The theory of sexual selection, first proposed by Charles Darwin, is often used to understand why male birds have brighter colors or more elaborate adornments than females. These attributes would normally make the males more conspicuous to predators, or less agile when moving around, and could be seen as disadvantages in terms of survival. Darwin proposed that males carrying these “disadvantages” were more often selected as mates by females, who thus drove the selection for these features.
In a new study, researchers used a genetic analysis of male yellowthroats to investigate the link between conspicuous coloration and genes for enhanced survival, such as for robust immunity or stress resistance. This type of investigation has only become possible in the past five years, due to advances in DNA analysis technology.
Yellowthroats are small songbirds that have olive backs, wings and tails but bright yellow throats. In addition to having bold yellow patches on their chests, males also have a very noticeable black mask that runs from the sides of the face, across the eyes and up to the forehead. These features stand out, visually, and certainly make male yellowthroats very conspicuous. The aim of the study was to find out whether the size of these adornments signals the presence of male genes that are superior in terms of survival.
The research involved a comparative study of two different populations of yellowthroats. Peter Dunn and Linda Whittingham at the University of Wisconsin, Milwaukee, tracked female preferences for male ornaments in Wisconsin, while Corey Freeman-Gallant at Skidmore College studied preferences in upstate New York. Interestingly, the females in these two populations prefer different male ornaments.
“We found that the particular ornaments that females tended to prefer in each of our locations didn’t match,” said Dunn, distinguished professor of biological sciences, “even though both characteristics are found among males in both areas.”
Females in the Wisconsin population preferentially selected males with a large black facial mask, while females in the New York population chose males with large yellow “bibs.”
To investigate the relationship between these sexually selected ornaments and genes that influence male fitness, the researchers analyzed the genetic basis of the ornaments in birds with larger-sized features. They found that, despite being produced by different pigments in different parts of the body, the size of the ornament preferred by females in each population was linked to numerous genes that influence beneficial survival traits.
This finding indicates that a large, yellow chest patch or a large black mask present in a male actually signals the presence of other genes for enhanced fitness in that male. A female choosing to mate with a male having a large ornament of this type would thus be choosing superior genes for her offspring.
There have been very few studies that examined the female appeal of male ornaments in different populations. Dunn refers to studies of swallows that indicate females in some locations prefer longer tails, while in other locations they preferentially select males with browner bellies. And even fewer studies have investigated the size of ornaments at the level of genes.
“With this study, we not only found genes related to ornaments, but we also showed that similar genes can be linked to different types of ornaments in different populations,” Dunn said. “It brings us much closer to understanding the function of ornaments.”
The findings don’t explain why the females have different preferences geographically, said Dunn, but they have implications for evolution. The availability of more than one ornament as a mating signal allows females to potentially respond to a different choice if their environment changes.
Although theory predicts that flashy or large ornaments in males are present because they are selected for by females, this may not always be true. Ornaments may also be present because they serve some other purpose, for example antlers and horns may be used in conflicts between males. For this reason, it was necessary to prove the link between female selection of features and the presence of genes for enhanced fitness.
“What we consider flashy characteristics may not be limited to mate choice. For example, they may be used in interactions between two males,” explained Dunn.
In this study, the presence of large ornaments was related to the presence of superior genes and therefore an enhanced chance of survival in offspring. Even though females from the two populations selected different ornaments in males, both of these ornaments were linked to the same superior genes. By choosing males with large ornaments, the females were thus also choosing the best possible genetic input for their offspring.
Nicholas Sly, a postdoctoral researcher at UW-Milwaukee, was first author on the research paper.
The study is published today in the journal Proceedings of the National Academy of Sciences.