The nocturnal bottleneck hypothesis is a hypothesis to explain several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively nocturnal through most of their evolutionary history, from their origin 225 million years ago to after the Cretaceous–Paleogene extinction event, 66 million years ago. ^[1] While some mammal groups later adapted to diurnal (daytime) lifestyles to fill newly unoccupied niches, the approximately 160 million years spent as nocturnal animals has left a lasting legacy on basal mammalian anatomy and physiology, and most mammals are still nocturnal. ^[2]

Evolution of mammals

Mammals evolved from cynodonts, a group of superficially dog-like synapsids in the wake of the Permian–Triassic mass extinction. The emerging archosaurian groups that flourished after the extinction, including crocodilians and dinosaurs and their ancestors, drove the remaining larger cynodonts into extinction, leaving only the smaller forms. ^[3] The surviving cynodonts could only succeed in niches with minimal competition from the diurnal dinosaurs, evolving into the typical small-bodied insectivorous dwellers of the nocturnal undergrowth. ^[4] While the early mammals continued to develop into several probably quite common groups of animals during the Mesozoic, they all remained relatively small and nocturnal.

Only with the massive extinction at the end of the Cretaceous did the dinosaurs leave the stage open for the establishment of a new fauna of mammals. Despite this, mammals continued to be small-bodied for millions of years. ^[5] While all the largest animals alive today are mammals, the majority of mammals are still small nocturnal animals. ^[6]

Mammalian nocturnal adaptions

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

Senses

• An acute sense of hearing, with coiling cochleae, external pinnae and auditory ossicles in the ear.
• Very good sense of smell, well developed nasal turbinates. Most mammals have a large olfactory bulb.
• Well-developed sense of touch, particularly the whiskers. ^[7]
• With the exception of higher primates, very large cornea, giving a less acute visual image compared to birds and reptiles. ^[8]
• Limited colour vision. ^[9]

Physiology

• Endothermia that enabled early mammals to become independent of solar radiation and environmental factors. ^[1]
• Unique type of brown 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 produce gadusol, a chemical which protects against the sun. ^{[13] [14]}
• The photolyase 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]}

Behaviour

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

References