Gartersnakes / Trouser snakes (ha ha) (250) It’s not just size that matters…
One of the most noticeable things about snakes is that they don’t have any arms or legs. So it’s hard to imagine how, without a pair of limbs to prefer one of, a snake could possibly be described as being ‘right-handed’. Until, that is, you learn that males have two penises, or hemipenes, and that the right is bigger than the left.
Rick Shine from the University of Sydney and colleagues studied male red-sided gartersnakes, measuring the size of their sex organs and looking to see if larger hemipenes are used preferentially. This might be expected because large organs, connected to a bigger sperm supply, should mean a better chance of fertilising eggs, and fathering more offspring. What’s more, males deposit a ‘copulatory plug’ after mating, which blocks the female’s genitalia and prevents other males from adding competing sperm. The bigger the male’s organs, the more effective the plug.
By setting up mating experiments, the researchers could record how often each hemipenis was used. They found that, despite the potential advantages of using the bigger organ, males use their left and right hemipenes equally overall. Under warm conditions, however, males use their larger right hemipenis instead of their left. The team suggest that warmth increases muscle suppleness and manoeuvrability, making it easier for a male to align his right hemipenis with the female’s cloaca without losing his position in the scramble to mate. Males use their hemipenes alternately, though, because the store of vital secretions from each side is limited.
(Behavioural Ecology, vol.11, No.4, p411-5)
Richard Northover
Background box (100) Snake sex
Garter snakes generally mate in March or April, when they emerge from hibernation. The increase in temperature in spring triggers male sexual behaviour.
Vast snake orgies ensue, with writhing balls of males all attempting to mate with a single female. Males chose the largest females to mate with, as they are likely to produce the most offspring (in this case, live young). Females therefore try to look bigger than they really are by hyperventilating.
When one lucky male has successfully aligned his body with the female’s, she raises her tail, allowing him access to her cloaca.