![]() ![]() This layer of fat also serves as extra insulation. Brown fat has more mitochondria than white fat, and these mitochondria act like fat-burning stoves which produce heat quickly. In winter, they can generate heat by releasing the energy stored in this brown fat through metabolism. Like other mammals, white-footed mice build a layer of brown fat ( brown adipose tissue) in response to colder weather in mid-September. This hemoglobin performance boost is the same process that got US bikers in trouble for “blood doping” at the 1984 Olympic Games in Los Angeles. With more oxygen, mice can increase their metabolic rates and their ability to keep warm. Hemoglobin is the protein in blood that transports oxygen into and carbon dioxide out of every cell. These nocturnal rodents typically enter torpor at sunrise and are active by late afternoon.Īs fall weather becomes colder, white-footed mice produce new red blood cells with higher hemoglobin content. White-footed mice enter a state of daily torpor when the air temperature falls below 3˚C (37˚F), at which time they will lower their body temperature by about 17 degrees and slow their metabolism by 75%. Torpor is an energy-conserving adaptation where an animal reduces the need to burn calories by slowing all non-essential activities to a bare minimum. White-footed mice will occasionally cache food in the subnivean zone, but this can be a risky gamble because a January thaw could expose their precious food supply, leaving them less able to survive the bitter weather still to come. The mice will build a network of tunnels that allow them to forage for food while protected from the bitter cold and most predators. If the snow is deep enough to provide insulation, the temperature in the subnivean zone will remain just a few degrees below freezing, even if the air above is well below zero. The subnivean zone is the narrow boundary between the top of the soil and the bottom of the snow layer. White-footed mice are one of several species to take advantage of the subnivean zone. An abandoned woodpecker hole or hollow branch are typical locations for mice nests, but they are just as likely to build their nest under the hood of a diesel tractor. The insulation often consists of plant material but could also be from the upholstery of a winterized camper or a collection of feathers from a chicken coop. To conserve life-sustaining energy on cold days, white-footed mice will huddle together in groups of two to six in nests insulated with whatever soft, fluffy material they can find. Juvenile white-footed mice reach sexual maturity at about 44 days. At about three weeks, the young mice are weaned and independent of their mother. The pups are born naked and blind ( altricial) after a gestation period of 22 to 28 days and open their eyes after two weeks. Both males and females will breed with multiple mates ( polygamous), so one litter often has newborns from different fathers. White-footed mice produce 2 to 4 litters per year, each with 2 to 9 offspring. They rely on their food caches and brown fat deposits for survival. White-footed mice are active all winter but remain in their nests during subzero temperatures. The female continues to produce pups throughout the summer.Īfter the last litter has been weaned and dispersed, mice will prepare for the hardships of winter by caching food and building a layer of brown fat. The spring thaw reveals the tunnel system that protected these non-hibernating rodents from predators and extreme weather. White-footed mice breed from March to October. ![]()
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