Author: Josh

Why is digestion so energetically costly: the case of the snake

Snakes are carnivorous vertebrates which often feed on prey large in size. As they feed infrequently, the digestive systems of snakes are often in a reduced state . Rapidly
reactivating the digestive system after feeding and then returning it to a reduced state once digestion is complete is very energetically costly. The costs of this must be met with energy already stored in the snake, further complicating its energy budget. Adaptations such as doubling intestinal mass after a large meal, as seen in the Burmese python (Python molurus), is an example of the physiological changes a snake may undergo during digestion. Furthermore, As they are unable to tear up their prey prior to feeding, snakes must eat their prey whole. This creates a very low surface area to volume ratio for digestive enzymes to work on and so a challenge for efficient digestion arises. The total cost of digestion is called the Specific Dynamic Action or SDA.

Southern Pacific Rattlesnakes (Crotalus oreganus helleri) achieve 92% assimilation efficiency when digesting their prey.

Snakes can consume animals ranging from 50% to as much as 160% of their own body weight.

What is SDA?
Specific Dynamic action (SDA) is the energy expenditure from the capture, ingestion, digestion and assimilation of the given prey. SDA is an essential part of an organism’s energy budget, especially in ectotherms. For example; SDA accounts for 19-43% of the daily energy
expenditure of free ranging snakes. SDA is strongly
influential over the daily lives of ectotherms, particularly snakes. It is affected by a variety of factors such as meal type and size, with the breakdown of proteins producing the most heat, and the body size and temperature of the organism itself playing a role. Snakes are not frequent hunters and mostly will only catch prey every other week or so. However, when they do catch their prey, their
metabolic rate increase is drastic, with a maximum average
increase of 687% .

Many snakes consume so rarely that they will even regurgitate their food if fed too frequently.

http://www.reptileknowledge.com/news/corn-snake-eat-127/

‘Digestion is an energetically demanding
activity. The rattlesnake Crotalus durissus, for example, uses 12-18% of the ingested
assimilated energy on digestion.’

http://visual.merriam-webster.com/animal-kingdom/reptiles/snake/anatomy-venomous-snake.php

Digestion as a process is more complex for snakes than most other animals due to several factors of a snake’s physiology and behaviour. These factors combine to make digestion in snakes more energetically costly in comparison with other
animals. Digestion in snakes is often the most energetically costly activity the snake
performs, rather than the effort involved in capturing prey. Due to this a snake’s energy budget is different to other animals; Snakes have had to evolve
ultra-efficient physiological characteristics (such as poisoning prey) to deal with the energetic cost of digestion. Although digestion in snakes IS more costly relative to other animals it clearly is effective. Snakes’ digestion is in fact remarkably
efficient considering the difficulties they must overcome. A snake’s
characteristics, such as being ectothermic and the way it swallows its food, mean digestion is a challenging obstacle in its life. This obstacle is overcome in an
expensive yet effective way.

Oxygen consumption and Thermal preference
Snakes are ectothermic, so their rate of digestion is affected by temperature.
Because they cannot chew, snakes are very
vulnerable during digestion as they are bloated and full which impairs their movement.
Digestion occurs faster at higher temperatures.
Preferred body temperature increases during
digestion, so the meal will digest faster.
Oxygen consumption (VO2) increases with
temperature.
Maximum VO2 value attained sooner at higher
temperature.
Maximal VO2 increases with meal size (regardless of temperature).
Resting and active oxygen consumption increases with body temperature.

During digestion, heart rate increases by as much as double.

Acknowledgements and references
Greenwald. The Effect of Body Temperature on Oxygen Consumption and Heart Rate in the Sonora Gopher Snake Copeia 1971 Secor, S.M. 2009. Specific dynamic action, a review of the postprandial metabolic response. J. Comp. Physiol. 179:1-56. Secor, S.M. and K.A. Nagy. 1994. Bioenergetic correlates of foraging mode for the snakes Crotalus cerastesand Masticophis flagellum. Ecology 75 Stevenson, R. D., C. R. Peterson, et al. (1985). "The Thermal Dependence of Locomotion, Tongue Flicking, Digestion, and Oxygen Consumption in the Wandering Garter Snake." Physiological Zoology
Toledo, Luís F., Augusto S. Abe, et al. (2003). "Temperature and Meal Size Effects on the Postprandial Metabolism and Energetics in a Boid Snake."Physiological and Biochemical Zoology http://the-best-top-desktop-wallpapers.blogspot.com/2011/02/snakes-wallpapers.html

Secor, S.M., Diamond, J.M. 2000 Evolution of regulatory responses to feeding in snakes Physiological and Biochemical Zoology
Secor, S.M., Fehsenfeld, D., Diamond, J., Adrian, T.E. 2001 Responses of python gastrointestinal regulatory peptides to feedingProceedings of the National Academy of Sciences of the United States of America 98