There are a few steps to successful reproduction. First, you need to pick a member of the correct species. Then you have to pick the correct sex to mate with, with bonus points for picking an attractive mate. In some species, males and females are visibly different from one another, with males typically being larger or more ornamented than females. In other species, males and females are similar in appearance, and need a different way to tell each other apart. The red-sided garter snake (Thamnophis sirtalis parietalis) is such a species, where males and females appear almost identical. These snakes solve the problem of telling males and females apart through pheromones, secreted hormones that serve as chemical signals. These ‘eau de snake’ pheromones are multi-purpose, and communicate the species, the sex and even the attractiveness of an individual.
So what makes a snake smell like an attractive female? Rockwell Parker from Oregon State University, USA, and Robert Mason from Washington and Lee University, USA, decided to probe these pheromones more deeply and investigate how pheromone signals are regulated. In an initial study, the researchers learned that by injecting male snakes with estrogen, a hormone that is typically associated with females, they could induce the males to start producing female pheromones. Therefore, they knew that estrogens were important for stimulating the production of female pheromones. However, the researchers then wanted to know whether there was also a role for testosterone, a hormone that is typically associated with males. Is the absence of testosterone alone enough to activate the expression of the female pheromone?
To investigate this question, the researchers compared the pheromone production of castrated males with males that underwent a sham surgery without having their testes removed. The scientists then compared these snakes to both castrated males and intact males that received supplementary testosterone. For all of the males, they measured total pheromone production, as well as assessing the composition of the pheromones. The researchers also measured how attractive the treated male snakes were to wild male snakes, by counting how many wild males would leave a female to investigate and court the treated males.
The scientists first determined that castration reduced the total circulating testosterone compared with control males. Both castrated and intact snakes that were injected with testosterone had testosterone levels that were slightly higher than control males, but within the reasonable range of testosterone for this species. Castrated males produced more overall pheromone than intact males, and the pheromone profile was very similar to that of an attractive female. Interestingly, injecting the castrated males with testosterone cancelled this effect, and these males had a pheromone profile that was restored to that of a typical male. Finally, wild males were attracted to the castrated males, and frequently tried to court them. However, the wild males were much less interested in sham-operated males, or castrated males that were injected with supplementary testosterone.
Since castrated male snakes have no specific hormone profile, and yet produce a female pheromone, the results of this study indicate that the default signalling in red-sided garter snakes is female, and male pheromone profiles are only produced when there is active inhibition of the female signalling by testosterone. Therefore, these results suggest that pheromone signalling in snakes is regulated not by one hormone, but by an active battle of hormones.