There's A Sexual Arms Race Taking Place Between Garter Snakes

In Canada garter snakes form large mating balls as males crawl over each other to mate with females (head raised, top center), who are keen for lots of mating, as long as it is brief. Christopher Friesen

Canadian red-sided garter snakes have developed remarkable genital armory to try to gain an advantage over their mates. A series of studies has revealed these reptiles can rival humans for kink, with the latest research revealing just how much evolution deems fair when love and war collide. Understanding sex under such extreme conditions may help us grasp how it works for species where the process is less obvious.

Thamnophis sirtalis infernalis spend the bitter Canadian winter hibernating in limestone caves. When they awake they engage in the world's largest snake orgy, where competition amongst males is fierce and females mate multiple times in quick succession.

Research on sexual conflict tends to explore the way males evolve advantages over rivals, and the basis on which females choose mates. However, Dr Christopher Friesen of the University of Sydney has found that in garter snakes the female does far more than just choose the male she wants and fend off the rest.

Instead, the females mate with lots of males while the going is good. Males have an interest in ensuring that it is their sperm that fathers the next generation, producing plugs that block the female's cloacas (multipurpose genital openings) to prevent any subsequent males from inseminating a female he got to first. Earlier this year Friesen reported the effort males put into this, devoting up to 18 per cent of their daily energy expenditure in producing these plugs.

Females meanwhile try to ensure that they get semen from as many males as possible, keeping mating with each male as short as possible so he lacks the time to deposit a large plug. Friesen told IFLScience it is not clear if there is a process for selecting the preferred sperm once it is inside her, perhaps through adjustment of acidity in the genital tract, or if the female simply takes a “may the best sperm win” approach.

Last year Friesen undertook the thankless task of manipulating the genitals of both male and female snakes. He revealed that the males have a basal spine that hooks into the female to keep a grip on her, allowing him to extend mating after she was done with him. And you thought Fifty Shades of Grey was rough.

The females have apparently responded by developing an extremely muscular cloaca whose contractions can get rid of males that have overstayed their welcome, while possibly also squeezing plugs out. Females also body roll to shake off unwanted suitors.

Friesen (for science!) removed the spines of the males and anesthetized the female cloacas to see what happens when each sex loses its primary weapon. As expected, plugs were smaller when the males were trimmed, but the effect of anesthetizing the females was more complex, with sex lasting longer, but plugs not decreasing in size.

At the Behavior 2015 conference, Friesen reported on combining these actions, telling IFLScience he mated “manipulated females with manipulated males”. His latest work confirmed the existence of an arms race between the sexes, that the females' capacity to control the length of mating has evolved along with the males' genitals so that neither can entirely determine whose genes get propagated.

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