Research Scientist / Adjunct Professor Ministry of Northern Development, Mines, Natural Resources and Forestry / Trent University, Canada
In 2013, ultrasonic vocalizations (USVs; vocalizations exceeding the range of human hearing; >20kHz) were first detected in two species of North American flying squirrels (Glaucomys sabrinus and G. volans). USVs have since been discovered in a variety of other gliding taxa, including colugos and giant flying squirrels. However, the purpose of ultrasonic communication in the nocturnal gliders remains unclear. We used the species first observed to produce USVs to investigate two confirmed functions of ultrasonic calls in other species: echolocation and social communication. If echolocation (or the simpler echonavigation) is the primary function of ultrasound, we predicted that USVs should be associated with solitary exposure to novel environments and gliding behaviours. If social communication is the primary function of ultrasound, we expected USVs to be associated with proximity to and interactions with conspecifics. We used a series of experiments to test these hypotheses: we brought squirrels into captivity and exposed them to a novel enclosure both alone and with other squirrels, we released squirrels in a T-maze to see if calls were associated with decision-making, and finally, we recorded squirrels during gliding events by releasing them from a platform in their woodlot and recording calls before, during, and after gliding.
We found that ultrasonic vocalizations (USVs) were associated with social communication, with no evidence for echolocation (or echonavigation) in these nocturnal, gliding mammals. USVs were produced while squirrels were paired with conspecifics in the novel enclosure, but not when squirrels were isolated. Squirrels also produced USVs before and after the gliding platform but were silent while preparing for and actively gliding. No calls were recorded in the T-maze. The lack of evidence for ultrasonic calling while in isolation, in a novel environment, or in association with gliding in a semi-natural environment suggests that North American flying squirrels do not use ultrasonic calls in association with echolocation. Rather, the increase in production of ultrasonic calls while in proximity to conspecifics and the variation in call types while interacting with conspecifics supports our hypothesis that the function of USVs in gliding mammals is primarily social communication. The unique predators, environment, and behaviours of nocturnal gliding Mammalia all likely play a role in why ultrasound has been selected for and these factors should be further investigated.