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Here, I present an hypothesis (Endothermic Pulses Hypothesis) which argues that the evolution of the scrotum was driven by <jats:styled-content style=\"fixed-case\">C<\/jats:styled-content>enozoic pulses in endothermy, that is, increases in normothermic body temperature, which occurred in <jats:styled-content style=\"fixed-case\">B<\/jats:styled-content>oreotheria (rodents, primates, lagomorphs, carnivores, bats, lipotyphylans and ungulates) in response to factors such as cursoriality and climate adaptation. The model argues that stabilizing selection maintained an optimum temperature for spermatogenesis and sperm storage throughout the <jats:styled-content style=\"fixed-case\">C<\/jats:styled-content>enozoic at the lower plesiomorphic levels of body temperature that prevailed in ancestral mammals for at least 163\u00a0million years. Evolutionary stasis may have been driven by reduced rates of germ\u2010cell mutations at lower body temperatures. Following the extinction of the dinosaurs at the <jats:styled-content style=\"fixed-case\">C<\/jats:styled-content>retaceous\u2013<jats:styled-content style=\"fixed-case\">P<\/jats:styled-content>alaeogene boundary 65.5\u00a0mya, immediate pulses in endothermy occurred associated with the dramatic radiation of the modern placental mammal orders. The fitness advantages of an optimum temperature of spermatogenesis outweighed the potential costs of testes externalization and paved the way for the evolution of the scrotum. The scrotum evolved within several hundred thousand years of the <jats:styled-content style=\"fixed-case\">K<\/jats:styled-content>\u2010<jats:styled-content style=\"fixed-case\">P<\/jats:styled-content>g extinction, probably associated initially with the evolution of cursoriality, and arguably facilitated mid\u2010 and late <jats:styled-content style=\"fixed-case\">C<\/jats:styled-content>enozoic metabolic adaptations to factors such as climate, flight in bats and sociality in primates.<\/jats:p>","DOI":"10.1111\/jeb.12373","type":"journal-article","created":{"date-parts":[[2014,4,15]],"date-time":"2014-04-15T13:06:53Z","timestamp":1397567213000},"page":"801-814","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":22,"title":["Cool sperm: why some placental mammals have a scrotum"],"prefix":"10.1093","volume":"27","author":[{"given":"B. 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