Small Mammals Compared with Large Mammals the size of mice and rats have only very modest capabilities

Thermoregulatory Development:
Small Mammals Compared with Large
Mammals the size of mice and rats have only very modest capabilities
to thermoregulate when they are first born. The white-footed
mouse (Peromyscus leucopus), one of the most abundant native
small mammals in North America, provides a typical example of
the course of development in such animals. The species occurs in
northern states such as Michigan and Wisconsin, as well as into
Canada. In these areas, the mice give birth to their first litters of
young each year in March and April, when the cold of late winter
still prevails. Their litters consist of 4–6 young born without fur.
In the days immediately following birth, young white-footed
mice can respond to cold exposure by increasing their metabolic
rates to a small extent, and if all the young in a litter huddle together
within the nest their mother provides, they are able collectively to
thermoregulate reasonably well for a few hours even when the air
outside the nest is near freezing. A newborn litter, therefore, can
stay relatively warm for a while when its mother is away foraging.
However, if a single newborn mouse is removed from the nest and
studied by itself, it cannot marshal a high enough metabolic rate to
stay warm even when the air temperature is +25°C—a temperature
higher than “room temperature” in American buildings. At an
ambient temperature of 25°C, the body temperature of a solitary
2-day-old soon drops from 37°C to about 28°C—not because the
animal is in some sort of controlled hypothermia, but because its
thermoregulatory abilities are overwhelmed. The young of lemmings
and other small mammals characteristic of the Far North are not
much different; to thermoregulate in their first days of life, they