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, François 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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François Mougeot, Stephen M. Redpath, Stuart B. Piertney, and Peter J. Hudson, "Separating behavioral and physiological mechanisms in testosterone mediated trade-offs" 166:2 August 2005.