The common shrew weighs about 9 grams, or 3 pennies, making it one of the smallest mammals on the planet by mass. In order to survive, shrews have to eat about 8 times their body weight every day. Their heartbeats 1,200 times per minute, compared to humans 80 beats per minute. They constantly need to be eating, and can starve to death within hours. They need a lot of energy!
Many small mammals increase their metabolism--or how fast they turn food into energy--at cold temperatures to stay warm. Oxygen is needed for metabolism to occur, so when metabolism increases, so does oxygen use. Scientists discovered that at low temperatures, oxygen consumption in shrews did not change, nor did their activity rate, meaning their metabolism did not change. How then, could they still stay warm?
The shrew has an interesting adaptation to conserve energy without changing their metabolism: during cold seasons, their brains reduce 10-20% in mass, their skulls shrink up to 25% to accommodate the smaller brain, and they lose significant mass in several major organs. Even stranger is that in the spring, these losses are restored. It is unclear how these mechanisms work, but it is believed that part of the reasoning behind this evolution is to reduce the surface area heat can escape from and to reduce the amount of energy needed in organ function, particularly in energy “expensive” organ tissues, such as the brain.
Consuming less energy in the winter is critical for the shrew because of how much they have to eat in order to survive. The decrease in energy allows them to survive on the scarce food presented in the winter months. This adaptation seems extreme, but shrews are living at the very edge of what is possible. Smaller mammals do not exist because they cannot exist--any smaller and they wouldn’t be able to stay warm enough to survive. With a tiny body and enormous energy demands, shrews are a testament to nature’s resilience and ingenuity for survival.
Schaeffer, P. J., O’Mara, M. T., Breiholz, J., Keicher, L., Lázaro, J., Muturi, M., & Dechmann, D. K. (2020). Metabolic rate in common shrews is unaffected by temperature, leading to lower energetic costs through seasonal size reduction. Royal Society Open Science, 7(4). https://doi.org/10.1098/rsos.191989