Fish-eating myotis pay metabolic cost of protein diet

Most animals don't think twice about the cost of dragging a large digestive tract around, but for tiny bats and birds the costs can be substantial. Kenneth Welch, from the University of Toronto Scarborough, Canada, and colleagues Aída Otálora-Ardila, Gerardo Herrera and José Flores-Martínez from the Universidad Nacional Autónoma de México, explain that flying vertebrates tend to have much shorter digestive tracts than similarly sized land-based animals. However, the bat's reduced digestive system may restrict the animal's ability to fuel their costly aerial lifestyle. Puzzled by the apparent paradox, Welch and his colleagues began investigating how one bat, the fish-eating myotis (Myotis vivesi) from the Sea of Cortés deals with shrimp meat meals.

Feeding the minute mammals (∼28 g) 1.5 g and 3 g shrimp banquets (∼5% and ∼10% of their body mass), the team measured the bats’ O₂ consumption and CO₂ production rates for up to 5 h, and found a massive increase in their O₂ consumption within 20 min of tucking in. O₂ consumption rocketed by 3 times after the 1.5 g meal and soared to 4.3 times the resting metabolic rate while the bats were digesting the enormous 3 g meal. Could the immense metabolic investment accelerate digestion to cut their overall energy costs? However, when the team tracked how long it took the tiny animals to process the meals, they were surprised to see that instead of extracting the goodness faster, the bats were taking up to 5 h to digest the largest meal. And when the team analysed the ratio of CO₂ production to O₂ consumption, they were surprised to see that instead of rising (as a result of the increased levels of bicarbonate in the blood associated with acid production for digestion), the ratio inexplicably fell.

So, fish-eating myotis pay a high metabolic price for their protein diet, and the team says, ‘The rate of prey digestion may limit food intake rate’. They also add that the cost of digestion may vary seasonally as the bats switch diet from lipid-rich fish in the autumn to almost total reliance on crustaceans in winter, so the team hopes to learn more about the complex interactions between the ecology and physiology of these tiny animals.