Nocturnal bottleneck explained

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 to after the Cretaceous–Paleogene extinction event, .^[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

See main article: article and 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.
• 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]

Notes and References

1. Gerkema. M. P.. Davies. W. I. L.. Foster. R. G.. Menaker. M.. Hut. R. A.. The nocturnal bottleneck and the evolution of activity patterns in mammals. Proceedings of the Royal Society B: Biological Sciences. 3 July 2013. 280. 1765. 20130508. 10.1098/rspb.2013.0508. 23825205. 3712437.
2. Web site: Sinn. J.. New Study Shows Effects of Prehistoric Nocturnal Life on Mammalian Vision. University of Texas. 24 November 2014.
3. Book: Michael Benton

   . Benton. Michael J.. Michael Benton. Vertebrate palaeontology. 2004. Blackwell Science. Oxford. 978-0-632-05637-8. 3rd.

4. Book: Kielan-Jaworowska. Zofia. Cifelli. Richard L.. Luo. Zhe-Xi. Mammals from the age of dinosaurs : origins, evolution, and structure. limited. 2004. Columbia University Press. New York. 978-0-231-11918-4. 5.
5. Web site: Than. K.. Rise of Modern Mammals Occurred Long After Dinosaur Demise. 28 March 2007 . LiveScience. 24 November 2014.
6. Gamberale-Stille. G.. Hall. K. S. S.. Tullberg. B. S.. Signals of profitability? Food colour preferences in migrating juvenile blackcaps differ for fruits and insects. Evolutionary Ecology. 20. 5. 479–490. 10 August 2006. 10.1007/s10682-006-0015-y. 45267536.
7. Vibrissal behaviour and function. Scholarpedia . 6. 6642 . 10.4249/scholarpedia.6642 . 2011 . Grant . Robyn . Mitchinson . Ben . Prescott . Tony . 10 . 2011SchpJ...6.6642P. free .
8. Hall. M. I.. Kamilar. J. M.. Kirk. E. C.. Eye shape and the nocturnal bottleneck of mammals. Proceedings of the Royal Society B: Biological Sciences. 24 October 2012. 279. 1749. 4962–4968. 10.1098/rspb.2012.2258. 23097513. 3497252.
9. Davies. Wayne I. L.. Collin. Shaun P.. Hunt. David M.. Molecular ecology and adaptation of visual photopigments in craniates. Molecular Ecology. July 2012. 21. 13. 3121–3158. 10.1111/j.1365-294X.2012.05617.x. 22650357. 9077192. free. 2012MolEc..21.3121D .
10. Cannon. B.. Brown Adipose Tissue: Function and Physiological Significance. Physiological Reviews. 1 January 2004. 84. 1. 277–359. 10.1152/physrev.00015.2003. 14715917.
11. Brand. M. D.. Couture. P.. Else. P. L.. Withers. K. W.. Hulbert. A. J.. Evolution of energy metabolism. Proton permeability of the inner membrane of liver mitochondria is greater in a mammal than in a reptile.. The Biochemical Journal. 1 April 1991. 275. 81–6. 1850242. 1. 10.1042/bj2750081. 1150016.
12. Ringvold. Amund. Aqueous humour and ultraviolet radiation. Acta Ophthalmologica. 27 May 2009. 58. 1. 69–82. 10.1111/j.1755-3768.1980.tb04567.x. 6773294. 24655348.
13. https://evolution.berkeley.edu/evo-news/mammals-nocturnal-past-shapes-sun-sensitivity/ Mammals’ nocturnal past shapes sun sensitivity
14. https://www.npr.org/sections/health-shots/2015/05/13/404444731/why-would-a-fish-make-its-own-sunscreen Why Would A Fish Make Its Own Sunscreen? - NPR
15. Lucas-Lledó JI, Lynch M . Evolution of mutation rates: phylogenomic analysis of the photolyase/cryptochrome family . Molecular Biology and Evolution . 26 . 5 . 1143–53 . May 2009 . 19228922 . 2668831 . 10.1093/molbev/msp029 .
16. News: Clues from a Somalian cavefish about modern mammals' dark past . 11 October 2018 . Science Daily . Cell Press . October 11, 2018.
17. Gerkema. M. P.. Davies. W. I. L.. Foster. R. G.. Menaker. M.. Hut. R. A.. The nocturnal bottleneck and the evolution of activity patterns in mammals. Proceedings of the Royal Society B: Biological Sciences. 3 July 2013. 280. 1765. 20130508. 10.1098/rspb.2013.0508. 23825205. 3712437.
18. Menaker. M.. Moreira. L.F.. Tosini. G.. Evolution of circadian organization in vertebrates. Brazilian Journal of Medical and Biological Research. March 1997. 30. 3. 305–313. 10.1590/S0100-879X1997000300003. 9246228. free.
19. Damiani. R.. Modesto. S.. Yates. A.. Neveling. J.. Earliest evidence of cynodont burrowing. Proceedings of the Royal Society B: Biological Sciences. 22 August 2003. 270. 1525. 1747–51. 12965004. 10.1098/rspb.2003.2427. 1691433.