Courting male garter snakes (Thamnophis sirtalis parietalis) use multiple cues to identify potential mates

Mating aggregations of red-sided garter snakes (Thamnophis sirtalis parietalis) in Manitoba provide a unique opportunity to identify the cues that attract a male snake and induce him to court. The snakes are abundant, tolerate human presence, and males direct courtship to a subset of other males ("she-males") as well as to females. Previous work has emphasised the role of pheromonal cues (skin lipid profiles) as sexual attractants in snakes. However, pheromones are so widely distributed throughout the den area that these chemical cues may be difficult for males to use to localise females. Our field studies show that males and females differ in several other attributes such as size (females are larger than males), body temperature (very cold snakes are often females), muddiness (females are often covered in mud) and whether or not they are solitary (most females are surrounded by courting males). Experiments show that males use all of these cues to identify possible sexual partners. Visual and thermal cues are particularly important for solitary mate-searching males; but after a mating ball forms around a female, pheromones may be most important. Our study also reveals substantial variation among individuals: females vary in attractiveness, and males vary in their intensity of courtship as well as in the magnitude of their preference for one potential partner versus another. Thus, a male snake's "decision" whom to court depends not only on visual and thermal as well as chemical cues, but also on the male's own preferences and on subtle differences among potential sexual "targets".