Additionally, birds and mammals can increase their metabolic rates to maintain a constant body temperature in the cold, but reptiles cannot. Therefore, some sleep researchers have speculated that REM sleep is related in some way to temperature regulation. In addition, because we are not certain what the function of sleep is in mammals, birds, reptiles and other species, we cannot be sure that sleep serves the same function in all animals. One prominent theory proposes that how much an animal sleeps is largely determined by its status as prey or predator-that prey animals sleep less because sleep makes them vulnerable. This theory is difficult to evaluate because predator-prey status is not generally as obvious as in the case of the lion and zebra, or the wolf and caribou.
Some species are both predator and prey, and judgments of their vulnerability are often subjective.
Also, it is far from clear that sleep significantly increases vulnerability to predation. The victims of predators are generally the very young, the sick, and the old, even when they are wide awake. The reasons for the relationship between sleep and size are not entirely clear, but it is thought to have something to do with energy conservation, which is a much greater problem for smaller mammals than for larger mammals. For example, the energy expense for locomotion is greater in smaller mammals. Because a smaller proportion of their weight is comprised of fat, smaller mammals have a more limited capacity for energy storage.