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Reptile romance a field of conflict

Red-sided garter snake

A matter of time: Male red-sided garter snakes compete to copulate (Source: Christopher Friesen)


The study, published today in Proceedings of the Royal Society B, shows the idea that males and females have to co-operate during mating does not apply to some species.

Instead mating can be characterised as a form of sexual conflict, says first author Dr Christopher Friesen, of the University of Sydney's School of Biological Sciences.

His study reveals male and female red-sided garter snakes, Thamnophis sirtalis, have conflicting views about the ideal length of copulation.

Females tend to favour shorter copulations, says Friesen, as it increases their ability to choose other mates to father offspring and reduces the risk of predation.

"For males however, longer copulations can serve not only to transfer more sperm, but also to 'mate-guard' the female," he and his US colleagues write.

This evolutionary conflict has led the snakes to develop gender-specific behaviours and anatomy to help them gain the upper hand during mating, says Friesen.

To test this theory, Friesen and his colleagues manipulated genital traits in wild male and female red-sided garter snakes that were key to their control of copulation.

The team removed a small hook-like spine from the male garter snake's hemipene (snake's penis) that Friesen theorised helped stop the female from evicting him during mating.

In a second experiment, the researchers anaesthetised the females' vaginal pouch to stop them using powerful muscles to terminate copulation.

Friesen says when the male's "hook" was removed, copulation time was significantly shortened and the male's ability to deposit a copulatory plug that stops sperm leakage, among other things, was limited.

The mating was mostly terminated by the female rolling off the male, he says.

Inversely anaethetisation of the females' vaginal pouch increased copulation time as the females' ability to eject the male was reduced.

Natural phenomena

The experiments were conducted as the snakes emerged from winter hibernation in Manitoba, central Canada. The event is a tourist attraction and natural phenomena.

Friesen says some snake dens can contain up to 10,000 snakes in an area the size of a living room with the ratio strongly male biased.

In larger dens between 20 to 50 males can congregate around a newly emerged female, forming 'mating balls' in which they compete to copulate.

He says the females are cold and hungry when they emerge and cannot move quickly. In this environment mating is effectively random and the female is unable to choose larger, fitter males.

Friesen says the results show how sexual conflict plays a role in evolution.

"The evolution of the basal spine (the hook) allows males to gain more control over copulation duration, forcing females to evolve some counter trait to regain some control," he writes.

Friesen says it is in the females' interest to limit copulation duration as a shorter mating time results in a smaller copulatory plug, which also stops the female from mating with another male and acts as a spermatophore from which sperm are released.

These plugs dissolve over a couple of days, so the smaller the plug the shorter the time before the female can mate again, he says.

This becomes important in terms of mate choice, Friesen says, as the female red-garter snake moves away from the den after about three days.

"As the female moves away from the den, the plug from her first mating dissolves and she may mate again in the ... small aspen groves surrounding the den where larger males are more prevalent and successful," he says.

Tags: animal-behaviour, evolution, zoology, reptiles