YELLOW-LEGGED FROGS. Rana boylii Baird⁴⁴

Field characters.—No conspicuous raised glands on hind neck or back; hind toes long and slender, fully webbed, without discs at tips. Total length 3 inches or less. Upper surface of body rough-surfaced, with scattered low points. (See pl. 60b.) Coloration above blackish, dark green or brown, with markings few and indistinct; lower surface yellow or whitish, sometimes mottled on throat. No light line along upper jaw; ear region not darker than rest of head.

Occurrence.—Common resident practically throughout the Yosemite region. Recorded from Pleasant Valley, eastward to near Mono Lake. In Transition Zone on both sides of mountains, and in Upper Sonoran on west side, is subspecies boylii; in Canadian and Hudsonian zones is subspecies sierrae (see footnote for details). Lives in, and on the banks of, ponds and streams.

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⁴⁴Two subspecies of Yellow-legged Frog are found in the Yosemite region. These occupy separate parts of the region and also present characters which make possible identification in the field.

CALIFORNIA YELLOW-LEGGED FROG, Rana boylii boylii Baird, resident throughout the Upper Sonoran Zone of central California, was found at Pleasant Valley and thence eastward to near Feliciana Mountain and to Smith Creek, east of Coulterville; it recurs in vicinity of Mono Lake (Farrington Ranch). It is distinguished by having many small points or roughnesses on the ear membrane and by a relatively long hind leg (when leg is bent forward the bent 'instep' reaches to or beyond nostril).

SIERRA YELLOW-LEGGED FROG Rana boylii sierrae Camp, resident in boreal portions of the Sierra Nevada, was found in the Canadian and Hudsonian zones from Peregoy Meadow and Porcupine Flat eastward to Tuolumne Meadows and the head of Lyell Cañon. It has a relatively smooth ear membrane, and when the hind leg is bent forward the bent 'instep' does not usually reach beyond the nostril.

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The Yellow-legged Frog is the commonest amphibian in most parts of the Yosemite section. Its total range is slightly less than that of the Pacific Tree-toad; but its numbers, especially at the higher altitudes, far exceed those of the smaller species. This frog is the species most likely to come to the attention of fishermen and others who may walk along the banks of Sierran streams and lakes.

The Yellow-legged Frog may best be identified by the characters which it lacks when compared with other species of frogs and toads from the same region. (See pl. 60b.) It differs from the Spadefoot in having a round instead of an elliptical pupil, from the California and Yosemite toads in the absence of enlarged glands on its shoulders and of prominent 'warts' on the back, from the tree-toad in not having expanded adhesive discs on its fingers, and from the Red-legged Frog in having no dark spot on the ear region, no ridges along the back, and no red in its coloration (save in individuals afflicted with the disease known as "red-leg"). The characteristics of the two local subspecies of Yellow-legged, Frog are set forth in footnote 44.

During the daytime these frogs are to be seen sitting quietly on rocks or other places close to the water. If a person is walking along the shore of a stream or pond his attention is usually first drawn to the animals when one of them 'plops' into the water and makes for the bottom. In the higher zones one's progress along the bank of a pool is announced by a series of splashes ten to twenty-five feet ahead, as the numerous frogs in quick succession take to the safety of the water. Once under the surface a few quick strokes of the hind legs with their broad foot-webs put the frog under some sheltering rock. The mottled pattern of the upper surface is quite protective in character when the animal comes to rest. When once on the bottom the frog is likely to remain there quietly unless further disturbed. When the frog is in motion, either jumping or swimming, the yellow color on the legs shows contrastingly against the dark upper surface of the body, but it is almost entirely masked when the frog is at rest.

Certain of the lakes in the higher parts of the Yosemite contain large numbers of Yellow-legged Frogs in both the tadpole and adult conditions. It is a commonly repeated observation that frogs, in tadpole form at least, do not occur in lakes which are stocked with trout. Adult frogs are sometimes found around the margins of such lakes and they occur in numbers along the shores of streams inhabited by trout, but the advent of fish in a lake sooner or later nearly or quite eliminates the frogs. It seems probable that the fish prey upon the tadpoles, so that few or none of the latter are able to reach the stage at which they transform. The frogs which live along the streams probably spawn in small temporary pools in the meadows which the trout cannot reach.

The spawning season of the Yellow-legged Frog varies with altitude, although in each locality the adults, as a rule, probably lay their eggs when the season is locally 'spring.' Thus tadpoles of considerable size were seen in Blacks Creek near Coulterville on May 10, 1919, and the one adult female of breeding age collected at Smith Creek on June 3, 1915, had finished laying. An exceptional case, perhaps, was that of an adult female taken near Feliciana Mountain on November 1, 1915, which contained well developed eggs. Most of the high mountain frogs (sierrae) collected at Peregoy and Mono meadows on June 22, 1915, had already laid, and tadpoles were seen in some of the creeks. Some females collected at and near Tuolumne Meadows during the first half of July, 1915, had already deposited their eggs; others contained eggs ready to lay.

In the foothill district, where there is a long spell of warm weather, the tadpoles (subspecies boylii) are able to grow to the size necessary for transforming into frogs in a single season. But with the high mountain animals (subspecies sierrae) the case is different. The eggs are not laid until June or July, and there is then but a short season, scarcely three months in length, before cold weather sets in again. Consequently the tadpoles which hatch from the eggs in any one season go through the winter still in the tadpole condition and do not transform into frogs until the following summer. Thus the numbers of tadpoles, 2 inches or more in length, found in Young Lake on July 8 and 9, 1915, came from eggs which had been laid in 1914. On the dates mentioned many of the tadpoles had the hind legs fully developed and in all probability would soon have completed their metamorphosis.

In such alpine lakes as are suited to occupancy by frogs (through the absence of fish) both adults and tadpoles are usually present. The frogs sit along the shore, on the ground or on rocks, whence they can reach the lake at one bound. When cakes of ice are floating in the water the frogs do not seem able to discriminate and in leaping lakeward they sometimes land on the ice instead of in the water. Where large numbers of frogs are present, a greater degree of safety is probably enjoyed by each individual, for all, of course, are on the alert, and thus the approach of any danger is the sooner realized from the action of a neighbor. In spite of this consideration, a person does not have much difficulty in capturing numbers of the frogs, and it seems likely that a coyote or other carnivore would be able to gather them in easily by prowling along the shore.

When undisturbed the tadpoles rest on the sandy bottom close to the shore, where the water is shallowest and warmed somewhat by the sun; but when frightened they wriggle off into the deeper parts of the lake.