Archaic mammals became exclusively nocturnal by the Late Triassic, and today, the majority of extant mammals remain nocturnal. Although there is ample morphological and physiological evidence supporting Late Mesozoic nocturnalism, a succinct hypothesis for why mammals became nocturnal remains elusive. Here, I propose a hypothesis that, with the onset of body size miniaturization in the Triassic and the concomitant evolution of fur and increased mass-specific metabolic rate and hence body temperature, small mammals became obligatorily nocturnal in order to avoid poor sperm quality, hyperthermia, and high rates of evaporative water loss and to maximize foraging time. The hypothesis hinges heavily on the assumption that, with the absence of externalized testes, the maximum optimum temperature of about 36°C for spermatogenesis was subject to strong stabilizing selection that placed a ceiling on increases in metabolic rate and body temperature. Heat-dissipating capacity during the daytime during the Triassic, Jurassic, and Cretaceous was thereby compromised. The release from the constraint of the optimum temperature of spermatogenesis occurred in placental mammals only with the advent of the externalization of testes in the scrotum in Boreotheria in the Cenozoic or, with the recent claim that the scrotum is plesiomorphic in mammals, as early as the Jurassic with the origin of the marsupials.

中文翻译：

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三叠纪古哺乳动物的强制性夜行动：保留精子质量？

三叠纪晚期，古哺乳动物仅是夜行性的，而今天，大多数现存的哺乳动物仍是夜行性的。尽管有足够的形态学和生理学证据支持晚中生夜行性，但是关于哺乳动物为何进入夜行性的简洁假设仍然难以捉摸。在这里，我提出一个假说，随着三叠纪开始出现体型小型化，伴随皮毛的进化以及质量比代谢率和体温的升高，为了避免精子质量差，体温过高，小型哺乳动物不得不夜间活动。 ，蒸发水分流失率高，并能最大限度地延长觅食时间。该假设很大程度上取决于以下假设：在没有外部睾丸的情况下，精子发生的最高最佳温度约为36°C，需要进行强烈的稳定选择，从而限制了代谢率和体温的增加。因此损害了三叠纪，侏罗纪和白垩纪白天的散热能力。胎盘哺乳动物只有在睾丸外生于新生代的阴囊中才出现睾丸外化作用时才释放出精子发生的最佳温度，或者最近有人声称阴囊在哺乳动物中是多形的。侏罗纪与有袋动物的起源。