Lean Mass Dynamics in Hibernating Bats and Implications for Energy and Water Budgets

Hibernation requires balancing energy and water demands over several months. Many studies have noted the importance of fat for hibernation energy budgets, but protein catabolism in hibernation has received less attention, and whole-animal changes in lean mass have not previously been considered. We used quantitative magnetic resonance body composition analysis to measure deposition of fat and lean mass of cave myotis (Myotis velifer) during the prehibernation period and decreases in fat and lean mass of Townsend’s big-eared bats (Corynorhinus townsendii) during hibernation. For cave myotis, lean mass represented 25% and 38% (female and male, respectively) of prehibernation mass gain. In hibernating Townsend’s big-eared bats, lean mass decrease was similar for females and males. We used values for Townsend’s big-eared bats to explore the functional implications of lean mass change for water and energy budgets. Lean mass accounted for a substantial proportion of mass change during hibernation (female: 18%, male: 35%), and although not accounting for a large proportion of the energy budget (female: 3%, male: 7%), lean mass catabolism represented an important contribution to water production (female: 14%, male: 29%). Although most mammals cannot rely on protein catabolism for metabolic water production because of the water cost of excreting urea, we propose a variation of the protein-for-water strategy whereby hibernators could temporally compartmentalize the benefits of protein catabolism to periods of torpor and the water cost to periodic arousals when free drinking water is typically available. Combined, our analyses demonstrate the importance of considering changes in lean mass during hibernation.