Some birds listen, instead of look, for mates

Andrew DeWoody, a Purdue University associate professor of forestry and natural resources, found that the higher the pitch of a male bird's song, the more genetic diversity that bird has, making him a better mate for breeding. His study was published Dec. 2 in the early online edition of PLoS Biology.

"If you have a diverse set of genes, that can translate into physiology and morphology diversity as well," DeWoody said. "Animals that are heterozygous, or have genetic diversity, are often bigger, stronger or can run faster."

DeWoody and former Purdue graduate student Johel Chaves-Campos studied ocellated antbirds in the tropical forests of Central America. The antbirds survive by tracking army ants, which hunt in large swarms and are capable of killing just about anything in their paths. The birds flit ahead of the swarms and collect arthropods that flee for their lives.

"They wait at the front for the ants to flush out a grasshopper, for example," DeWoody said.

The antbirds have several calls, some to let fellow antbirds know where the army ants are heading, others to attract mates and still others that are defensive or aggressive to protect turf. DeWoody's research involved recording those calls and matching them to DNA samples of the birds. The results suggest that genetic diversity in antbirds affects their physical abilities to produce certain sounds.

"Our results are consistent with the idea that some sound frequencies are biomechanically difficult to produce. Males that are genetically diverse, and therefore expected to be in better physical condition, are able to produce sound frequencies that males with less genetic variation are unable to reach," Chaves-Campos said.

DeWoody said females can pick up on the pitch of the males' songs to decide which birds will make the best mates.

"Females may prefer to mate with males that hit the highest notes because their offspring will have more genetic diversity," DeWoody said. "Male calls could be honest indicators of their genetic diversity."

Antbirds also can use pitch information in disputes. For instance, a bird could decide not to escalate a conflict over territory if it decides the other bird is in better physical condition. The bird also could decide to intensify the conflict if it believes its opponent is weaker.

Funding for the research came from the National Science Foundation. Chaves-Campos said the next step is to show that the finding in antbirds holds true in other birds.

Yimen Araya-Ajoy, a graduate student from the University of Costa Rica, collaborated with DeWoody and Chaves-Campos on the research and was first author on the paper.