Nice Guys Come First If Female Sheep Get To Choose

That don't impress me much. The rams that would get the most sex if ewes could exercise free choice are not the same ones that do the most mating in reality. Image Credit: Andrea Izzotti/Shutterstock.com

Female sheep prefer to mate with males lower down the pecking order, researchers have found. Far from being attracted to the alpha status of the stronger ram, ewes will pick a subordinate ram over the king of the hill every time. Care always needs to be taken when extrapolating from animal behavior to humans, but the finding does raise some interesting questions.

Today's sheep usually have their sex lives controlled by farmers wishing to breed for specific traits, even when artificial insemination isn't used. However, when mating is left to flocks of domestic sheep, females mate with multiple rams in a fertility cycle, but far more often with dominant ones. Indeed, research has shown it is common for the most dominant males to mate so often in a breeding season they actually run out of sperm before they lack partners to mate with. A case perhaps of their eyes being bigger than their testicles.

Zoologists suspect this undermines herd fitness. Besides meaning some females don't become pregnant in a cycle, it could also lead to inbreeding, raising the question of if this was a price worth paying for the spread of the dominant males' superior genes.

Dr Agustin Orihuela of Mexico's Universidad Autónoma del Estrado de Morelos noted that when dominant males start shooting blanks females often mate more with the less dominant ones. This raised the question of what ewes would do if they were free to make their own choices, rather than having dominant males but in on potential encounters with subordinate males.

In Applied Animal Behavioral Science Orihuela and co-authors describe identifying dominant and subordinate rams and tethering one of each at opposite ends of a field. The rams had enough rope to not impede mating with females who approached them, but not enough to assert their influence over the entire field.

The author's induced estrus in 28 ewes using progestagen-impregnated sponges and let them loose in the field seven at a time, giving them a choice of spending time with either ram or in a neutral shag-free zone.

The ewes always preferred the subordinate ram, and a quarter of them avoided letting the dominant male mate with them entirely. Previous studies have produced the opposite results, but these were done with rams free to roam, raising the possibility the dominant ram was coercing females, while Orihuela gave them genuine choice.

If dominance is hereditary, having offspring by the dominant male should be the most effective way to secure grandchildren outside the trial's rare circumstances. That, however, ignores the importance of herd health. If ewes have developed a preference for subordinate rams, such that they will mate with them when the opportunity presents, it should enhance the flock's genetic diversity and therefore the survival of all. Subordinate males are also less likely to have exhausted their sperm supplies, which may make them better prospects for producing a lamb.

Alternatively, the authors speculate, dominant rams' greater aggression may be off-putting to ewes, who perhaps prefer more gentle love-making. The paper suggests this is a topic deserving future investigation.

 

H/T phys.org

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