Sexual dimorphism is a widespread phenomenon among animals (Darwin, 1871; Andersson, 1994). Differences between the sexes come in many forms, in- cluding both morphology (e.g., size, shape, coloration) and behavior (e.g., risk-taking or defensive behavior). Sexual differences in physiology also occur, with the best known examples being hormonal differences associated with reproduction. Sexual dimorphism also may take other forms, and here we report on sexual differences in the odor of musk secretions produced by two species of snakes.
We used black rat snakes (Elaphe obsoleta) and eastern garter snakes (T. sirtalis) for this experiment simply because both species were subjects of our current research. We conducted a double-blind experiment. One experimenter prepared pairs of cotton swabs with musk secretions from one male and one female of the same species. We used 10 pairs of swabs with musk from black rat snakes and 10 pairs of swabs with musk from eastern garter snakes. The snakes in each pair were selected randomly from among a group of snakes recently captured in the wild and each individual was only used once. We conducted this experiment in early May before females should have ovulated, so that we could not assess whether females were reproductive. Musk was obtained by palpating the base of the tail. Care was taken to apply the same quantity of musk (estimated visually) to each pair of swabs in a given trial. The swabs were labeled "one" or "two", with numbers assigned randomly by sex. All swabs were prepared just prior to their presentation to the observers.