Garter snake dens in the Interlake region of Manitoba, Canada, are the scene of a mating frenzy each spring and pfrovide an opportunity for researchers to work out the mechanisms by which snake's choose their mates. The following is based upon an article published in The Journal of Experimental Biology that experimentally demonstrates that oestrogen triggers to production of the female sex pheromone to attract males to females. Males detect the phromone with theri vomeronasal organ and collect the female's molecules by tongue flicking. Now researchers at Oregon State University have decoded the secrets of chemical signalling in red-sided garter snakes (Thamnophis sirtalils parietalis). The hormone oestrogen activates sexual signaling expression in birds and other tetrapods and M. "Rocky" Parker and Robert Mason thought oestrogen may initiate the production of the sexual signaling pheromone in garter snake They reasoned that, if oestrogen was the key to their puzzle, exposing male snakes to oestrogen should make males smell like females and therefore irresistible to other males. To test the hypothesis, they collected male redsided garter snakes in Manitoba and surgically inserted oestrogen implants into the males’ body cavities in their Oregon lab. The following spring, they returned the snakes to Manitoba to test whether oestrogen had made them alluring. Placing the altered males in an outdoor arena,they found wild males courted the implanted males. But, seeing that the altered males tried to avoid their love-struck suitors, Parker and Mason devised an additional test. Starting a mating ball composed of a female surrounded by courting males, they placed an altered male near the mating ball and counted how many males lost interest in the female and directed their attention to the altered male instead. Again, wild males courted implanted males; tricked into believing they were pursuing females. Parker and Mason found the effects were reversible, the wild males were no longer fooled by altered males once their implants were removed. Oestrogen functions as an ‘on/off switch’ for female pheromone production. When they laid scent trails in a Y-maze – by rubbing male and female snakes along the arms of a maze, they were astonished to find implanted males were more attractive than small females. ‘Longer females have more babies, so it’s best to court large females’, said Parker. ‘For some reason, oestrogen made males as alluring as large females.’ To find out why, Parker and Mason collected altered males’ skin lipids and examined their pheromone composition using mass spectrometry. Garter snake sex pheromones are made up of light and heavy methyl ketones, with large females producing mostly heavier ketones. When the researchers plotted the pheromone profiles, they saw that altered males had a heavy pheromone composition, just like large females. ‘It turns out that oestrogen triggers the reproduction of the heaviest, and therefore sexiest, methyl ketones’, Parker concluded. So, in red-sided garter snakes, oestrogen triggers female pheromone production. This may offer an explanation for the puzzling existence of ‘she-males’ – wild males who naturally produce female sex pheromone but have negligible circulating oestrogen levels. ‘Exposure to oestrogen- mimicking pollutants could explain the presence of she-males’, says Parker; bad news for a species whose reproduction depends solely on chemical cues. ‘But the good news is that the changes are at least reversible’, he adds.
Parker, M. R. and Mason, R. T. (2012). How to make a sexy snake: estrogen activation of female sex pheromone in male red-sided garter snakes. Journal of Expiremental Biolology 215: 723-730.