Ontogenetic changes in body size are weakly associated with basal metabolic rate in a maternity colony of Myotis velifer
This study reveals that while body mass scales positively with basal metabolic rate in *Myotis velifer*, individual variation in metabolic rate is only weakly explained by body size, age, or reproductive status, suggesting that other intrinsic and ecological factors drive energy expenditure within maternity colonies.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Every living creature that maintains a constant body temperature, from a hummingbird to a human, faces a fundamental biological challenge: staying warm while staying still. To do this, they must burn energy even when they are resting, sleeping, or simply sitting in a comfortable spot. Scientists call this minimum energy cost the basal metabolic rate. It is the engine idling in the background, the baseline fuel consumption required just to keep the heart beating and the lungs breathing. While we know that larger animals generally burn more fuel than smaller ones, the story becomes much more complicated when we look at how this energy use changes as an animal grows from a tiny infant into a full-grown adult. Does the engine simply get bigger in proportion to the body, or does the way the engine runs change as the animal matures? Understanding this is crucial because energy is the currency of life; how an animal spends it dictates how it grows, how it reproduces, and how it survives the harsh realities of the wild.
In a cave system in central Mexico, researchers set out to watch this energy story unfold in real time. They focused on a colony of bats, specifically a species known as Myotis velifer, which gathers in large numbers to raise their young. These caves provide a warm, humid nursery where mothers nurse their pups and the young grow rapidly, eventually learning to fly. The scientists wanted to see if the simple rule of "bigger body, more energy" held true as these bats developed from helpless newborns into independent juveniles and finally into adults. They measured the resting energy use of nearly one hundred individual bats, capturing them gently from the cave walls and placing them in a controlled environment to record how much oxygen they consumed while resting. By comparing these energy readings against the animals' body weight and the length of their forearm—a reliable measure of their skeletal size—they hoped to find a clear pattern linking growth to energy expenditure.
The results revealed a story of two different kinds of growth happening at once. As the bats grew, their body weight did increase in step with their forearm length, but the relationship changed dramatically depending on their age. For the youngest pups and the growing juveniles, gaining weight was tightly linked to building new bone and muscle; as they got heavier, their skeletons grew in a predictable, steady rhythm. However, once the bats reached adulthood, this tight connection broke. In the grown-ups, body weight no longer tracked with skeletal size. Instead, the extra weight in an adult bat likely represented fat reserves or the physical demands of pregnancy and nursing, rather than the construction of a larger frame. This means that for a baby bat, being heavier usually means being structurally bigger, but for an adult, being heavier might just mean having more energy stored up for a rainy day.
When the researchers looked at the energy use, or basal metabolic rate, they found that while heavier bats did tend to use more energy overall, body weight was a surprisingly poor predictor of exactly how much. The weight of the bat explained only a tiny fraction of the differences in energy use between individuals. Two bats of the exact same size could have very different resting energy costs. Furthermore, the scientists found that factors like whether the bat was male or female, whether it was a pup or an adult, or whether a female was currently nursing a baby did not significantly alter the energy cost once the size of the animal was taken into account. The expected spikes in energy use for growing young or nursing mothers were not the dominant drivers of variation in this group.
This suggests that the energy bill for these bats is not set by a simple formula based on size or age. Instead, the variation in energy use appears to be driven by a complex mix of individual differences that are harder to see from the outside. Some bats might simply be more efficient at burning fuel, while others might be carrying different amounts of internal reserves or facing different levels of stress. The study indicates that within a single population, especially one full of animals at different stages of life, the rules of energy expenditure are flexible and highly individual. Rather than a rigid relationship where size dictates cost, the metabolic rate of these bats seems to be a dynamic trait, shaped by a hidden variety of physiological and environmental factors that allow each animal to manage its energy in its own unique way.
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