Sperm competition roles and ejaculate investment in a promiscuous mammal

Theoretical models of sperm competition predict how males should allocate sperm and seminal fluid components to ejaculates according to their mating role (dominant vs. subordinate). Here we present a detailed analysis of ejaculate expenditure according to male roles in the bank vole (Myodes glareolus). Sperm competition occurs regularly in this species and dominant males typically achieve higher fertilisation success than subordinates. Contrary to theoretical predictions, we found that dominant male bank voles invest more sperm per ejaculate than subordinates, both absolutely and relative to body and testes mass. The testes of dominant males were also absolutely (although not relatively) larger than those of subordinates. However, we found no evidence that subordinate males compensate for lower sperm numbers per ejaculate by increasing ejaculation frequency or sperm velocity. Similarly, we found no evidence for differential investment in copulatory plug size according to male roles in sperm competition, although dominant males had significantly larger seminal vesicles (both absolutely and relative to body mass) compared to subordinates. We conclude that sperm competition roles can have significant but unexpected influences on ejaculate investment in mammals with clearly defined differences in male social status.