The colour of the frog enables it to harmonize with a background of vegetation, and so helps it to escape notice by its enemies. Further, by the alteration of the pigment cells of the skin the colour of the frog changes according to the type of background. In dark situations the pigment in the pigment cells expands, and the frog assumes a deeper colour. A well-lit background causes the pigment to contract, and the frog becomes lighter in colour.
The skin contains glands which produce a secretion unpalatable to many animals.
The frog avoids the cold weather of winter by hibernating.
Movement on Land
When on land the frog usually moves by leaping, and an adaptation in connection with this mode of progression is seen in the great length of the hind legs. These are folded in the form of a Z when the animal is at rest, and are suddenly straightened when it jumps. The front legs are short, and are used in landing after a jump and in propping up the front end of the body. The frog has no tail, for this would interfere with the jumping.
Movement in the Water
When it is swimming the barge-like shape of the frog enables the animal to cleave the water easily. When swimming quickly the front legs of the frog are held closely to the sides and the swimming is carried out by the powerful hind legs. The toes of the hind leg are joined by a web of skin which is expanded during the swimming stroke, but the toes are automatically folded together when the legs are drawn up to the body again. At the end of a stroke the animal is arched, and for this reason the animal goes deeper into water. But when paddling this does not occur, and so the animal remains in the surface water.
The protuberance of the eyes from the head and theof the nostrils on slight prominences on the snout are adaptations to an aquatic existence, and make it possible for the frog to float completely submerged except for the eyes and nostrils which show above the surface.
Mode of Feeding
The very wide mouth and the flap-like tongue of the frog are adaptations concerned with its mode of, for the frog swallows its food alive and without mastication. The small teeth on the upper jaw, the vomer teeth on the roof of the mouth and the power of withdrawing the eyes into the roof of the mouth all help to prevent the escape of a slippery prey.