Larger testes produce more sperm and therefore improve reproductive success in the face of sperm competition. Adaptation to social mating systems with relatively high and low sperm competition are therefore likely to have driven changes in relative testes size in opposing directions. Here, we combine the largest vertebrate testes mass dataset ever collected with phylogenetic approaches for measuring rates of morphological evolution to provide the first quantitative evidence for how relative testes mass has changed over time. We detect explosive radiations of testes mass diversity distributed throughout the vertebrate tree of life: bursts of rapid change have been frequent during vertebrate evolutionary history. In socially monogamous birds, there have been repeated rapid reductions in relative testes mass. We see no such pattern in other monogamous vertebrates; the prevalence of monogamy in birds may have increased opportunities for investment in alternative behaviours and physiologies allowing reduced investment in expensive testes.

中文翻译：

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一夫一妻制鸟类中相对睾丸质量迅速下降，而其他脊椎动物则没有。

更大的睾丸会产生更多的精子，因此面对精子竞争会提高生殖成功率。因此，对精子竞争相对较高和较低的社会交配系统的适应可能会推动相对睾丸大小朝相反方向变化。在这里，我们将有史以来收集的最大的脊椎动物睾丸质量数据集与系统发育方法相结合，以测量形态演变的速率，从而为相对睾丸质量如何随时间变化提供了第一个定量证据。我们检测到分布在整个脊椎动物生命树中的睾丸质量多样性的爆炸性辐射：快速变化的爆发在脊椎动物进化史中屡见不鲜。在社会上一夫一妻制的鸟类中，相对睾丸质量已反复快速下降。在其他一夫一妻制脊椎动物中，我们没有看到这种模式。一夫一妻制在禽类中的流行可能会增加对替代行为和生理的投资机会，从而减少对昂贵睾丸的投资。