By Susan Milius
Taking an uncommon perspective on fashions that entice the opposite sex, European researchers have documented the first case of females in the wild decked out to advertise their good genes to picky males.
The more sexy black spots that a female barn owl sports on her breast feathers, the more disease resistance she passes to her offspring. This link between spots and good genes could explain male taste for spottier females, Alexandre Roulin, now at the University of Cambridge in England, and his colleagues report in the May 7 Proceedings of the Royal Society of London B.
“This is the first female signal of genetic quality,” Roulin says. Evolutionary theorists have long explained male fashion exuberance, such as peacock tails, as competitive advertising to win females. Evidence is building that some of the flashiness signals prime fathering quality.
“People were always focusing on the male,” Roulin fusses. “This is really like an obsession.” Now, it’s time for a female perspective, proclaim such leading researchers as Trond Amundsen of the Norwegian University of Science and Technology in Trondheim. Understanding what pressures drive female ornaments “is essential for a complete and realistic understanding of animal mating dynamics,” Amundsen argues in the April Trends in Ecology And Evolution.
Research is turning up evidence that female showiness does not come just as some side effect of male showiness but may have links to more useful qualities, Amundsen notes. For example, female pied flycatchers with brighter forehead patches suffer fewer parasite infections, and female cardinals with flashier underwings feed their offspring more often.
Female barn owls vary in spottiness, and the variation seems genetically determined, says Roulin, who spent years at the University of Bern in Switzerland monitoring European owls. Neither environmental stresses nor the birds’ health correlates with their spot differences, he and his colleagues observed. These owl spots contrast with peacock tails, which vary with health.
Successive females nesting with the same male tend to rank similarly in spottiness, and owl sons mate with females of similar spottiness to their mothers. These and other signs suggest that males shop carefully for mates, Roulin says.
To check for genetic benefits, Roulin’s research team switched hatchlings to foster nests to rule out any differences in rearing. Researchers then injected 175 young owls with sheep red blood cells, a harmless substance that kicks up an immune response. Roulin linked high spottiness in the mothers to the most intense responses in the hatchlings.
Many people have asked him how dark spots could serve as a mating advertisement for largely nocturnal animals, he chuckles. He responds that owls can check out a prospective partner in daylight, too.
Another researcher who writes about immunity and signaling, Marlene Zuk of the University of California, Riverside, says that the study by Roulin has given her a new appreciation of ornamentation. “I’ve looked at barn owls for ages, and it’s never occurred to me that female spottiness could be considered ornamentation,” Zuk says.
She laments that previous work on female ornamentation has mostly occurred in sex-role-reversed birds like jacanas, where a big bold female defends a territory and monopolizes the males within it (SN: 3/6/99, p. 149). “What’s neat about this study is that it’s your run-of-the-mill bird,” she says.
But is it? Roulin suggests that male barn owls might be picky because females leave their mates partway through rearing a brood. The first male finishes feeding the first family, while she and a second male start another nest.
