Metabolic Rate Limits the Effect of Sperm Competition on Mammalian Spermatogenesis

Sperm competition leads to increased sperm production in many taxa. This response may result from increases in testes size, changes in testicular architecture or changes in the kinetics of spermatogenesis, but the impact of each one of these processes on sperm production has not been studied in an integrated manner. Furthermore, such response may be limited in species with low mass-specific metabolic rate (MSMR), i.e., large-bodied species, because they cannot process energy and resources efficiently enough both at the organismic and cellular levels. Here we compare 99 mammalian species and show that higher levels of sperm competition correlated with a) higher proportions of seminiferous tubules, b) shorter seminiferous epithelium cycle lengths (SECL) which reduce the time required to produce sperm, and c) higher efficiencies of Sertoli cells (involved in sperm maturation). These responses to sperm competition, in turn, result in higher daily sperm production, more sperm stored in the epididymides, and more sperm in the ejaculate. However, the two processes that require processing resources at faster rates (SECL and efficiency of Sertoli cells) only respond to sperm competition in species with high MSMR. Thus, increases in sperm production with intense sperm competition occur via a complex network of mechanisms, but some are constrained by MSMR.

精子竞争（sperm competition）会在众多类群（taxa）中导致精子产出量增加，该适应性响应可能源自睾丸大小增加、睾丸结构（testicular architecture）改变或是精子发生动力学（kinetics of spermatogenesis）发生变化，但目前尚未有研究对上述各过程对精子产出的影响开展整合性分析。此外，这类响应在单位体重代谢率（mass-specific metabolic rate, MSMR）较低的物种，即大体型物种中可能受到限制，因为这类物种无法在个体和细胞层面高效地处理能量与资源。本研究对99种哺乳动物进行了比较分析，结果显示更高水平的精子竞争与以下特征呈正相关：a) 生精小管（seminiferous tubules）占比更高；b) 更短的生精上皮周期时长（seminiferous epithelium cycle lengths, SECL），该指标可缩短精子生成所需时间；c) 支持细胞（Sertoli cells）效率更高（支持细胞参与精子成熟过程）。这些针对精子竞争的响应最终会带来更高的每日精子产出量、附睾（epididymides）中储存的精子更多以及射出精液中的精子数更多。然而，两项需要以更高速率处理资源的过程（生精上皮周期时长与支持细胞效率），仅在单位体重代谢率较高的物种中会随精子竞争水平提升而产生响应。综上，伴随剧烈精子竞争而来的精子产出提升，是通过一套复杂的机制网络实现的，但其中部分过程会受到单位体重代谢率的约束。