![]() Predation pressure and inter-species competition are thought to have stimulated the development of endothermia, a major adaptive change that enabled early mammals to become nocturnal and independent of solar radiation and environmental temperature. Thought to be mainly ectothermic, these reptiles would have had to restrict their activity to the daytime because solar radiation was essential to increase their body temperature to operational levels. The nocturnal bottleneck hypothesis suggests that early eutherian mammals faced competition with diurnal reptiles (e.g. ![]() Further evidence to support the hypothesis arose from consistent findings in mammalian evolution, including the evolution of endothermia and the adaptation of photosensory systems (e.g. hypothalamus, pineal gland, parietal eye). The main focus of this review is the ‘nocturnal bottleneck’ conjecture that was inspired by the fact that mammals have only one (retinal) photic input pathway to their circadian pacemaker system, whereas non-mammalian species have several parallel retinal and extra-retinal circadian photic input systems (e.g. Menaker and co-workers formulated a specific ‘nocturnal bottleneck hypothesis’, although some researchers have suggested the involvement of a period of mesopia (medium-light levels) in the evolution of eyes in response to light-restricted habitats, as observed in other species (e.g. Our conclusion presently is that arguments in favour of the nocturnal bottleneck hypothesis in eutherians prevail.Ī general view that early mammals were small insectivores, living on trees and being only night active, has become commonplace in textbooks, with some articles devoted specifically to vision, whereas others are more general in nature. We also assess genes, relating to light detection (visual and non-visual systems) and the photolyase DNA protection system that were lost in the eutherian mammalian lineage. ![]() ![]() We review the ecological plausibility that the activity patterns of (early) eutherian mammals were restricted to the night, based on arguments relating to endothermia, energy balance, foraging and predation, taking into account recent palaeontological information. This review describes the status of what has become known as the nocturnal bottleneck hypothesis, giving an overview of the chronobiological patterns of activity. Walls based this concept of a longer episode of nocturnality in early eutherian mammals by comparing the visual systems of reptiles, birds and all three extant taxa of the mammalian lineage, namely the monotremes, marsupials (now included in the metatherians) and placentals (included in the eutherians). In 1942, Walls described the concept of a ‘nocturnal bottleneck’ in placental mammals, where these species could survive only by avoiding daytime activity during times in which dinosaurs were the dominant taxon. ![]()
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