Getting old has its sexual privileges - at least for males

If you're a rooster, it doesn't matter if you're incredibly old - you can still make it with all the hens you want. Even if you're shooting blanks. But how is this good for the future of chickenkind?

The simple answer is that it isn't. A group of scientists studied feral chickens and found that age doesn't prevent a rooster from achieving high status - which means he gets first dibs on sex with all the hens. And that puts roosters and hens directly at odds with each other, since the rooster doing the most mating also has (due to age) the lowest probability of fertilizing eggs. Apparently this scenario is at its worst in groups of chickens dominated by hens. The scientists write:

By experimentally manipulating the sex ratio of replicate groups of nine birds, we studied the relationship between male age and male social status under different intensities of intrasexual competition, and we found a signal of senescence in male social status only under intense competition. In groups in which six males competed over access to three females (6:3), socially subordinate males were older than males of higher status . . . However, in the more relaxed competition of female-biased groups (3:6), male social status was independent of male age.

In other words, when many roosters are fighting each other, the younger and more fertile roosters get the most sexual access. Luckily for hens, though, it's typical to mate several times with different roosters - and their sperm competes with each other to get access to the hen's eggs. So even if the old guys get the first shot, their sperm may still get beaten out by a younger bird's genetic material.

Getting old has its sexual privileges - at least for males


[In the chart above: (A) Copulation propensity. (B) Probability of ejaculation. (C) Total number of sperm ejaculated. (D) Sperm swimming velocity (average path velocity, VAP). Solid lines represent predicted values from the model; dashed lines represent confidence intervals.]

According to zoologist Rebecca Dean, a co-author of the study:

In evolution there are many battlegrounds, but nothing is more important than successfully reproducing. So, for hens, being monopolised by an impotent old rooster who will cause them to lay many infertile eggs is a disaster and amounts to a declaration of war. Our study shows that this sort of sexual decline is an engine driving sexual conflict in animals.

via Current Biology (Spotted on Science Daily)