Do mother birds play God?
Imagine that you’re considering having children. Upon visiting a genetic counselor, you discover that you and your partner both carry the same rare, recessive genetic mutation. While neither you nor your partner shows any symptoms, there is a 25% chance that your child will suffer from a debilitating genetic disorder. Then imagine that the counselor tells you that new embryo-selection technology can ensure that you’ll have a healthy baby; with early-stage genetic testing, doctors can pick a candidate embryo with the right genes and discard the others. This day is probably closer than you realize, and the ethical issues surrounding such technology will no doubt be contentious; some will argue that we shouldn’t “play God” with our reproduction.
But there is a bird in Australia that does just that.
The Gouldian Finch (Erythrura gouldiae) is an almost comically beautiful bird of open woodlands in northern Australia, but ironically, it is perhaps better known as the bird featured in the ViewSonic logo — a fitting emblem for a company that sells LCD displays. As if its rainbow-colored body plumage weren’t spectacular enough, the Gouldian Finch comes in three head-color morphs: black, red, and yellow. These morphs occur side by side in natural populations, although the yellow-headed morph is quite rare. Dr. Sarah Pryke, a behavioral ecologist at Macquarie University in Sydney, studies Gouldian Finches, focusing on how the common red- and black-headed morphs coexist in nature.
Gouldian Finches don’t mate indiscriminately with respect to head color. Instead, red-headed females tend to pair with red-headed males, and likewise for black-headed individuals. Biologists call this pattern assortative mating. But what does a Gouldian Finch gain from being choosy about its mate’s head color?
As it turns out, it gains quite a lot. In a paper published in the journal Evolution last year, Pryke showed that offspring of mixed pairs (e.g. a red-headed female paired with a black-headed male, or vice versa) were severely compromised compared to offspring whose parents both belonged to the same head-color morph. Mixed-morph eggs were less likely to hatch and, once hatched, mixed-morph chicks were more likely to die in the first 140 days of life. Among mixed-morph offspring, female embryos and chicks were especially likely to perish. To understand why, you need to know a little bit about bird genetics.
In humans, females possess two X chromosomes, while males have an X and a Y. This makes males the heterogametic sex – “heterogametic” because they have two different sex chromosomes. In birds, however, the situation is reversed; males are ZZ and females ZW. Back in 1922, evolutionary biologist J. B. S. Haldane showed that when a genetic incompatibility existed between parents (e.g., the male and female belonged to different species, or to different morphs within a species), heterogametic offspring typically suffered the greatest viability or fertility disadvantage. This trend became known as Haldane’s Rule, and although its genetic mechanisms are still debated, Haldane’s Rule holds true in most animals.
A female Gouldian Finch, therefore, should pair with a male of her own color morph if she can. But if she can’t find a male with her head color, she may be forced to pair with a less desirable male — in fact, as many as 30% of wild Gouldian Finch pairs are mixed-morph pairs. In theory, one way a female finch could make the best of this unfortunate situation is to produce mostly male chicks, since male offspring of mixed-morph pairs are far more likely to survive than female offspring. But can a female Gouldian Finch control the sex ratio of her brood?
Pryke tested this idea in an experiment published last year in the journal Science. She paired females with red- and black-headed males, and examined the resulting offspring. Amazingly, females paired with mates of a different head-color morph produced significantly male-biased broods — 82% male, on average! Does this prove that females can manipulate the sex ratio of their offspring? Not quite… Male-biased broods could result from female embryos dying early in development, and this wouldn’t constitute sex ratio manipulation on the female’s part.
To resolve this uncertainty, Pryke painted the heads of red-headed males black, and paired these phony black-headed males with red- and black-headed females. In this situation, black-headed females produced broods with an unbiased sex ratio, despite their genetic incompatibility with their mates. And red-headed females, mating with compatible males painted to look like incompatible males, produced more sons than daughters. Sure enough, in an effort to maximize their reproductive success, females were actively manipulating the sex ratio of their brood. And they were doing so based on nothing but the appearance of their mates!
The physiological mechanism by which females accomplish this feat is not yet known; this is one of many unanswered questions about these remarkable birds. But our opportunities to answer these questions are dwindling. Gouldian Finches once occurred throughout northern Australia, but they have declined dramatically in the last half-century, mostly because of large-scale habitat alteration by humans. Now only about 2,500 Gouldian finches remain in the wild. Dr. Pryke’s experiments often involve captive birds, minimizing impacts on remaining wild populations.
Studying the reproductive biology of an Australian finch might seem an esoteric pursuit. But in evolutionary biology, lessons learned from one species can be applied to others. And if the Gouldian Finch is any indication… Well, maybe “playing God” isn’t so unnatural after all.
Update 12/14/10: This post was selected as one of two winners of the NESCent contest! You can find all the entries (there were many good ones) here. I’m excited to be attending ScienceOnline2011 in January, and I’ll definitely be blogging about it. Thanks for your continued support of my blog; I’ll keep writing as long as people are reading it!
Pryke, S. R. and S. C. Griffith. 2009. Postzygotic genetic incompatibility between sympatric color morphs. Evolution 63(3):793-798.
Pryke, S. R. and S. C. Griffith. 2009. Genetic incompatibility drives sex allocation and maternal investment in a polymorphic finch. Science 323(5921):1605-1607.
N.B. Please do not interpret this post as an endorsement of eugenics. It is not.