Nocturnal bottleneck

The tapetum lucidum of a badger reflects the photographers flash, one of many nocturnal traits ubiquitous in mammals

The nocturnal bottleneck hypothesis is an hypothesis to explain several mammal traits. The hypothesis states that 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 dinosaurs 65 millions years ago. While some mammal groups have later evolved to fill diurnal niches, the 160 million years spent as nocturnal animals has left a lasting legacy on basal anatomy and physiology, and most mammals are still nocturnal.^[1]

Evolution of mammals

Brasilitherium a very advanced near-mammalian cynodont, were probably itself burrowing and nocturnal

Mammals evolved from cynodonts, a group of superficially dog-like mammal-like reptiles 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.^[2] 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.^[3] While the early mammals continued to develop into several probably quite common groups of animals during the Mesozoic, they all remained 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.^[4] While all the largest animals alive today are mammals, the majority of mammals are still small nocturnal animals scurrying around in the undergrowth.^[5]

Mammalian nocturnal adaptions

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

Several different features of mammalian physiology appear to be adaptations to a nocturnal lifestyle, mainly related to the sensory organs. These include:

Senses

• 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.^[6]
• With the exception of higher primates, very large cornea giving a less acute visual image compared to birds and reptiles.^[7]
• Limited colour vision.^[8]

Physiology

• Unique type of brown adipose tissue allowing mammals to raise heat fast.^[9]
• Mitochondria with five to seven times higher respiration rates compared to reptiles of similar sizes.^[10]
• Fur to assist on thermo-regulation in a cold (night) environment.
• Lack of shielding mechanism against (diurnal) ultraviolet light.^[11]

Behaviour

• Circadian rhythm and behaviour patterns in all basal groups are nocturnal, at least in placentals.^[12][13]
• Burrowing lifestyle allowing sheltering from climate and diurnal predators appear to be a basal mammalian trait.^[14]

References

1. Sinn, J. "New Study Shows Effects of Prehistoric Nocturnal Life on Mammalian Vision". University of Texas. Retrieved 24 November 2014.
2. Benton, Michael J. (2004). Vertebrate palaeontology (3rd ed.). Oxford: Blackwell Science. ISBN 978-0-632-05637-8.
3. 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 0-231-11918-6.
4. Than, K. "Rise of Modern Mammals Occurred Long After Dinosaur Demise". LiveScience. Retrieved 24 November 2014.
5. 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. doi:10.1007/s10682-006-0015-y.
6. Grant, Robyn; Mitchinson, Ben; Prescott, Tony (2011). "Vibrissal behaviour and function". Scholarpedia 6 (10): 6642. doi:10.4249/scholarpedia.6642. Retrieved October 29, 2011.
7. 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. PMID 23097513. Retrieved 24 November 2014.
8. 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. doi:10.1111/j.1365-294X.2012.05617.x.
9. 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.
10. Brand, MD; Couture, P; Else, PL; Withers, KW; Hulbert, AJ (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. PMID 1850242.
11. 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.
12. 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): 20130508–20130508. doi:10.1098/rspb.2013.0508.
13. Menaker, M.; Moreira, L.F.; Tosini, G. (March 1997). "Evolution of circadian organization in vertebrates". Brazilian Journal of Medical and Biological Research 30 (3). doi:10.1590/S0100-879X1997000300003.
14. Damiani, R; Modesto, S; Yates, A; Neveling, J (22 August 2003). "Earliest evidence of cynodont burrowing." (PDF). Proceedings of the Royal Society B: Biological Sciences 270 (1525): 1747–51. doi:10.1098/rspb.2003.2427. PMC: 1691433. PMID 12965004. Retrieved 15 December 2014.