In most birds, beauty is more than skin deep. Chicks with ugly fathers not only lose out on superior genes, but their deadbeat dads also tend to bring less food to the nest than attractive fathers do. A new study of house finches, however, shows that these apparently unlucky chicks may be better off than thought: Their mothers endow their eggs with extra hormones and vitamins, counteracting the dads' deficiencies. The findings contradict work in most other species of birds, which have described the opposite--that females devote extra resources to the offspring of good-looking males.
Female zebra finches, for example, put more growth-boosting testosterone in their eggs and feed their young ones more food if their mates have colorful plumage. Scientists think that this is one way that females increase their young ones' chances of survival and ensure that mom's genes are perpetuated through the next generation. A few studies have also hinted that the same strategy is applied in the opposite scenario, where females take extra care of their chicks when their mates are unattractive, with plumage that's less impressive. Still, the evidence for this has been scant.
Reproductive physiologist Kristen Navara of Ohio State University in Columbus and her colleagues decided to look at the phenomenon in house finches, and their results took them by surprise. Examining levels of testosterone and antioxidants (which include vitamins A and E) in fertilized eggs from 15 finch nests, they found that females with ugly mates, not attractive ones, had higher doses of testosterone and antioxidants in their eggs. In fact the levels of antioxidants were 2.5 times higher in eggs sired by unattractive males than those sired by good-looking ones.
While testosterone is known to ramp up growth rates and improve food-begging efficiency, antioxidants increase chicks' immunity and reduce the amounts of harmful molecules called free radicals, which damage tissues. House finches might use a strategy opposite to that of many other birds because the females are very short lived, surviving for just 1 to 2 years compared to up to 20 years in other birds, says Navara. "They can only make one or two breeding attempts in [a] lifetime," she says. With just a couple shots at offspring, a female house finch might find it even more important to compensate for an ugly mate. The team reports its findings in the November/December issue of Physiological and Biochemical Zoology.
The antioxidant measurements in particular make the findings compelling, says David Harper, an evolutionary biologist at the University of Sussex, U.K. The study, he adds, is the first to link antioxidant levels in eggs to the attractiveness of the father. One principal question now is how the female perceives male attractiveness and translates it to chemical concentrations in her eggs.