Short photoperiods increase ultrasonic vocalization rates among male Syrian hamsters

Two experiments investigated the effects of daylength on the emission of 35 kHz ultrasonic (US) calls among male hamsters. In Experiment 1, castrated males received Silastic implants subcutaneously that contained either low doses of testosterone in oil or oil alone; US calls were recorded when these males were paired with receptive females. Males exposed to eight hours of light per day (short photoperiod) called more often than males exposed to fourteen hours of light per day (long photoperiod). This was true whether or not they received testosterone. In Experiment 2, a similar testing and photoperiod exposure paradigm was used, but the subjects were gonadally intact. Among males exposed to short photoperiods, US call rates increased while endogenous testosterone levels decreased. In contrast, hamsters exposed to long photoperiods maintained stable calling rates and testosterone levels. These findings are related to recent studies concerning the neural mechanisms that regulate ultrasonic vocalizations and to the possible role of photoperiod in modulating conspecific aggression.