There are several important features of tropical mammals and their habitats that differentiate them from temperate-zone mammals. First, tropical mammals face different environmental stresses than do temperate-zone mammals, and they respond to stresses in different ways. Many temperate-zone mammals, of course, must endure extreme variation within a year; from cold winters with snow and low food supplies to hot summers with dry weather and abundant food. Many mammals respond with hibernation, staying more or less dormant for several months until conditions improve. Tropical mammals, except in the high-altitude mountains, do not encounter such extreme annual changes, but they do face dry seasons, up to five months long, that sometimes severely reduce food supplies. For some surprising reasons, they cannot alleviate this stress by hibernating, waiting for the rainy season to arrive with its increased food supplies. When a mammal in Canada or Alaska hibernates, many of its predators leave the area. This is not the case in the tropics. A mammal sleeping away the dry season in a burrow would be easy prey to snakes and other predators. Moreover, a big danger to sleeping mammals would be army ants. These voracious insects are very common in the tropics and would quickly eat a sleeping mouse or squirrel. Also, external parasites, such as ticks and mites, which are inactive in extreme cold, would continue to be very active on sleeping tropical mammals, sucking blood and doing considerable damage. Last, the great energy reserves needed to be able to sleep for an extended period through warm weather may be more than any mammal can physically accumulate. Therefore, tropical mammals need to stay active throughout the year. One way they counter the dry season's reduction in their normal foods is to switch food types seasonally. For instance, some rodents that eat mostly insects during the rainy season switch to seeds during the dry season; some bats that feed on insects switch to dry-season fruits. The abundance of tropical fruit brings up another interesting difference between temperate and tropical mammals: a surprising number of tropical mammals eat a lot of fruit, even among the carnivore group, which, as its name implies, should be eating meat. All the carnivores in Brazil, save pumas, jaguars, and otters, are known to eat fruit on occasion. Upon reflection, however, it makes sense that these mammals consume fruit. Fruit is very abundant in the tropics, available throughout much of the year, and, at least when it is ripe, easily digested by mammalian digestive systems. A consequence of such frugivory (fruit eating) is that many mammals have become, together with frugivorous birds, major dispersal agents of fruit seeds, which they spit out or which travel unharmed through their digestive tracts to be deposited in feces far from the mother tree. Some biologists believe that, even though the carnivores plainly are specialized for hunting down, killing, and eating animal prey, it is likely that fruit has always been a part of their diet. Finally, there are some differences in the kinds of animals inhabiting tropical and temperate regions. For instance, in tropical regions there are few social rodents like beavers and prairie dogs and very few rabbit species. On the other hand, some groups occur solely in the tropics and do extremely well there. There are about 75 to 100 species of New World monkeys (depending on which primate specialist you consult), all of which occur in tropical areas. Arboreal (tree-living) mammals such as monkeys and sloths are plentiful in tropical forests, probably because there is a rich, resource-filled, dense canopy to occupy and feed in. The closed canopy blocks light to the ground, which allows only an undergrowth that is sparse and poor in resources, and consequently permits few opportunities for mammals to live and feed there. Bats thrive in the tropics, being very successful both in terms of number of species and in their abundances. Nine families of bats occur in Brazil, including more than 140 species; only four families and 40 species occur in the entire United States, an area similar in size to Brazil. While most North American bats feed on insects, the diets of Brazilian bats are more varied and include fruit, nectar, and fish.