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This image shows a male, left, and a female peacock. Differences between the sexes in a host species can pose very different challenges and opportunities to their parasites, causing them to adapt to one host sex.

Researchers have observed that certain disease-causing parasites tend to favor one sex over the other in a host species and have suggested that differences in immune responses or behavior between the sexes is the main reason why.

A Cornell postdoctoral researcher proposes that such differences as morphology, physiology, behavior, diet and life history traits between the sexes in the host species can, in fact, pose very different challenges and opportunities to their parasites, causing them to adapt to one host sex. Sex-specific adaptations in parasites may also occur when parasites routinely encounter one host sex more frequently than another. Parasites adapted to male or female hosts may help explain sex differences in parasite prevalence and disease expression.

The "ideas" paper, which proposes the hypothesis, is published in the Feb. 28 issue of the Public Library of Science Biology.

David Duneau, the lead author and a postdoctoral researcher in entomology at Cornell, wrote the paper while he was a graduate student at the University of Basel, Switzerland, and is now conducting experiments to test his ideas in the lab of Brian Lazzaro, a Cornell associate professor of entomology. Zoologist Dieter Ebert, professor at University of Basel, is a co-author of the paper; its work was supported by the Swiss National Science Foundation.

"Our hypothesis may help explain the widespread phenomenon of host sex-biased parasitism and disease expression," said Duneau. "We suggest a new perspective on host-parasite interactions, taking parasite evolution into account."

The paper outlines different scenarios in parasite evolution that can lead to sex-specific disease. These include "sex-specific adaptations" with subpopulations of the parasites either adapted to females or males, "single sex specialization" with the parasite specifically adapted to one host sex, and "plastic sex-specific disease expression" with the parasite adapted to respond one way to male hosts and another way to female hosts.

Further research may help explain questions from many varied fields, such as why effects of vaccines can be sex-specific; how parasites among hosts and at large are distributed; and why parasites can be locally adapted to certain host sexes.