The
red-sided garter snake (Thamnophis sirtalis parietalis) demonstrates an unusual reproductive behavior involving
multiple morphs of the animal’s male form. In order for either male morph to
elicit mating behavior from the female, it must display several different
behaviors of its own. Upon encountering the female, the male will press his
chin into her dorsal skin and flick its tongue rapidly. He will then undulate
his body along the female. If the female is sexually receptive, intromission
will occur. This behavior commonly occurs at a ratio of many males to one
female in a mating ball.
Female
garter snakes also play a role in sexual receptivity. Females produce a
pheromone that delivers chemical cues to the male garter snake’s vomeronasal
organ (VNO). Previous studies suggest that the male’s tongue flicking delivers
the compound to the VNO.
The
present study aimed at identifying this compound through a bioassay. Chemical
extracts were presented on paper towels to courting males and their responses
were recorded. Female pheromones were obtained and fractionated and the active
compounds were presented to the males one at a time. The pheromone was
identified as methyl keytone, a nonvolatile contact pheromone.
When
attempting to mate, normal males typically ignore other normal males because
the normal morph does not produce the same quantity of methyl keytone as the
female. The normal male also produces a chemical called squalene. An
interesting component of the red-sided garter’s pheromone system is perhaps in
the alternate male morph, referred to in the literature as the “she-male.” The
she-male is indistinguishable from the normal male morph and elicits mating
behavior from the normal male. The she-male is a male mimic of the female and
has comparable levels of methyl keytone to the female and does not produce any
squalene.
The
normal male is sexually receptive to the she-male and will attempt to mate with
the she-male morph in the same way that it does the female. When in the mating
ball, the she-male will induce some of the normal males to move away from the
female. The normal males begin their unproductive courting of the she-male and
the she-male begins courting the female.
Previous
research suggests that the she-male is more than twice as successful at
copulating with the female as the normal male. The she-male also contains about
three times as much testosterone as the normal male. We typically think
of testosterone as masculine and a characteristic that would be unattractive to
the normal male morph. The article hypothesizes that although there is a higher
level of testosterone in the she-male, this testosterone is converted by
aromatase into estrogen in the skin, resulting in mating behavior from the
normal male.
4 comments:
Very interesting, isn't it? Sneaky snakes, indeed. Cool post!
Wow! So is there any advantage to being a 'regular' male in this population?
The she-male has the chemosensory advantage but the normal male occurs at a higher frequency than the she-male in the population. So even though the she-male can trick a number of normal males away from courting the female, it still has to compete with anywhere from 10-100 other snakes in the mating ball.
I also don't know how a she-male is affected by other she-males in the mating ball either. I'm pretty sure there isn't even any data on that. Someone presented a few papers in class on this species that suggested a size dimorphism between the male and female but not the male and she-male. Apparently males prefer larger females and that might come into play when a smaller she-male is attempting to distract a number of normal males who are going after a larger female.
And there's another hypothesis that the she-male chemosensory cue is not to distract the normal male in the mating ball. Instead it is to attract the normal male to ball up around the she-male after hibernation and warm the she-male up. Then the she-male has a temperature advantage that becomes a movement advantage over the normal male so the she-male can rapidly approach the female and outrun the normal male. I don't particularly believe that one though.
Very cool (or should I say, warm)! Thanks for the additional info, Zach!
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