Thenocturnal bottleneck hypothesis is anevolutionary biologyhypothesis to explain the origin of severalmammalian traits. In 1942,Gordon Lynn Walls described this concept which states thatplacental mammals were mainly or even exclusivelynocturnal through most of their evolutionary history, from their origin225 million years ago during theLate Triassic to after theCretaceous–Paleogene extinction event,66 million years ago.^[1] While some mammalian groups later adapted todiurnal (daytime) lifestyles to fillniches newly vacated by theextinction ofnon-avian dinosaurs, the approximately 160 million years spent as nocturnal animals has left a lasting legacy onbasal mammaliananatomy and physiology, and most mammals are still nocturnal.^[2]

Mammals evolved fromcynodonts, a group of superficially dog-liketherapsidsynapsids that survived thePermian–Triassic mass extinction. The emergingarchosauriansauropsids, includingpseudosuchians,pterosaurs anddinosaurs and their ancestors, flourished after the Early TriassicSmithian–Spathian boundary event andcompetitively displaced the larger therapsids into extinction, leaving only the smallerburrowing cynodonts.^[3] The surviving cynodonts could only succeed in leftover niches with minimal competitions from the moredominant,diurnal dinosaurs, evolving into the nocturnal, small-bodied,insectivorous andgranivorous dwellers of theforestundergrowths.^[4] While the early mammals continued to develop into several probably quite common groups of animals during theMesozoic, they all remained relatively small and nocturnal.

Mammals experienced a significantradiation from theangiosperm revolution during the Middle/LateCretaceous, but only with the massiveend-Cretaceous extinction event did the dinosaurs' demise leave the stage open for the establishment of new mammalianfaunae. Despite this, mammals continued to be small-bodied for millions of years.^[5] While all thelargest animals alive today are mammals, the majority of mammals are still small nocturnal animals.^[6]

Numerous features of mammalian physiology, especially features relating to the sensory organs, appear to be adaptations to a nocturnal lifestyle. These include:

• An acutesense of hearing, with coiledcochleae, sound-collectingauricles on theouter ear and sound-amplifyingossicles in themiddle ear.
• Very goodsense of smell, well developednasal turbinates. Most mammals have a largeolfactory bulb.
• Well-developedsense of touch, particularly thewhiskers.^[7]
• With the exception ofhigher primates, very largecornea, giving lessvisual acuity compared to birds and reptiles.^[8]
• Limitedcolour vision.^[9]

• Endothermia that enabled early mammals to become independent of solar radiation and environmental factors.^[1]
• Unique type ofbrown adipose tissue, allowing mammals to generate heat quickly.^[10]
• Mitochondria with respiration rates five to seven times higher than those of reptiles of similar size.^[11]
• Fur to assist in thermo-regulation in a cold (night) environment.
• Lack of an ocular shielding mechanism against (diurnal)ultraviolet light.^[12]
• Loss of the ability to producegadusol, a chemical which protects against the sun.^[13][14]
• Thephotolyase DNA repair mechanism, which relies on visible light, does not work in the placental mammals, despite being present and functional in bacteria, fungi, and most other animals.^[15][16]

• Circadian rhythm and behaviour patterns in allbasal mammalian groups are nocturnal, at least inplacentals.^[17][18]
• Burrowing lifestyle allowing sheltering from climate and diurnal predators appears to be a basal mammalian trait.^[19]

• Behavioral ecology
• Biology theories
• Chronobiology
• Circadian rhythm
• Evolutionary biology
• Evolution of mammals
• Night
• Prehistoric mammals
• Sleep

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• Short description matches Wikidata