THE PROBLEM WITH COURTING A CYLINDRICAL OBJECT: HOW DOES AN AMOROUS MALE SNAKE DETERMINE WHICH END IS WHICH?

Abstract

[In order to achieve copulation, a male snake must align his body in the same direction as his mate. This may not be a trivial problem in an elongate limbless animal, especially when the male is much smaller than the female (and hence, his head is far from hers) and when she is simultaneously being courted by many other males (so that much of her body is not visible). This is exactly the situation experienced by red-sided gartersnakes ( Thamnophis sirtalis parietalis ) at communal dens in Manitoba. To clarify this question, we modified cues available to males by manipulating various attributes of females (direction of movement, presence of head and tail, concentration of sex pheromones, etc.). Male alignment during courtship does not depend on the visual stimulus provided by the female′s head or tail. Instead, male alignment is influenced by the female′s direction of movement, and by pheromonal stimuli from her skin. We suggest that courting male snakes obtain directional information via their characteristic 'chin-pressing′ behaviour. By pressing firmly on each scale with his chin as he passes over it, a male moving anteriorly (but not posteriorly) along the female′s body will open a small gap between that scale and the next most anterior one. This gap enables his tongue-tips to penetrate more easily towards the scale-hinge area (overlain by the more anterior scale) where sex pheromones are concentrated. Thus, the direction of scale overlap provides pheromonal as well as physical cues that permit males to align correctly for copulation., In order to achieve copulation, a male snake must align his body in the same direction as his mate. This may not be a trivial problem in an elongate limbless animal, especially when the male is much smaller than the female (and hence, his head is far from hers) and when she is simultaneously being courted by many other males (so that much of her body is not visible). This is exactly the situation experienced by red-sided gartersnakes ( Thamnophis sirtalis parietalis ) at communal dens in Manitoba. To clarify this question, we modified cues available to males by manipulating various attributes of females (direction of movement, presence of head and tail, concentration of sex pheromones, etc.). Male alignment during courtship does not depend on the visual stimulus provided by the female′s head or tail. Instead, male alignment is influenced by the female′s direction of movement, and by pheromonal stimuli from her skin. We suggest that courting male snakes obtain directional information via their characteristic 'chin-pressing′ behaviour. By pressing firmly on each scale with his chin as he passes over it, a male moving anteriorly (but not posteriorly) along the female′s body will open a small gap between that scale and the next most anterior one. This gap enables his tongue-tips to penetrate more easily towards the scale-hinge area (overlain by the more anterior scale) where sex pheromones are concentrated. Thus, the direction of scale overlap provides pheromonal as well as physical cues that permit males to align correctly for copulation.]