Short- and long-term effects of litter size manipulation in a small wild-derived rodent

Iteroparous organisms maximise their overall fitness by optimising their reproductive effort over multiple reproductive events. Hence, changes in reproductive effort are expected to have both short- and long-term consequences on parents and their offspring. In laboratory rodents, manipulation of reproductive efforts during lactation have however revealed little short-term reproductive adjustments, suggesting that female laboratory rodents might express maximal rather than optimal levels of reproductive investments as observed in semelparous organisms. Using a litter size manipulation (LSM) experiment in a small wild-derived rodent (the common vole; Microtus arvalis), we show that females altered their reproductive efforts in response to LSM, with females having higher metabolic rates and showing alternative body mass dynamics when rearing an enlarged rather than reduced litter. Those differences in female reproductive effort were nonetheless insufficient to fully match their pups’ energy demand, pups being lighter at weaning in enlarged litters. Interestingly, female reproductive effort changes had long-term consequences, with females that had previously reared an enlarged litter being lighter at birth of their subsequent litter and producing lower quality pups. We discuss the significance of using wild-derived animals in studies of reproductive effort optimisation.

多次繁殖生物（Iteroparous organisms）通过优化多次繁殖事件中的繁殖投入，以最大化整体适合度。因此，繁殖投入的变化预计会对亲本及其后代产生短期与长期的双重影响。不过，针对实验啮齿类的哺乳期间繁殖投入操控实验，仅发现了极少量的短期繁殖调整现象，这提示实验啮齿类雌性可能如同单次繁殖生物（semelparous organisms）一般，展现出最大化而非最适水平的繁殖投资。本研究针对小型野生啮齿类——普通田鼠（Microtus arvalis）开展窝仔数操控（litter size manipulation, LSM）实验，结果显示，雌性个体的繁殖投入会响应LSM发生改变：相较于抚育缩减窝仔的个体，抚育扩增窝仔的雌性具有更高的代谢率，且呈现出差异化的体重动态变化。尽管雌性的繁殖投入出现上述差异，但仍不足以完全匹配幼崽的能量需求，扩增窝仔中的幼崽在断奶时体重更轻。值得注意的是，雌性繁殖投入的变化会产生长期影响：此前抚育过扩增窝仔的雌性，在其后续产仔时的初生体重更低，且产下的幼崽质量更差。本研究最后讨论了在繁殖投入优化研究中使用野生来源动物的重要意义。