Social thermoregulation does not explain heterospecific nesting in North American flying squirrels

Contemporary climate change has increased range overlap between northern (Glaucomys sabrinus) and southern (Glaucomys volans) flying squirrels, which has in turn led to interspecific hybridization. Mating in flying squirrels begins in late winter, at a time when squirrels tend to thermoregulate socially (i.e. they huddle in communal nests for warmth). We tested the hypothesis that the proximate cause of hybridization was a requirement for social thermoregulation that would lead squirrels at the range periphery to nest socially with heterospecifics. We conducted a captive preference experiment with pairs of same-sex, wild-caught flying squirrels to test the relative frequency of conspecific and heterospecific nesting. Contrary to our expectations, ambient temperature did not predict the willingness of squirrels to thermoregulate socially. Instead, G. volans nested as a conspecific pair in all trials, whereas G. sabrinus nested in conspecific pairs less often than G. volans. Heterospecific pairs occurred less often than conspecific pairs of either species. A low level of heterospecific social nesting occurred in all conditions and at all times of year, however, suggesting that the behaviour of social nesting is not a perfect reproductive barrier and might lead to some opportunities for hybridization.