Limbs in Mammalian Evolution

The more than 4,000 living species of mammal have infiltrated almost every habitat in the world. From alpine mountaintops to plains grasslands, from aerial heights to the depths of the ocean, from slender forest branches to narrow subterranean burrows, mammals occupy an extraordinary variety of substrates. Some are capable of running, swimming, or swinging at great speeds, while others creep slowly along limbs or push laboriously through the soil. The limbs of mammals reflect the diversity of their habitats (fig. 15.1). The long, slender legs and two-toed feet of the antelope allow it to survey the plains for predators and bound off at their approach. The short, muscled arms and broad, thick-clawed hands of the mole scrape through soil as the animal searches for worms and grubs. Spidery, elongated hands and fingers with webs of skin between them propel bats on their erratic chases after night-flying insects. Dense, paddlelike fins steer whales through the watery course on which they are propelled by their massive tail flukes. Limbs are crucial for mammalian locomotion, social behavior, and feeding.