Individual physiological state is influenced by the physiological state of conspecifics within red squirrel acoustic neighbourhoods

There is mounting evidence from many species that physiological state can be reflected in external cues, such as the structure of vocalizations. North American red squirrels ( Tamiasciurus hudsonicus (Erxleben, 1777)) produce individually distinct territorial vocalizations, called rattles, containing information relating to both a proximate stressor and reproductive condition. Given the importance of these vocalizations in communicating territory ownership, and their ability to contain cues relating to physiological state, we hypothesized that information regarding individual circulating glucocorticoid concentrations is transmitted between conspecifics within an audible range. We analysed fecal cortisol metabolite (FCM) concentrations from 129 red squirrels, comparing focal individual FCMs to conspecific FCMs within an acoustic neighbourhood. We found support for our hypothesis through a significant positive relationship between the FCM concentrations of focal squirrels and their neighbours independent of local density. These findings demonstrate that individual physiology interacts with conspecific physiology at ecologically relevant spatial scales and suggest that the social relationships between individual red squirrels may play a larger role in dictating individual responses to the environment.