Evolutionary biologists often study the investment males make in producing sperm by measuring the size of their testes. And with good reason: relative testis size is considered the “witness of the mating system“, with high levels of female promiscuity (and consequent sperm competition) favouring increased investment into sperm production (to outcompete rivals in the conflict over who fertilises the female’s eggs). But this is hardly the only way that the testis – and the process of spermatogenesis that occurrs within it – can be shaped by sperm competition. Lukas Schärer and I have just written a review in which we pull together and attempt to organise how we should think about the various ways in which sperm competition might affect testicular function other than via gross testis size. We argue that by more carefully considering the biology of spermatogenesis, we can better understand and predict adaptive features of testicular function influenced by sperm competition.
Ramm SA & Schärer L. 2014. The evolutionary ecology of testicular function: size isn't everything. Biological Reviews, doi: 10.1111/brv.12084