Who Has Been Horsing Around?
Researchers in the Czech Republic recently distributed a questionnaire, tracking the results of 81 pregnancies, comparing the outcomes of three groups.
One group consisted of females who were living with the male who was the genetic parent. Among these 36 cases, all pregnancies were carried to term
A second group, consisting of 13 cases, consisted of females who were living near males who were not the genetic parent, but the living arrangements were such that the females and males did not have physical contact and so, of course, were not having sex. Of these females, only six carried to term. The remaining seven were aborted.
A third group, N=32, consisted of females who were not only living near males who where not the genetic father, but were sharing quarters, which afforded a certain degree of sexual contact. Of these, only 22% failed to carry to term.
Here is one explanation for this pattern of results. Suppose that males might harm, even kill, offspring who they know with certainty are not their own. For females, of course, this is a reproductive disaster, sinking the cost of bringing a fetus to term, only to have the offspring killed. One counterstrategy is to have sex with the males with whom one finds oneself living during the early stages of pregnancy. This might lead a male to believe that the offspring might be his own, reducing the chance of violence.
But now consider the females in the second group, in the position of living near males, but unable to mate with them because of the living conditions. These females are in a bind. Their offspring are going to be at risk, but the chance of “confusing” the males living nearby by having sex with them is precluded. And though the sample size is small, this is the group that shows the largest fraction of abortions, suggesting a decision to save the investment in the current fetus, set against the possibility of a later one.
Indeed, this is the argument that Bartoš et al. make, in a recent paper, published online March 26th. They make the claim that the females mating with the males in the third group “could be adaptive,” a behavior designed to confuse paternity, decreasing the risk of violence to her offspring. When this isn’t possible, they write, “Termination of pregnancy … may thus be seen as an adaptation to save energy and avoid likely future infanticidal loss of their progeny.”
Summarizing, the authors write: “We interpret our results as suggesting that where possible, a [female] manipulates the male’s paternity assessment by promiscuous mating. If she has no chance to do that, she may abort the current foetus.”
So, first of all, that’s pretty cool. Females behave in a way that anticipates the violence done to offspring living near adult males who are not the biological parent.
Now I know what you’re thinking. Here we have some modest cell sizes, claims about what is “adaptive,” and no measure of heritability or fitness. (Not to mention the fact that it was all done by survey.) I’m sure that we’ll hear howls from the blogosphere about this one for all the usual reasons. Right?
Oh. Wait. Maybe not. I think I left something out.
Did I mention that the subjects in this study were not members of Homo sapiens, but rather Equus ferus caballus, aka, horses?
Figure from Bartos et al.
Citation
Bartoš, L., Bartošová, J. Pluháček, J. & Šindelářová, J. (in press). Promiscuous behaviour disrupts pregnancy block in domestic horse mares. Behavioral Ecology & Sociobiology.
Comments (9)