Reproductive Ecology and Biology Group, Museo Nacional de Ciencias Naturales (CSIC), José Gutierrez Abascal 2, 28006 Madrid, Spain.
Proc Biol Sci. 2011 Oct 22;278(1721):3135-41. doi: 10.1098/rspb.2011.0275. Epub 2011 Mar 9.
The hypothesis that sperm competition should favour increases in sperm size, because it results in faster swimming speeds, has received support from studies on many taxa, but remains contentious for mammals. We suggest that this may be because mammalian lineages respond differently to sexual selection, owing to major differences in body size, which are associated with differences in mass-specific metabolic rate. Recent evidence suggests that cellular metabolic rate also scales with body size, so that small mammals have cells that process energy and resources from the environment at a faster rate. We develop the 'metabolic rate constraint hypothesis' which proposes that low mass-specific metabolic rate among large mammals may limit their ability to respond to sexual selection by increasing sperm size, while this constraint does not exist among small mammals. Here we show that among rodents, which have high mass-specific metabolic rates, sperm size increases under sperm competition, reaching the longest sperm sizes found in eutherian mammals. By contrast, mammalian lineages with large body sizes have small sperm, and while metabolic rate (corrected for body size) influences sperm size, sperm competition levels do not. When all eutherian mammals are analysed jointly, our results suggest that as mass-specific metabolic rate increases, so does maximum sperm size. In addition, species with low mass-specific metabolic rates produce uniformly small sperm, while species with high mass-specific metabolic rates produce a wide range of sperm sizes. These findings support the hypothesis that mass-specific metabolic rates determine the budget available for sperm production: at high levels, sperm size increases in response to sexual selection, while low levels constrain the ability to respond to sexual selection by increasing sperm size. Thus, adaptive and costly traits, such as sperm size, may only evolve under sexual selection when metabolic rate does not constrain cellular budgets.
精子竞争应该有利于精子大小的增加，因为这会导致更快的游动速度，这一假说得到了许多分类群研究的支持，但对于哺乳动物来说，这一假说仍然存在争议。我们认为，这可能是因为哺乳动物谱系对性选择的反应不同，这是由于体型的巨大差异，而体型差异与比代谢率有关。最近的证据表明，细胞代谢率也与体型成正比，因此小型哺乳动物的细胞从环境中处理能量和资源的速度更快。我们提出了“代谢率约束假说”，该假说认为，大型哺乳动物的比代谢率较低可能限制了它们通过增加精子大小来应对性选择的能力，而小型哺乳动物则不存在这种限制。在这里，我们表明，在具有高比代谢率的啮齿动物中，精子大小在精子竞争下会增加，达到了真兽亚纲哺乳动物中发现的最长精子大小。相比之下，体型较大的哺乳动物谱系的精子较小，虽然代谢率（按体型校正）影响精子大小，但精子竞争水平没有影响。当所有真兽亚纲哺乳动物被联合分析时，我们的结果表明，随着比代谢率的增加，最大精子大小也会增加。此外，比代谢率低的物种产生均匀的小精子，而比代谢率高的物种则产生广泛的精子大小范围。这些发现支持了这样一种假设，即比代谢率决定了精子生产的可用预算：在高水平下，精子大小会因性选择而增加，而在低水平下，会限制通过增加精子大小来应对性选择的能力。因此，适应性和昂贵的特征，如精子大小，只有在代谢率不限制细胞预算的情况下，才会在性选择下进化。
