Fetuses maintained in the mother’s body rely on her to provide warmth and regulate temperature. Most birds keep their eggs warm by using a specially vascularized area of skin (the brood patch), which transfers heat efficiently to the eggs. Even after hatching or birth, the young of many species cannot regulate body temperature very well, mainly because they are small (and lose heat quickly) and have limited energy resources. So, they must continue to be protected by their parents.
Because rat pups are born without hair, they have a hard time maintaining body temperature when exposed to cold. The rat mother keeps her pups protected in a warm nest and warms them with her own body heat. Newborn rat pups are able to generate heat by using brown-fat deposits like the one between the shoulder blades, shown in Figure 1a (Blumberg et al., 1997).
Nonetheless, one problem for newborn rats is insulating their hairless bodies to conserve the heat they generate. To tackle this problem, they huddle together (see Figure 1b). The effectiveness of this strategy is easily demonstrated: placed in a room-temperature environment, an isolated 5-day-old pup will soon cool to less than 30°C, but as part of a group of four, the same pup can maintain a temperature above 30°C for 4 hours or more (Alberts, 1978), while also using less metabolic fuel. Pups frequently change their positions in the huddle, regulating their temperature by moving to the inside or the outside of the clump. All the pups benefit from this cooperative thermoregulation.
Alberts, J. R. (1978). Huddling by rat pups: Multisensory control of contact behavior. Journal of Comparative and Physiological Psychology 92: 220–l230.
Blumberg, M. S., Sokoloff, G., and Kirby, R. F. (1997). Brown fat thermogenesis and cardiac rate regulation during cold challenge in infant rats. American Journal of Physiology 272: R1308–R1313.