Physiological resonance - where the physiological state of a subject generates the same state in a perceiver - has been proposed as a proximate mechanism facilitating pro-social behaviours. While mainly described in mammals, state matching in physiology and behaviour could be a phylogenetically shared trait among social vertebrates. Birds show complex social lives and cognitive abilities, and their monogamous pair-bond is a highly coordinated partnership, therefore we hypothesised that birds express state matching between mates. We show that calls of male zebra finches Taeniopygia guttata produced during corticosterone treatment (after oral administration of exogenous corticosterone and during visual separation from the partner) provoke both an increase in corticosterone concentrations and behavioural changes in their female partner compared to control calls (regular calls emitted by the same male during visual separation from the partner only), whereas calls produced during corticosterone treatment by unfamiliar males have no such effect. Irrespective of the caller status (mate/non-mate), calls' acoustic properties were predictive of female corticosterone concentration after playback, but the identity of mate calls was necessary to fully explain female responses. Female responses were unlikely due to a failure of the call-based mate recognition system: in a discrimination task, females perceive calls produced during corticosterone treatment as being more similar to the control calls of the same male than to control calls of other males, even after taking acoustical differences into account. These results constitute the first evidence of physiological resonance solely on acoustic cues in birds, and support the presence of empathic processes.