Straying voles:
There has been some interesting work on voles. (Gene Robinson, Dissecting Diversity in the Social Brain, Science vol. 350 no. 6266 December 11, 2015 page 1310 and Mariam Okhovat et al., Sexual Fidelity Trade-offs Promote Regulatory Variation in the Prairie Vole Brain page 1371 in the same issue)
Sources are cited that indicate that less that 5% of mammals are monogamous. I suppose one might be forgiven for assuming that promiscuity is the natural state and monogamy develops rarely, but I take the opposites tack. Monogamy is probably the default setting with promiscuity a superimposed behavior. That is not just because the risks of exogamy are greater than the risk of inbreeding. Inbreeding can be corrected in one generation while outbreeding depression must take longer if it is reversible at all. Also the short term advantage of promiscuity for a male (positive selection because of more offspring) is so obvious that the selective advantage of monogamy is little known.
In the voles studies, pair bonding is just about universal, but offspring outside of the bond is common, about 25%. Also any male will tend to be aggressive toward – I wish I could say will beat the snot out of – any male discovered intruding on his territory.
We all know the deathless rhyme:
Hogamous, higamous,
Men are polygamous;
Higamous, hogamous,
Women monogamous.
This is so drilled (not in that form) into our minds that I have within not so many years seen it seriously recorded that men are more promiscuous than women, on average. Ignoring homosexuality, a taboo subject back then, and assuming equal numbers of males and females, unions between unmarried couples, whether they are married to someone else or not, must be exactly the same.
It seems that slandering voles is a bit safer than slandering ones wife. Anyway, female voles tend to be receptive to the advances of any old male that comes along. Fidelity is up to the male.
And the males come in two flavors. Some are quite faithful and some less so. This appears to have a genetic basis of such a nature as to suggest a balanced polymorphism: if everybody else is promiscuous, stay at home and fight for your mate; if everybody else is holed up being the jealous guard, step out a bit and hope for the best.
Most interestingly, it turns out that those males that sire extra-bond offspring have a poor sense of direction. They’re probably just wandering around lost and getting waylaid by amorous females.
Now vole populations vary widely in the course of a year between something like 25 up to 600 voles per hectare (a kind of metric acre that amounts to 2.471 real acres). We knew about those population swings from the European counts, and of course the study I myself published shows them in frit flies and in a computer model using only post-zygotic infertility. So the question arises as to whether fidelity responds to population density. Since it is genetically caused, this seems to call for evolution going lickety-split.
Robinson, but not Okhovat, suggests that in crowded circumstances there will be more males acting promiscuously. That would work; it’s a way of getting the population under control that is in addition to pure kinship considerations. It would also, if it is a common mammalian trait, tend to explain the enthusiasm of urban human populations to pursue outbreeding.
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