The steroid hormone testosterone regulates the expression of reproductive behaviors and sexual traits of many animal species. While high levels of testosterone are required for reproductive activities or for the expression of sexual traits, these are often costly, and can lead, for instance, to increased parasite infection. Such costs would also ensure honesty in sexual signals or behavior used by animals for assessing the healthiness of a potential mate.
The mechanisms linking elevated testosterone and increased parasite infection remains little know in wild animals. One possibility is that testosterone modifies behavior and increases exposure to infective parasite larvae. An alternative mechanism is related to the physiological actions of testosterone, which cause increased susceptibility to parasites.
In an article in the August 2005 issue of The American Naturalist, Franois Mougeot and colleagues test to determine which mechanism was most likely in a wild game bird of the UK, the red grouse. Using a new treatment, which increased testosterone concentration while blocking its direct actions (by blocking testosterone receptors), they show that the mechanism linking testosterone to parasite infection is via increased susceptibility, and that the action of testosterone on host parasite defense was most likely indirect, via cascading effects mediated by other hormones.
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