Mammals range in size and shape from diminutive, insect-eating shrews to massive herbivorous elephants (the largest of modern land animals), and the structural adaptations displayed by these hairy, milk-bearing vertebrates vary enormously. From the big antler rack of a wapiti bull to the prehensile tail of a New World monkey, the form and features of mammals reflect myriad ways of living in the world.
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Adaptations for Locomotion
Mammals use an impressive variety of locomotion styles. Some have returned to their ultimate ancestral home of aquatic environments, which have necessitated extreme structural adaptations. Otters have webbed paws and sleek, streamlined forms for underwater foraging; seals, sea lions and walruses are similarly contoured and sport flippers for propulsion and steering; and whales, dolphins and porpoises have lost hind legs entirely, moving around with powerful tail flukes and breathing via blowholes at the top of their heads. Worlds away, moles have heavy foreclaws for burrowing in soil. Many ungulates and large carnivores have lean forms and long legs for running or pouncing. The long hind legs of jackrabbits, kangaroo rats and jerboas help propel these small creatures across open country to escape their many predators. Bats are the only true flying mammals, their wings formed from thin flaps of skin stretched between highly elongated forelimbs and clawed feet.
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Carnivorous mammals have formidable weaponry for dispatching prey. Cats have retractable claws and sharp fangs, so they employ both paws and jaws in grappling with their victims -- which range, depending on the species, from insects and rodents to big ungulates and primates. Dogs, meanwhile, have relatively blunt, non-retractable claws (except for the grey fox). Unlike the stalk-and-ambush predatory style of a cat, dogs usually run down prey and kill it with powerful, tearing bites. To deal with the tough cell walls of plants, herbivores display a number of digestive adaptations, including the multi-chambered stomach and cud-chewing of ruminant ungulates.
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Human beings belong to the order of primates, which are some of the most specialised of the larger mammals. Most, from tarsiers to orang-utans, are mainly arboreal, well-adapted to foraging and taking shelter in the canopy. The long, powerful arms of a chimpanzee help the ape brachiate, swinging in monkey-bar fashion from branch to branch. The gangly limbs and prehensile tail of the spider monkey accommodate a slower, more dexterous negotiation of the treetops. Humans are some of the few bipedal animals, having evolved as long-distance walkers and runners. Primates have binocular vision for acutely judging distances and, in the higher ranks, opposable thumbs -- equally good for clutching branches or fashioning tools.
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Horns and Antlers
The males and some females of many hoofed mammals sport notable headgear: either horns, which are permanent bony protuberances sheathed in keratin, or antlers, structures of bone shed and regrown annually. Often these are employed as weapons of intraspecific (within-species) conflict. A bull caribou, for example, will lock antlers with a rival male for breeding rights, as will an impala buck. Some of the most spectacular structures and battles are found in the various kinds of mountain sheep, which have massive, curled horns for ramming against one another in head-cracking contests for ewes.