Flippers, wings and legs —a moving subject

Press, 4 October 1985, Page 18

Flippers, wings and legs —a moving subject

By

GEOFFREY TUNNICLIFFE

Some visitors to the museum may find a display featuring animal skeletons a somewhat bewildering or even an unsavoury topic to contemplate. Such exhibits, however, can prove a most fascinating experience — especially if one examines the skeleton in relation to the environment in which the animal lives. Mammals have colonized a diversity of environments in the course of their evolution. They abound, not only in the various terrestrial habitats, but some of them have become successfully adapted to aquatic and aerial life. We should not be surprised, therefore, to find among mammals and vertebrates as a whole, a great variety of means of locomotion. These are usually grouped under three major headings: walking, swimming and flying. Under the caption “Ways of Life” the museum has featured a selection of vertebrate skeletons: one reptile (an Indian python), and six mammals — fur seal, common dolphin, hedgehog, fox, collared flying fox (a bat) and a Capuchin monkey. At first glance such an array of

articulated skeletons may prove a little perplexing for some visitors. As a starting point, therefore, it is suggested that attention be directed to examining the bones of the fore and hind-limbs. If we look at the fore-limbs first it will become apparent that all the mammals exhibited have clearly defined paired appendages in front, whereas the python lacks these. At the rear of the dolphin’s and python’s skeletons'it will be noticed that these are the only vertebrates displayed without hind-limbs. The python and the dolphin are of particular interest to zoologists as both have vestigial pelvic bones which confirm that their ancestors once walked on all fours. The conspicuous hind and fore-limbs of the fur seal provides more convincing evidence that these predominantly aquatic creatures also evolved from a four-legged terrestrial animal. The absence of girdles is consistent with vertebrates which lack limbs as. it is these structures which privide skeletal support in the body for the attachment of limbs.

Without limbs the question arises as to how such a creature as a python moves about on land. The large number of vertebrae in the python’s backbone, and associated ribs, coupled with the muscles attached to these, produce movement by the alternate mechanisms of contraction and relaxation; this gives rise to a series of waves which enable the snake to move forward. The slow movements of snakes are made with the help of muscles that connect with the ventral or “belly” scales. The flying fox (a large bat) has become strictly aerial; it is entirely dependent on flying in its search for food and for other activities. The bones which support the bat’s wing are those of the typical forelimb: the humerus from shoulder to

elbow, then the radius and ulna, carpals, metacarpals, and digits. Whereas a bird’s inner arm supports the bulk of the wing, in bats it is the hand which supports the greater part of the wing. As in the bird and bats the locomotory power of a fur seal (when it swims) comes from its fore-limbs; the hind-limbs are rarely active. The whole forearm and hand of the fur seal projects from the body and forms the large flipper used for propulsion. Unlike the fur seal, dolphins and their relatives are powered completely by their tails; the flippers are involved only in steering and balance. Just as the dolphin tail is often described as an extra limb, so too has the tail of New World monkeys (such as the Capuchin) been interpreted. Sometimes referred to as a “fifth hand,” the tails of monkeys are used to grasp distant objects, freeing the hands and feet for other activities such as feeding. The tail also facilitates balance. All of these functions combined are very useful adaptations for animals which spend most, or all of their lives in trees. The method most frequently practised by the majority of land mammals is four-footed or quadrupedal locomotion— the walk. The machanics of the walk in most hoofed animals and members of the cat and dog families may be schematized as follows: the animal raises the two diagonally opposite feet, for example the front right and hind left; it advances them while the other diagonal pair supports and propels the body forward. The animal then replaces on the ground the feet it has advanced and raises the other two. All mammals have limbs that follow the basic pattern seen in reptiles and birds. In the most primitive condition, running mammals stand on the whole length of the foot. This method of locomotion is still used by man, apes, monkeys and bears. The first action in running is to raise the heel and instep so that only the lower surface of the digits touches the ground. This is the position adopted by dogs and cats. Some mammals stand right on the tips on their toes, which are then provided with immensely enlarged nails or hooves. This condition is found in elephants, pigs, horses and cows. Space does not permit a complete coverage of the various ways in which animals move, for example pacing, trotting, galloping, leaping, brachiation (two-handed locomotion), flapping flight, and gliding. It is important to realise that nature does not recognise rigid classifications and it is possible to walk, swim and fly in many ways. The two important systems associated with locomotion — the muscles and Skelton — are entirely interdependent and the particular features of the skelton are an i expression of the requirements of ('the musculature.