Eastern Wood Frog — Part 2

Rana sylvatica Le Conte

The Wood Frog may measure 2 1/2 or 3 inches in length. Its head is pointed. The body is broad and flat. The legs are extremely long, measuring twice the length of the head and body together. The hind feet are strongly webbed. The upper parts are light or dark reddish, yellowish, or greyish brown in colour. This frog is distinguished within its own range by the dark-brown or black cheek-patches. A distinct dark line reaches from the front of the eye to the end of the muzzle. A golden yellow line extends along the sides of the upper jaw and is continued to the shoulder. The eyes, which are larger than the ears, are very prominent, and possess an alert but gentle expression. The golden iris shows a lower half much darker than the upper. The light-brown lateral folds, extending from the eyes to the posterior end of the body, are very conspicuous. (Colour Plate XIV.) The male Wood Frog, when held in the hand, talks in a vigorous purring voice, something like that of the Common Leopard Frog.

Very early in the spring, the Wood Frogs come from their winter sleep under the leaves and mossy logs. In fact, they are the first to be enticed by the spring sunshine. In February or March, after only a few days of moderate temperature (40 deg. to 60 deg. F.), the frogs are out and the eggs are laid March 23, April 8, 1890. T.H Morgan, Baltimore, Md in the pools of the woods or of the open country adjacent. The eggs are laid in masses which measure about four or five inches in diameter, and which contain from one thousand to three thousand eggs each. The masses may be attached to twigs and grasses in shallow water, or they may be free.

After the eggs have been in the water a week or more, the mass flattens and spreads greatly, and rests on the surface of the water. There are likely to be several masses close together in the water. The jelly about the eggs becomes green in colour, and thus the egg-masses bear a close resemblance in position and appearance to the floating masses of green pond-scum. The green colour of the jelly about the eggs is due to the presence of innumerable microscopically small green plants. The relation between these plants and the developing egg is one of mutual advantage. The plants feed upon the large amount of carbon dioxide breathed out by the young tadpoles, and the tadpoles get, as their share in the partnership, the free oxygen that the plants give out as a waste product from their starch-forming process. This oxygen must be of infinite value, produced everywhere in the midst of the egg-mass, because it supplies sufficient pure air for breathing, in spite of the crowded condition of the two thousand or more growing tadpoles.

These egg-masses are not easily found, partly because they are so inconspicuous, and partly because our attention is rivetted on the very conspicuous jelly masses of the spotted salamanders, Amblystoma punctatum or Amblystoma or of the marbled salamanders Amblystoma punctatum or Amblystoma opacum (which choose the same time and often the same place for depositing eggs). Eggs that are laid in water that afterwards freezes are not killed, and will develop as soon as a higher temperature returns.

As has been said, the hoarse croaking of the Wood Frog is heard only at the breeding-season. At that time, however, dozens of the frogs croak together in a most clamorous fashion. The repeated notes are low-pitched (about an octave below middle C). The notes have been compared to the quacking of ducks, but near at hand, at least, they are much less unmusical. The males alone do the croaking. They have no external vocal pouches, but, as in the Pickerel and Green frogs the throat and sides of the body over the lungs distend considerably as the sounds are produced. The breeding-season is likely to be over before the first of May. After that time their voices are never heard.

In colour, the eggs are chocolate-brown above and white below. If laid early, they develop very slowly, requiring at least a month to reach the hatching stage. If laid later, when the temperature of the water is much higher, the development is so much more rapid that the tadpoles may hatch in nine or ten days. The development is especially rapid in shallow temporary pools. The young tadpoles are nearly black in colour. The external gills become considerably longer and more branched before their absorption than do those of the Leopard Frog.

The tadpoles of the Wood Frog eat The mouth structure of R. sylvatica is like that of R. catesbiana except that the lower lip is broader and bears four rows of teeth, instead of three not only the green jelly-mass from which they themselves hatch, but also the soft green spheres within the jelly-masses vacated by young salamanders. Like other tadpoles, they act as scavengers by greedily devouring all dead animal matter of the pond.