it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts. mammals born with well-developed regulatory systems, such as the horse and giraffe, tend to have less rem sleep than the species which are less developed at birth, such as cats and rats. this appears to echo the greater need for rem sleep among newborns than among adults in most mammal species. many mammals sleep for a large proportion of each 24-hour period when they are very young. the giraffe only sleeps 2 hours a day in about 5–15 minute sessions. koalas are the longest sleeping-mammals, about 20–22 hours a day. however, killer whales and some other dolphins do not sleep during the first month of life. instead, young dolphins and whales frequently take rests by pressing their body next to their mother’s while she swims. as the mother swims she is keeping her offspring afloat to prevent them from drowning. this allows young dolphins and whales to rest, which will help keep their immune system healthy; in turn, protecting them from illnesses. during this period, mothers often sacrifice sleep for the protection of their young from predators. however, unlike other mammals, adult dolphins and whales are able to go without sleep for a month.