Friday, March 6, 2009

Sand lizards find excuse for being lazy lovers

Sand lizards engage in what are called serial copulation events. A single female copulates with multiple males in a sequence: male one, male two, male three, etc. In response to selective pressure exerted by mating with more than one male, the first male will insert a copulatory plug to serve as a barrier so the next male's sperm does not fertilize his female. We see this elsewhere in Animalia - namely, in humans! (Think "flowback")

Scientists noticed something peculiar was resulting from the serial copulation events. Sand lizards yield a disproportionate number of offspring that are sired by the multiple partners with whom the female mates. We would expect that the first male sire the majority of offspring, but that was not the case. There appeared to be something going on with the sperm or reproductive tract, but what?

Mats Olsson of University of Wollongong observed the male lizards exerting variable amounts of effort when copulating with a female. The amount of effort was positively correlated with clutch size (Figure 1 and 2, above). He sought to find what determined the time investment.

He and his team studied the second male and his ejaculation tactics. The second male would approach a female and obtain information about the prior male's relationship to her. If his rival was more genetically similar to the female than the second male was to the female, the second male stands a better chance of passing on his genetic material. The male sand lizards are able to sense an MHC (major histocompatibility compex)- related odor of the copulatory plug. With this information, the second male could ascertain the relatedness of the first male and female, and would spend more time in copula with the female if the first male was more related to her than he was to her. (Figure 3, below)

If his nemesis was less related to the female than the second male, the second male would engage in an informal "quickie" and then move on.

Olsson, M., T. Madsen, B. Ujvari, and E. Wapstra. 2004. Fecundity and MHC affects ejaculation tactics and paternity bias in sand lizards. Evolution 58: 906-909.

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