Abstract:
Sperm competition results from postcopulatory continuation of male–male competition for paternity. The level of
sperm competition is predicted to be highest in species with greater polyandry and weakest in monogamous pairs.
Sperm competition levels can be indexed using traits that reflect male investment in fertilization, particularly relative
testes mass (RTM). However, the relationship between RTM and levels of sperm competition may also be
influenced by precopulatory competition selecting for higher levels of testosterone, also produced by the testes.
To test the relationship between RTM and both pre- and postcopulatory male–male competition we compared
two bathyergid mole-rat species, the promiscuous Georychus capensis and the monogamous eusocial Fukomys
damarensis. The promiscuous species had not only larger RTM, but also a greater proportion of spermatogenic
tissue, maximizing germ cell production as well. Conversely, the eusocial species had smaller testes, but a higher
proportion of interstitial tissue (which contains the androgenic Leydig cells) and higher levels of testosterone.
Consequently, testicular traits as well as testes mass may be under selection, but these are not normally measured.
More research is required on relative investment in different testicular traits in relation to both pre- and postcopulatory
selection pressures.
