PACIFIC TREE-TOAD. Hyla regilla Baird and Girard

Field characters.—Size small; total length 2 inches or less; ends of toes with small rounded discs. Coloration extremely variable, ranging from pale light gray through vivid green and brown to nearly black; a dark streak is always evident on side of head extending from tip of nose at level of eye to behind ear membrane; under surface white, unspotted, blackish on throat in males; back may or may not be marked with dark streaks or spots. Voice: Song note a loud, raucous kreck-ek, repeated at frequent intervals; call note a single low prolonged, guttural kr-r-r-eck.

Occurrence.—Distributed throughout the Yosemite section without regard to life zone. Recorded from Snelling eastward across Sierra Nevada to Walker Lake and up to 10,600 feet altitude on Conness Mountain. Lives chiefly in damp situations and on the ground, seldom being found in trees or even in bushes. Essentially solitary except when spawning.

The Pacific Tree-toad is one of the very few species of animals in the Yosemite section which ranges uninterruptedly from the San Joaquin plains to the highest passes of the Sierra Nevada. Of other land vertebrates only the Gambel White-footed Mouse, the Red-shafted Flicker, and the Western Chipping Sparrow can be said to do the same. The range of the Pacific Tree-toad is most remarkable when we remember that it is a 'cold-blooded' animal which has a body temperature always close to that of its environment, while each of the other species mentioned has a heat-regulating mechanism which maintains its body at practically constant temperature irrespective of that of the surroundings.

At Snelling the air temperature at different seasons of the year varies from slightly below freezing to above 110° F., while on Tuolumne Meadows it undoubtedly goes below 0° F. in winter and may reach above 85° F. in summer; yet tree-toads are found in both of these places. The animals keep to moist situations near the surface of the earth, where the temperature fluctuations are somewhat less than those given, and in freezing weather must of necessity seek shelters below the frost line. Even so, the Tree-toads must be subject to considerable variation in the temperature of their surroundings. The hardihood of these diminutive creatures is indicated by the fact that in Yosemite Valley on February 28, 1916, when there was two feet or more of snow on the Valley floor, tree-toads were chorusing in open marshy ponds below the Royal Arches; and on Mount Hoffmann on June 29, 1915, their voices were heard coming from beneath deep snow banks on the north side of the mountain. On July 8, 1915, several were croaking in a small lake covered almost completely with ice on Conness Mountain at 10,600 feet altitude. By way of contrast it may be mentioned that on May 27, 1915, at Snelling, when the air was to us uncomfortably hot, tree-toads were heard in voice near the river margin.

Among all the toads and frogs of the Yosemite region the Pacific Tree-toad may be known at a glance by the expanded discs on the ends of all its toes and by the presence of a streak of dark color which extends along the side of its head, at the level of the eye, from the tip of the nose to behind the ear membrane. These features are evident at all ages, from the smallest individuals recently transformed from tadpoles to the largest adults.

Many amphibians use their voices almost exclusively during the spawning season, but the Pacific Tree-toad is likely to be heard at any time of year, being silent, if at all, only during the dry hot days of late summer. Thus, in Yosemite Valley, the notes have been heard by us in February, May, June, October, and December; at Snelling in January and May; at Chinquapin in May and June; about Tuolumne Meadows in July; at Vogelsang Lake on August 30; and at McGee Lake and Ten Lakes in October. During the fall months the single note is usually the only one heard, while from January to June the two-syllabled song note, as well as the call, is given commonly.

The call-note consists of a single prolonged guttural syllable, kr-r-r-r-eck, which it requires a second or two to pronounce. This note is uttered at irregular intervals, by isolated individuals when in their retreats and by members of a group in a pond or marsh when their song chorus has been interrupted.

The 'song' note is a two-syllabled kreck-ek given in faster time than the call and repeated at short intervals. There is slightly less emphasis on the second note, which drops in pitch and ends abruptly. When a number of Hylas are in a marsh or pool together, their notes tend to be given in unison, so that there is a continued series of notes, every alternate note being slightly stressed. Such a chorus may continue for a long period, but more often it is interrupted, ceasing rather abruptly and then beginning again after a period of quiet. If a person walks out into a meadow where tree-toads are chorusing, the voices soon cease. One or more call notes with an interrogative inflection are given, and then there is silence. If the observer stands absolutely still, the animals will shortly resume their singing. The chorus begins as it ended, with a few call notes; then one individual commences his song, to be quickly followed by another, and in five seconds or so the place resounds with the chorus once more.

The tree-toad is quite the strongest voiced of any of our amphibians, though it is the smallest in point of body size among the tail-less forms. Only the males sing, although females may give the low call note. The males when held in the hand are seen to possess loose folds of black skin on the throat. When the animal is singing, this skin is inflated to form a pouch which swells out beyond the chin and is kept continuously inflated while the notes are being given. The pouch evidently acts as a resonator and helps to give volume to the sound.

The eggs of the tree-toad are laid during the spring months. No egg masses were seen anywhere in the region, but it seems likely that the individuals living in the higher altitudes spawn at much later dates than those in the lowlands. On August 20 and 23, 1915, at Merced Lake, small tree-toads, measuring between one-half and three-fourths of an inch in length, were found in numbers. These were animals which had but recently changed to the adult condition, and which had undoubtedly come from eggs laid during the same season.

Among the specimens of this species collected in the Yosemite region are two individuals taken at Merced Grove Big Trees and Porcupine Flat, respectively, which after being preserved are nearly 2 inches (1-7/8 and 1-15/16) in total length and are fully as large if not larger than any we have seen from anywhere. The cup-like discs or pads at the ends of the toes on these animals are unusually large and conspicuous and in life must have been fully twice the diameter of the toes themselves.

In Yosemite Valley on October 13, 1915, scores of half-grown Hylas were seen in a saw grass swale. There were also many grasshoppers there and it was often difficult to distinguish a toad from a grasshopper until the individual was scrutinized closely. The tree-toads exhibited much variation in color, ranging from uniform bright greenish yellow to dark brown; some were coarsely mottled with dark and light brown.