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Abstract
One assumption of the immunocompetence handicap hypothesis is that androgens increase the expression of secondary sex characters while concomitantly suppressing immune function. Primary sex function, such as spermatogenesis, may also benefit from immune down-regulation since spermatozoa are not recognised as self by the immune system. Males with greater parasite resistance should therefore be able to experience higher androgens levels with a resultant increase in spermatogenic investment and ejaculate quality, plus greater suppression of the immune system. We investigate this possibility in two species of Australian bat. Testis size was used as a measure of ejaculate investment and quality, and spleen size was used as an indicator of immune function. Significant negative relationships were found between spleen and testis mass in one species and a negative trend was noted in a second. When the overall effect size was calculated, the negative association across both species was statistically significant. Our results are consistent with the immunocompetence handicap hypothesis since males able to withstand greater immune system down-regulation had larger testes.