Male Sparrows With Unfaithful Partners Feed Their Young Less Food

One of the male sparrows on Lundy, which acts like a natural laboratory. A. Sanchez-Tojar
Josh Davis 02 Jun 2016, 13:36

In most species, even those that are traditionally seen as being faithful to each other, there is often a bit of competition between the two sexes around whether or not one can sneak off and have a cheeky bit on the side. Well, it seems that male sparrows have an unusually brutal way of dealing with an unfaithful mate, simply by providing less food for their brood.

Researchers had previously noticed that lazy males tended to partner with unfaithful females, but were unable to ascertain whether or not they did this naturally, or whether the males somehow knew that their partners had had an affair. New research, however, has found that the males will alter their brood feeding behavior depending on how promiscuous his mate is, suggesting that he follows some sort of cue to figure out whether or not his partner has been cheating.

Why each sex ultimately wants to be unfaithful and mate with other individuals boils down to separate evolutionary strategies. For the male birds, which produce lots of sperm and thus lots of potential offspring, it is as simple as the more females he mates with, the more chances that they’ll have his chicks and continue his line. For the females, however, it is slightly different. Comparatively she produces way fewer eggs, and so needs to be more selective about who she lets father them, therefore she may seek out another male who she deems as being “fitter” than her current mate.

But this then raises the odds that some male birds will therefore be raising chicks that are not their own, wasting precious resources and energy in doing so. And so it seems that if a male suspects his mate has been doing the dirty on him, he’ll come down pretty harshly on them, and feed the resulting chicks less food. Yet how does he know when the female has been having an occasional dalliance with a more attractive male? Well, it seems that he may be keeping track of her whereabouts.

“If chicks were switched into a nest where the female was faithful, then the father at that nest kept up his hard work providing for the chicks, suggesting they have no mechanism, such as smell, to determine which chicks are theirs,” explains Dr. Julia Schroeder, lead author of the paper published in The American Naturalist. “Instead, the males may use cues from the female’s behavior during her fertile period – for example how long she spends away from the nest.”

The paper has come out of an impressive 12-year study of every single sparrow that lives on the isolated Lundy Island, in the Bristol Channel. The researchers have been following 200 males and 194 females, as they formed 313 pairs and hatched 863 broods. The island is in effect a natural laboratory, as over that period of 12 years, only four external sparrows are known to have made it from the mainland, allowing the scientists to DNA genotype every single bird and therefore build a detailed family tree of all the sparrows so they can then figure out exactly which females were the most unfaithful and which were the most loyal.

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