Nature

Males Can Manipulate Sperm Quantity Depending on Competition and on Female's Quality and Novelty

At least in the case of roosters

By Stefan Anitei, Science Editor

17th of May 2007, 12:52 GMT

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A male's mission is to mate with as many females he possibly can and as often as he can, n order to spread his genes as much as possible.

But it doesn't stop here.

If a male mates only with one female, his offspring won't have much genetic variation.

But roosters come with another method: they can control the quantity of sperm they waste during a sexual encounter, depending on two social factors: competition with other males for the same hen and if it has already mated or not with a particular hen. "Roosters distribute their sperm resources strategically according to several factors, including the reproductive value of the female," said Tommaso Pizzari of the University of Leeds, the United Kingdom, whose team investigated feral chickens at Tovetorp research station (Sweden).

An important sexual signal for hens is the size of her comb: the larger the comb, the bigger and more numerous the eggs. When more males mate with the same

female, only hazard chooses which one fathers the offspring.

The team discovered that when a dominant rooster experiences competition from other males, it will increase progressively the semen amount at each ejaculation. By increasing the number of sperm, the dominant male also increases its probability of fathering. That's why the subordinate roosters ejaculate less sperm if several dominant males compete with them for the same hen, saving their sperm for better occasions, as sperm production consumes energy.

But semen count also revealed that roosters also decrease progressively their amount of sperm when they repeatedly mate with the same female. When a new female is placed in the group, the rooster's sex drive increases but at the same time it allocates much more sperm when it copulates with the new mate.

The researchers are planning to track down the genes involved in the regulation of the semen amount.

But hens too were found to be very active in determining the paternity of their offspring. Hens are too weak to reject a rooster's sexual advance, but the same team found they have the ability of squirting semen back on a rooster after copulating, which lasts about a second. Still, the hens did not eject the sperm of the dominant roosters, selecting the high quality genes for their offspring.

Observations also revealed that females got increasingly bored by mating with the same rooster.

Another chapter in the battle of the sexes ...

TAGS:

sperm | semen | mate

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