杏吧原创

When female sheep get the choice, they would rather mate with a subordinate male than a dominant one.

Dominant rams usually mate with ewes much more often than lower-ranking rams do. But they can literally run out of sperm from mating so much in a short period of time.

And if they fathered all the lambs in a flock of sheep (Ovis aries), the group would have less genetic diversity, which can lead to malformations and inheritable diseases. So ewes, which generally mate with several males when they are in heat, could have their own strategy for preventing inbreeding, says Rodolfo Ungerfeld at the University of the Republic in Uruguay.

To test this, he and his colleagues first ranked male hierarchy among eight rams by placing two of them with a ewe in heat to see which one stood closest to her for the longest time. Domestically raised sheep usually show dominance by subtle body language instead of violent fighting.

Then they put the rams in pairs of one dominant and one subordinate male and tied them up at opposite corners of a large pen. They let the ewes loose in the pen, one at a time while they were in heat, until all 28 ewes had chosen between the rams.

The researchers found that the ewes spent more time with the subordinate ram 鈥� on average it was three times longer, says Ungerfeld. Ewes also mated with the subordinate rams twice as often as the dominant ones. And a quarter of the ewes refused to approach the dominant ram at all.

The findings make sense for a species in which sperm from different males compete for the female鈥檚 eggs in the reproductive tract, says Andrew McLean at the University of Melbourne in Australia. Ewes seeking out subordinate males to mate with ensure genetic diversity in their flock.

A second possibility might simply be that ewes prefer the gentler nature of subordinate rams, says Ungerfeld. 鈥淒ominant males display many behaviours that could be interpreted as aggressive by the ewe, with more abrupt movements or activities,鈥� he says.

McLean questions whether the results may have been skewed by the rams being stressed from being tied up, especially if they weren鈥檛 used to it. Submissive rams in particular might have cried out in a way that 鈥渕ay have attracted the ewes in the first instance to enable mating鈥�, he says.

Ungerfeld acknowledges that the rams might have found the situation 鈥渁 bit stressful鈥�, but notes that these rams were reared in a controlled environment and wouldn鈥檛 have mated with the ewes if they were overly stressed. Because they could still see and smell each other, the tethers didn鈥檛 prevent the two rams from acting in a dominant or subordinate way.

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