Obligatory Nocturnalism in Triassic Archaic Mammals: Preservation of Sperm Quality?

Abstract

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 367C 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.

Variable Climates Lead to Varying Phenotypes: “Weird” Mammalian Torpor and Lessons From Non-Holarctic Species

“…Often, tropical and subtropical heterotherms inhabit dry environments and water savings might even be more essential for survival than energy savings (Schmid and Speakman, 2000;Dausmann, 2014). Water loss, however, could be avoided if T b is regulated slightly above ambient, and the outward flow of heat is varied by insulation, therefore, a switch to a diurnal activity pattern in some species would have necessitated higher T b s (Crompton et al, 1978), which in turn were only made possible with the evolution of the scrotum (Lovegrove, 2019). The evolution of endothermy in mammals is an emergent property of the evolution of various characteristics that aid in either heat production (e.g., thermogenesis, UCP1) and heat dissipation (e.g., insulation, external scrotums) and happened to a different degree across the mammalian lineage (Lovegrove, 2012(Lovegrove, , 2019Seebacher, 2018;Jastroch and Seebacher, 2020).…”

“…Water loss, however, could be avoided if T b is regulated slightly above ambient, and the outward flow of heat is varied by insulation, therefore, a switch to a diurnal activity pattern in some species would have necessitated higher T b s (Crompton et al, 1978), which in turn were only made possible with the evolution of the scrotum (Lovegrove, 2019). The evolution of endothermy in mammals is an emergent property of the evolution of various characteristics that aid in either heat production (e.g., thermogenesis, UCP1) and heat dissipation (e.g., insulation, external scrotums) and happened to a different degree across the mammalian lineage (Lovegrove, 2012(Lovegrove, , 2019Seebacher, 2018;Jastroch and Seebacher, 2020). By studying the added level of variability in many thermoregulatory traits, torpor use in particular, observed in species in the tropics and subtropics can help shed further light on how endothermy evolved in mammals.…”

Nocturnal increase in cerebrospinal fluid secretion as a circadian regulator of intracranial pressure

“…Curiously, the elevated CSF dynamics occurred in the dark phase in both species, irrespective of the human diurnal activity vs. the rat nocturnal activity. Such paradox may be explained by a given diurnal rhythm of our common ancestors and the associated establishment of certain physiological processes [ 47 – 49 ]. With a later division into day- and night active species, such circadian regulatory properties may have prevailed during transitions to nocturnal versus diurnal behavior.…”

Evolutionary physiology at 30+: Has the promise been fulfilled?

“…We envision great strides in several areas. First, the linking of physiological traits to evolutionary fitness, across hierarchical levels from genes to ecosystems, in a causal way to inform mechanisms of evolutionary adaptation, constraint, and diversification (e.g., [ 26,27,124,126–129 ] ). Second, the use of adaptations in wild animals to develop new strategies for combating diseases and injuries.…”