We never get tired of finding out who’s cheating on who, if the headlines are any indication. Finding out how much cheating is going on in the animal kingdom was revolutionized with the introduction of DNA fingerprinting, and led to major rethinking of sexual behaviour, particularly among birds.
Go back before the introduction of DNA technology, and the general consensus was the most bird species were monogamous, at least during a breeding season. There were all kinds of good reasons that were given as to why this should be so, involving the ability of the males to support offspring and the certainty of paternity. When researchers were actually able to check relationships of eggs to parents, however, many bird species got redescribed as “socially monogamous” because chicks were routinely found that were not fathered by the male tending the nest. Not the majority of chicks, but a consistent portion of them.
At first glance, this paper on zebra finches (Taeniopygia guttata) didn’t seem to be bringing very much new to the table. Zebra finches have been studied a lot, and this paper mainly seeks to refine the value of what percent of offspring are the result of mating outside the social pair bond. There’s no hypothesis or experiment here; this is straight up descriptive natural history.
Griffith and company went out to the birds’ native habitat in New South Wales, Australia, watched their behaviour, took blood samples, and did the DNA fingerprinting to figure out if the chicks were related to the pair minding the nest. They found the number of chicks that were the result of “extra pair” matings was lower than had been previously reported, and in fact, was one of the lowest reported for bird: a little under 2%.
The finding that caught my attention, however, was that some eggs were not related to either of the birds at the nest. You may know about cuckoos and cowbirds, which lay their eggs in other species’ nests (brood parasitism). It turns out that zebra finches do much the same thing to other zebra finches, like the classic cartoon image of the baby left on the doorstep. This form of reproductive “cheating” (quasiparasitism) was more common than the extra pair matings, accounting for about 5% of the chicks. The introduction and discussion makes almost no real references to previous papers or theories about quasiparasitism, which is a trifle irritating for me.
I am not sure this paper adds very much new to our understanding of extra pair mating in birds generally. Still, I was pleased that I got to learn something that I either had never known or forgotten about. There are plenty of interesting questions bubbling to mind about that. What conditions make a female more likely to lay her eggs in another bird’s nest? What behaviours to birds take to prevent being exploited? How does it vary across species?
I guess I have a little reading to do.
Griffith, Simon C., Holleley, Clare E., Mariette, Mylene M., Pryke, Sarah R., & Svedin, Nina (2010). Low level of extrapair parentage in wild zebra finches Animal Behaviour : 10.1016/j.anbehav.2009.11.031
Photo by user Jörg Dickmann on Flickr, used under a Creative Contents license.