Sperm production and immune function in two Australian bats, Chalinolobus morio and Nyctophilus geoffroyi

One assumption of the immunocompetence handicap hypothesis is that androgen s increase the expression of secondary sex characters while concomitantly s uppressing immune function. Primary sex function, such as spermatogenesis, may also benefit from immune down-regulation since spermatozoa are not reco gnised as self by the immune system. Males with greater parasite resistance should therefore be able to experience higher androgens levels with a resu ltant increase in spermatogenic investment and ejaculate quality, plus grea ter suppression of the immune system. We investigate this possibility in tw o 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. Whe n the overall effect size was calculated, the negative association across b oth species was statistically significant. Our results are consistent with the immunocompetence handicap hypothesis since males able to withstand grea ter immune system down-regulation had larger testes.