Parasites have been shown to play a role in cyclic fluctuations of red grouse populations in the U.K., as has aggressive territorlal behavior by the birds.
Now Peter Hudson and coworkers have found that these extrinsic and intrinsic factors interact in this species (Lagopus lagopus scoticus).
Using anthelmintic drugs (to remove parasites), together with testosterone implants (to boost testosterone levels), they discovered that males with more testosterone not only defended larger territories than control birds, but also became infected with higher parasite intensities.
The mechanism for increased infection is still not clear: testosterone may be immunosuppressive, and / or the males' changed behavior may expose them to more parasites.
In a November 2005 paper describing their findings, the researchers point out that interactions between parasites and aggressiveness could influence the amplitude and period of grouse population cycles.
Written By: Linzi J. Seivwright, Stephen M. Redpath, Franн_ois Mougeot, Fiona Leckie, & Peter J. Hudson
Journal: 272: 2299-2304
Journal Reference: 272: 2299-2304
Paper Id: 10.1098/rspb.2005.3233