All animals face the risk of periods of food deprivation, which can lead to starvation and ultimately death. Most animals, especially mammals, are not well adapted to withstand food deprivation and extended periods of starvation. But some animals, such as penguins and ground squirrels, have developed strategies that enable them to survive multiple months without food. Snakes,however, are in a league of their own in their ability to deal with food limitation and can endure multiple years of starvation. Although this has been known for a long time, very little is known about the underlying biological mechanisms. To investigate this stunning phenomenon, Marshall D. McCue from the University of Arkansas, USA, examined changes in physiology,morphology and body composition in response to 168 days of starvation in three species of snakes: the ball python (Python regius), the ratsnake(Elaphe obsolete) and the western diamondback rattlesnake(Crotalus atrox).
It is not a simple task to define when fasting turns into starvation,especially in infrequently eating animals. In this study, McCue defined the starvation period as starting when animals were deprived of a meal they would otherwise voluntarily have eaten, which is around 2 weeks after a meal. With this in mind, the 62 snakes were subdivided into four groups: fasting, and 56,112 and 168 days of starvation. All animals had access to fresh water throughout the experiment. McCue then measured the effects of starvation on body composition, mass and length, and resting metabolic rate over a 24 h period.
Following 168 days of starvation, all snakes had lost a percentage of their initial body mass: ratsnakes 9.3%, pythons 18.3% and rattlesnakes 24.4%. Despite this serious weight loss, and in contrast to previous investigations on reptiles and fish, all three species increased in length by around 4%. This indicates that there is a rather high selection pressure on length in these sub-adult snakes – size apparently does matter. Starvation also induced a highly significant decrease in resting metabolic rate in all three species,especially in rattlesnakes, which had a metabolic depression of an astounding 72%. This is surprising, since snakes have a very low resting rate even before the onset of starvation, and it was not expected that they could reduce this much further.
To find out how starvation affected body composition, McCue measured the water content of dead snakes by freeze-drying and subsequently measured the amount of lipid, carbohydrate and protein in their bodies. Because the snakes had access to water during the experiment, relative water content increased in all the species by an average of 6%, despite their weight loss. The relative protein content increased in all species during starvation, whereas lipid and carbohydrate content decreased. This shows that all snakes preferably use fat stores over protein as an energy source during starvation. Comparing body composition between the species, McCue found that ratsnakes began to break down proteins faster than pythons and rattlesnakes. This is probably because ratsnakes generally have an abundant food supply in their natural habitat and are maybe not as adapted to starvation as the other species.
The results show that starving snakes reduce their resting metabolic rate and change to metabolising lipids while sparing their protein stores. This was done to a degree where all snakes were able to increase in length despite a significant weight loss. Further investigations are needed to determine whether the observed metabolic depression is achieved through reductions in protein synthesis, reducing nerve activity or by something else entirely. Nevertheless, this paper very elegantly demonstrates one of the reasons why snakes are such an evolutionary success – they are well adapted to survive in areas with a low density of prey.