Common Bullfrog — Part 2

Rana catesbiana Shaw

The colour varies greatly, not only according to the sex, but also among individuals of the same sex. The Bullfrog has power to change colour considerably. The general colour of the upper parts is dull olive-green, marked with irregular, dusky spots of brown. However, when the frog is in warm air, and exposed to bright light, the skin may become a beautiful spotless yellow-green, very light in tone. The Bullfrog just from the mud, or from some place of concealment in deep water, is so dark-coloured that he is nearly black. Experiments prove that light has much to do with these changes of colour; at least, that these changes in colour take place with changes of light, when temperature and moisture conditions remain the same. (See pp. 22 to 27 for discussion of change of colour in the Batrachia.)

The female is usually more brown and spotted, and the male more nearly plain green. The under parts of both are white, with distinct or indistinct mottlings of brown. The male has a bright yellow throat; that of the female is dirty white, mottled with brown. The ear of the male is much larger than that of the female.

This description holds regarding the Bullfrog of northeastern North America. In southern New York, and still farther south, the Bullfrog is more likely to be spotted, and the male lacks the bright yellow on the throat. The Bullfrog of Texas has the underparts vividly marked everywhere with irregular, connected black spots. This frog of the Southwest has also a more tubercular and leathery skin. There are probably three or four distinct varieties of this species in different localities of North America. It is certain that the Bullfrog of Wisconsin and adjoining regions is different enough to be classified as a variety. It is finely spotted with dark on all parts except the head. There is much more bright yellow in its colouration than in that of Bullfrogs from other parts of North America. The male has this colour not only on the throat, as in the case of the Eastern Bullfrog, but also along the sides of the body and on the legs and feet to the tips of the toes. The head is relatively more pointed than that of other Bullfrogs. The body is relatively longer; the head is contained three and one-fourth to three and one-half times in the total length, whereas the usual proportion is one to three. The eyes are surprisingly small and little elevated, giving the flat head an expression greatly lacking in intelligence and alertness. The longitudinal diameter of the eye in a frog 6 1/2 inches long is a short 3/8 inch, while it is fully 1/2 inch in an Eastern Bullfrog measuring 5 3/4 inches. The smaller size of the eye results in a greater space between the eye and the nostril and a longer fold of skin above the ear. In this frog the vocal sacs of the throat extend backward over the arm so that when the frog croaks, not only does the throat swell, but pouches appear between the ear and the arm as in the case of the Leopard Frog. The increase in vocal powers in this frog is shown also in the fact that the female croaks in a manner similar to that of the male, but without quite the same force and without the appearance of pouches at the sides of the head.

The Common Bullfrog is a powerful swimmer, with great strength and length of hind legs (which may measure from seven to ten inches long) and with very large webs. It is interesting to see one dive under the water. The legs are straightened powerfully, and then slowly drawn forward into position for the second stroke. The huge web is alternately extended into a flat resisting membrane and folded again as the leg is drawn forward. The eyes are shut; that is, they are flattened until they are level with the head, by being lowered in their sockets, which project downward from the roof of the mouth. This takes the eyes out of danger during the swift motion through the water, but also makes it necessary that the frog shall swim rapidly but a short distance at a time. He must stop, or slacken speed and open his eyes to see where he is, and then, perhaps, may make a new plunge with eyes lowered, if an enemy is near. The frog, like the toad, has no outer ear to hinder him in swimming. The ear-drum is at the surface of the head, covered and protected merely by the moist skin.

Something else is seen to take place, also, as the frog dives under water. Large bubbles of air are given off from the nostrils, which are then closed tightly. The frog does not use the lungs in breathing under water; the nostrils are kept closed, and the throat shows none of the swallowing movements so conspicuous when the frog is breathing air. The frog's moist skin is like a great gill stretched over the whole body. In consequence the frog not only can live under the water for months at a time, but will, by preference, spend a very large portion of his time lying, with flattened body and closed nostrils, at the bottom of the pond.