Sex, War And Disease: The Effects Of Infection On Horn Size And Intra-sexual Competition In The Broad-horned Flour Beetle, Gnathocerus cornutus
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Sexual selection is widely used to explain the evolution of mating systems where most often it is manifest as the competition among males for access to females. This competition takes the form of direct male-male interactions in broad-horned flour beetle Gnathocerus cornutus and often results in exaggerated male phenotypes. Sexually selected traits are interesting because in many cases their existence appears to contradict natural selection. For example, the "handicap hypothesis" suggests that there may be a trade-off between immune response and horn size because mounting an immune response necessary for survival (natural selection) may divert resources away from growing longer horns that are required to garner mating opportunities (sexual selection). In contrast, the "good genes hypothesis" suggests that the degree of expression in secondary sexual traits is indicative of males' overall fitness, and therefore should be positively correlated with other fitness related traits such as immunity. I tested these opposing hypotheses about sexual selection using the eggs of Hymenolepis diminuta (the rat tapeworm) to infect G. cornutus larvae and then measuring the correlation between adult horn size and immune protein level. As a result of infection with H. diminuta, the beetles developed shorter horns that imposed a direct disadvantage to them in male-male competition. Growth and maintenance of secondary sexual trait in the form of beetle horns in males did not impose a trade-off in the constitutive levels of immune protein in their bodies but rather advertised their increased ability to resist the detrimental effects of parasitism.