All hibernating bats need to eat enormous amounts of food during autumn to build up a substantial fat store for hibernation. These large fat stores supply the immense amount of energy needed to survive without access to food and water. However, little is known about how the breakdown of proteins from muscles and organs contribute to the hibernating animal's energy stores and possibly their water budget. Liam McGuire from Texas Tech University, USA (now at the University of Waterloo, Canada), and colleagues from Montanan State University, Texas Tech University and the Wildlife Conservation Society, all in the USA, set out to investigate how proteins from the muscles and organs of two species of bats are used for energy and water during hibernation.
In October 2016, before the bats entered hibernation, the team observed 184 cave myotis (Myotis velifer) in a cave in Oklahoma, taking measurements of the animals’ forearm length (a measurement of body size), determining their sex and using a mobile laboratory to measure the amount of fat, muscle and organ masses. Next, the team collected the same measurements from 65 Townsend's big-eared bats (Corynorhinus townsendii) at two caves in eastern Nevada during both the fall and winters of 2017–2019 to track changes in the fat, muscle and organ masses as the animals hibernated. Then, the team calculated the amount of energy and water generated by the Townsends’ big-eared bats from the decreases in fat, muscle and organ mass.
McGuire and colleagues found that prior to entering hibernation, muscle and organ masses in the cave myotis was ∼25–38% of the animals’ total body mass. The team suspect that during this period, the bats build up their digestive organs to allow them to binge and build up their fat reserves in addition to bulking up their muscles to carry the large fat store. However, larger muscles and organs also provide bats with access to protein throughout hibernation. In addition, the males had proportionally larger muscles and organs, whereas the females had more body fat, which the researchers suggested may be due to differences in the animals’ reproduction strategies. Females need to conserve energy reserves throughout hibernation in preparation for possible pregnancies following hibernation. With their larger protein stores, the males have greater flexibility when arousing from hibernation to burn through organs and muscle to fuel their energy and water requirements, as protein breakdown produces more water compared to fat. But how did the hibernating big-eared bat reserves hold during their long hibernations?
The males consumed approximately twice the amount of muscle and organ tissue than the females, accounting for 35% of the males’ total body mass loss and contributing 7% of the energy they used during hibernation. The females, however, had more body fat and were able to generate more energy as fat produces larger amounts of energy relative to protein. The researchers calculated how much water the animals generate as they both broke down muscle and organs and discovered that, after they subtracted the additional water that the animals lost excreting the toxic waste products generated by consuming protein, the female bats benefited by retaining four times more of the water they generated than the males. The team argues that bats could potentially use protein breakdown for hydration during hibernation.
McGuire and colleagues have successfully shown that bats build up their muscles and organs prior to hibernation, which could be used as a minor source of energy but could also play a major role in the production of water. Relatively little is known about how hibernating bats using protein as a source of water, but this new study opens the flood gates to new avenues in bat hibernation research.
