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 story, starting with their origin 225 million years ago, and only ending with the demise of the non-avian dinosaurs 66 million years ago. While some mammal groups have later evolved to fill diurnal niches, the approximately 160 million years spent as nocturnal animals has left a lasting legacy on basal anatomy and physiology, and most mammals are still nocturnal.
Evolution of mammalsEdit
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 crocodiles and dinosaurs and their ancestors, drove the remaining larger cynodonts into extinction, leaving only the smaller forms. 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. 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. While all the largest animals alive today are mammals, the majority of mammals are still small nocturnal animals.
Mammalian nocturnal adaptionsEdit
Several different features of mammalian physiology appear to be adaptations to a nocturnal lifestyle, mainly related to the sensory organs. These include:
- Acute sense of hearing, including external pinnae and auditory ossicles.
- Very good sense of smell, well developed nasal turbinates.
- Well developed sense of touch, and particularly the whiskers.
- With the exception of higher primates, very large cornea, giving a less acute visual image compared to birds and reptiles.
- Limited colour vision.
- Unique type of brown adipose tissue, allowing mammals to generate heat quickly.
- Mitochondria with five to seven times higher respiration rates compared to reptiles of similar sizes.
- Fur to assist in thermo-regulation in a cold (night) environment.
- Lack of shielding mechanism against (diurnal) ultraviolet light. (This is not actually true, as fur is good for shielding from ultraviolet radiation.)
- The photolyase DNA 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.
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