Now, this is truly important applied science, in PLoS.
While many studies have investigated the occurrence of extra-pair paternity in wild populations of birds, we still know surprisingly little about whether individual females differ intrinsically in their principal readiness to copulate, and to what extent this readiness is affected by male attractiveness.
Oh, it?s about birds ? well, it?s still interesting.
For decades, and even to this day, much of the discussion of behavioral biology of mating systems has focused on the truism that males are selected to be promiscuous while females are selected to invest in offspring and be relatively non-promiscuous. This concept thrives right alongside the countervailing idea that genetic diversity and variation was highly adaptive (thus, sexual reproduction evolved and is maintained as a trait to generate and enhance this diversity). Should it ever have been surprising that in case after case, we find that monogamous birds are not so monogamous? A significant percentage of baby birds in a given nest are fathered by a male other than the male in that nest?s pair. Similar findings, but with less scientific certainty, pertain to monogamous human populations as well.
Clearly, while males may be seeking diverse mating opportunities in order to get away with having other males invest in their offspring, females are busy seeking diverse mating opportunities in order to get more genetic combinations among the products of their own reproductive efforts. So, if this is an important behavioral trait, under selection, it would be important to know about variation in this trait. The study published in PLoS concludes:
Intrinsic variation in female readiness to copulate as well as variation in the attractiveness of the extra-pair male but not the social partner decided the outcome of extra-pair encounters.
So, if you are a male in a pair, you have very little choice about what happens. In birds.
Here is the key graph, or part of it anyway, that makes the point:
The left side shows female preference, or lack there of, among males of greater or lesser attractiveness when the females are not par of a pair. In other words, this is bird casual sex. On the right, we see what happens with females who are in a pair, with respect to their choosiness regarding extra-pair males. Females are more choosy about these males.
One obvious explanation for this is the cost of extra-pair matings. Females always suffer greater costs for bad reproductive decisions than do males because of differential investment levels, but in-pair individuals (males or females, but probably more so for females) suffer the additional costs of abandonment or harassment of some kind.
So if you are going to have an extra pair copulation, it better be worth it.