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 s...精子竞争（sperm competition）的理论模型可预测，雄性应依据其交配角色（优势个体与从属个体），向每次射精产物中分配精子与精液组分。本研究针对棕背䶄（Myodes glareolus），系统分析了雄性不同交配角色对应的射精投入模式。该物种中精子竞争频发，且优势雄性的受精成功率通常高于从属雄性。与理论预测相悖的是，本研究发现：优势雄性棕背䶄单次射精的精子投入量，无论绝对数量还是相对于躯体与睾丸质量的相对比值，均显著高于从属雄性。优势雄性的睾丸同样在绝对质量上大于从属雄性（相对质量则无显著差异）。然而，本研究未发现证据表明，从属雄性可通过提升射精频率或精子运动速度，弥补单次射精的精子数量不足。类似地，本研究未发现雄性会依据自身交配角色，在交配栓（copulatory plug）大小上存在差异化投入的证据……