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Wednesday, February 23, 2011

Monogamy Can Be Stressful!

Remember your high school prom?  Some of you were fortunate to secure a date right away.  If so, this article is not about you.  But you can eavesdrop if you like.

This article is about the rest of us.  The girls who watched all the popular boys ask other girls first.  And maybe even the less popular boys ask other girls first.  Don't worry, it was just as bad for many boys but the rejection was more direct and unpleasant.  We got to ask the popular girls first and be told no.  And then we got to ask the less popular girls .. and be told no.

The good news is that most of us eventually got a date.  The bad news is that many of us got a date with someone who was not our first choice.  Or second choice.  Or third choice.

And that is how it works in species with monogamous breeding systems.  Just like prom sorts the high school population into nonrandom pairs of teenagers in formal wear, so too does monogamy sort a breeding population into pair-bonded couples.

Some of these pairs are quite happy with one another -- after all they got their first picks.  Some of these pairs, however, are not so pleased with the way things turned out.  They not only did not get their first or second pick, they ended up with a partner (maybe a partner for life!) that others had already rejected. 

The matter might end there if such species were both sexually and socially monogamous.  The females picked first may indeed be both.  After all, they got the best males.  But the males picked first have incentives to be socially, but not sexually, monogamous.  After all, it may only cost a little sperm to dally with females paired with other males.  Which places those picked-over females into the rather difficult position of deciding whether to mate only with their partner or cuckold the poor fellow.  (see my earlier post about the geometry of promiscuity)

This is why monogamy can be so stressful!

Australian scientists Simon Griffith and Sarah Pryke at Macquarie University and William Buttemer at Deakin University recently took an experimental approach to asking just how stressful monogamy can be.  They compared levels of the stress hormone corticosterone in female Gouldian finches (Erythrura gouldiae) that were able to pair with the male of their preference versus females paired with a less preferred male.  Stress hormone levels were four times higher in females paired with low-quality males! 

The stakes are particularly high for female Gouldian finches because they are genetically incompatible with males of certain (non preferred) color morphs and suffer elevated mortality rates among the offspring of such pairings.  It is not entirely surprising that stressed females with less-than-ideal mates are the most likely to cheat.

Therein lies the difference between social and sexual monogamy.  I'm getting stressed out just thinking about it!

Hey readers: leave a comment! What do you think of The Birds and the Bees?

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