Page 28 - Amphibian

Continued from previous page

Legs at full stretch
DIVE, DIVE, DIVE
Down, down, down toward the water’s
surface, the frog holds its arms backward
and its legs out straight with the toes
pointed. Now the frog’s body is almost
streamlined, and as it descends into the
water, it meets little air resistance.
In and out of the water

Water plays an essential part in a frog’s life. Many Typical cream-colored
ridge along upper jaw
species have to return to the water to mate and to lay and also along body
and fertilize their eggs (pp. 32–33). The young pass through
various aquatic tadpole stages (pp. 38–39) before they change into
land-dwelling adults. Because water is denser than air, it takes more effort
for a frog to move in water than on dry land. Frogs that live in fast-flowing
water, where there is a danger of being swept away by the current, have developed
grasping fingers and toes for clinging to weeds or rocks. Similarly, their tadpoles
have flat bodies that water can easily flow over and large sucker-like mouths with
which they can attach themselves to rocks and boulders. Once in the water, a frog must
overcome the water’s surface tension in order to leave it. Surface tension is caused by
water molecules at the surface clinging together so tightly that water seems to have
a thin, elastic film covering its surface. Northern leopard frogs use their powerful leg
muscles to leap free from the water, while tree frogs (pp. 50–53) climb out helped by
powerful suction pads on the ends of their fingers and toes.

Hind legs kicking out
and down together

An Oriental
fire-bellied toad
swimming

Drawing
knees up
Front legs
extending
backward

SWIMMING FROGS AND TOADS
When frogs and toads swim, they draw their hind legs
upward toward their bodies; then, in order to propel
themselves forward, they shoot them out backward in a
straight line. To aid this forward motion they bring their front
legs down to their sides to form a streamlined shape. People
swim in this way when they do the breast stroke. Tadpoles
swim like newts and aquatic salamanders (pp. 28–29), with
their newly formed limbs held against the sides of the body.
When a tadpole develops into a frog, it loses its tail and has to
learn to swim using its legs. The froglet’s body is too short for it
to continue swimming like a tadpole using “S-shaped”
movements, so the hind legs must become strong enough
to propel it through water. Arms held flat
against body

Free-floating Front legs stretched
when at rest forward, as hind
Front and hind legs kick out
legs move together