Nocturnal bottleneck

Artist's impression of the Purbeck lagoon at dusk: Durlstotherium (right and center foreground) and Durlstodon (left foreground) ventured out at night to hunt insects. The theropod Nuthetes holding a captured Durlstotherium (centre middle distance).

The tapetum lucidum of a European badger reflects the photographer's flash, one of many nocturnal traits ubiquitous in mammals.

The nocturnal bottleneck hypothesis is an evolutionary biology hypothesis to explain the origin of 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 during the Late Triassic to after the Cretaceous–Paleogene extinction event, 66  million years ago. ^[1] While some mammalian groups later adapted to diurnal (daytime) lifestyles to fill niches newly vacated by the extinction of non-avian dinosaurs, 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

Brasilitherium , a near-mammalian cynodont, was probably a nocturnal burrower.

Main article: Evolution of mammals

Mammals evolved from cynodonts, a group of superficially dog-like therapsid synapsids that survived the Permian–Triassic mass extinction. The emerging archosaurian sauropsids, including pseudosuchians, pterosaurs and dinosaurs and their ancestors, flourished after the Early Triassic Smithian–Spathian boundary event and competitively displaced the larger therapsids into extinction, leaving only the smaller burrowing cynodonts. ^[3] The surviving cynodonts could only succeed in leftover niches with minimal competitions from the more dominant, diurnal dinosaurs, evolving into the nocturnal, small-bodied, insectivorous and granivorous dwellers of the forest undergrowths. ^[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.

Mammals experienced a significant radiation from the angiosperm revolution during the Middle/Late Cretaceous, but only with the massive end-Cretaceous extinction event did the dinosaurs' demise leave the stage open for the establishment of new mammalian faunae. 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

The whiskers on a shrew, used in finding prey, navigation and socialization

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 coiled cochleae, sound-collecting auricles on the outer ear and sound-amplifying ossicles in the middle 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 less visual acuity 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

1. Gerkema, M. P.; Davies, W. I. L.; Foster, R. G.; Menaker, M.; Hut, R. A. (3 July 2013). "The nocturnal bottleneck and the evolution of activity patterns in mammals". Proceedings of the Royal Society B: Biological Sciences. 280 (1765): 20130508. doi:10.1098/rspb.2013.0508. PMC   3712437 . PMID   23825205.
2. Sinn, J. "New Study Shows Effects of Prehistoric Nocturnal Life on Mammalian Vision". University of Texas. Retrieved 24 November 2014.
3. Benton, Michael J. (2004). Vertebrate palaeontology (3rd ed.). Oxford: Blackwell Science. ISBN   978-0-632-05637-8.
4. Kielan-Jaworowska, Zofia; Cifelli, Richard L.; Luo, Zhe-Xi (2004). Mammals from the age of dinosaurs : origins, evolution, and structure . New York: Columbia University Press. p.  5. ISBN   978-0-231-11918-4.
5. Than, K. (28 March 2007). "Rise of Modern Mammals Occurred Long After Dinosaur Demise". LiveScience. Retrieved 24 November 2014.
6. Gamberale-Stille, G.; Hall, K. S. S.; Tullberg, B. S. (10 August 2006). "Signals of profitability? Food colour preferences in migrating juvenile blackcaps differ for fruits and insects". Evolutionary Ecology. 20 (5): 479–490. doi:10.1007/s10682-006-0015-y. S2CID   45267536.
7. Grant, Robyn; Mitchinson, Ben; Prescott, Tony (2011). "Vibrissal behaviour and function". Scholarpedia. 6 (10): 6642. Bibcode:2011SchpJ...6.6642P. doi: 10.4249/scholarpedia.6642 .
8. Hall, M. I.; Kamilar, J. M.; Kirk, E. C. (24 October 2012). "Eye shape and the nocturnal bottleneck of mammals". Proceedings of the Royal Society B: Biological Sciences. 279 (1749): 4962–4968. doi:10.1098/rspb.2012.2258. PMC   3497252 . PMID   23097513.
9. Davies, Wayne I. L.; Collin, Shaun P.; Hunt, David M. (July 2012). "Molecular ecology and adaptation of visual photopigments in craniates". Molecular Ecology. 21 (13): 3121–3158. Bibcode:2012MolEc..21.3121D. doi: 10.1111/j.1365-294X.2012.05617.x . PMID   22650357. S2CID   9077192.
10. Cannon, B. (1 January 2004). "Brown Adipose Tissue: Function and Physiological Significance". Physiological Reviews. 84 (1): 277–359. doi:10.1152/physrev.00015.2003. PMID   14715917.
11. Brand, M. D.; Couture, P.; Else, P. L.; Withers, K. W.; Hulbert, A. J. (1 April 1991). "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. 275 (1): 81–6. doi:10.1042/bj2750081. PMC   1150016 . PMID   1850242.
12. Ringvold, Amund (27 May 2009). "Aqueous humour and ultraviolet radiation". Acta Ophthalmologica. 58 (1): 69–82. doi:10.1111/j.1755-3768.1980.tb04567.x. PMID   6773294. S2CID   24655348.
13. Mammals’ nocturnal past shapes sun sensitivity
14. Why Would A Fish Make Its Own Sunscreen? - NPR
15. Lucas-Lledó JI, Lynch M (May 2009). "Evolution of mutation rates: phylogenomic analysis of the photolyase/cryptochrome family". Molecular Biology and Evolution. 26 (5): 1143–53. doi:10.1093/molbev/msp029. PMC   2668831 . PMID   19228922.
16. "Clues from a Somalian cavefish about modern mammals' dark past". Science Daily. Cell Press. October 11, 2018. Retrieved 11 October 2018.
17. Gerkema, M. P.; Davies, W. I. L.; Foster, R. G.; Menaker, M.; Hut, R. A. (3 July 2013). "The nocturnal bottleneck and the evolution of activity patterns in mammals". Proceedings of the Royal Society B: Biological Sciences. 280 (1765): 20130508. doi:10.1098/rspb.2013.0508. PMC   3712437 . PMID   23825205.
18. Menaker, M.; Moreira, L.F.; Tosini, G. (March 1997). "Evolution of circadian organization in vertebrates". Brazilian Journal of Medical and Biological Research. 30 (3): 305–313. doi: 10.1590/S0100-879X1997000300003 . PMID   9246228.
19. Damiani, R.; Modesto, S.; Yates, A.; Neveling, J. (22 August 2003). "Earliest evidence of cynodont burrowing". Proceedings of the Royal Society B: Biological Sciences. 270 (1525): 1747–51. doi:10.1098/rspb.2003.2427. PMC   1691433 . PMID   12965004.