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Pacific Tree Frog — Part 2
Not only does the background of colour vary thus greatly, but the colour pattern is so variable that at first sight it may be difficult to see what are the elements of the common pattern. This common pattern consists of a triangular patch between the eyes and lengthwise bands on the back. There may be small spots placed irregularly between the bands, and on the sides of the frog below the bands. But there is what seems to be nearly every possible disguise of the pattern. The triangle between the eyes may be very large, extending well up on the eyelids, and sending a point far back on the neck, or it may be decidedly small. It may be of solid colour, or only in outline. It may be separate or connected with the band in the midline of the back, or it may be broken into two or occasionally three spots. The longitudinal bands of the back may be distinct broad bands, two, three, or occasionally five in number, or they may be broken into spots rounded or elongated, regular or irregular in shape. The bands may have parallel sides, or may be fantastic in shape. They may be broad and conspicuous, and if there are spots instead of bands, these may be many and crowded, or bands and spots may be so reduced that they are scarcely noticeable.
Judging from the one hundred fifty specimens examined from regions about San Diego and San Francisco, California, and Seattle, Washington, those from the south are more likely to have the dark colour in lines or bands with few spots between and on the sides, and those from the north to have broken bands and crowded spots showing much more dark colour. The specimens from the north, besides having more dark colour, measure considerably larger than those at the south.
Hyla regilla has the power to change colour rapidly. Let us isolate two and observe them. They have been in wet moss in a dark place, and are nearly black in colour. When looked at more closely, one is seen to be a very dark green, and the other a rich dark brown. There is no trace of spots; only the light band above the jaw shows. On exposure to bright light, the colour immediately lightens; one becomes decidedly green, the other decidedly brown. Soon a pattern of spots and bands shows obscurely on their backs. This pattern becomes more and more distinct as the background grows lighter. When the medium shade of green or brown is at its brightest, the pattern shows most vividly. However, if they still continue to remain in bright light, we could scarcely find time to draw the pattern while it is vivid, for as the background continues to grow light, the spots begin to be dulled. They become less and less distinct, the background becomes still more light in tone, and the pattern wholly fades, and leaves one frog metallic fawn colour with golden reflections, and the other yellowish green of a very light tone. This change from a colour so dark that there are no spots to a light unspotted colouration may take place within eight or ten minutes. (See Colour Plate IX.) Putting the frogs into a dark situation again, they reverse the change, become vividly spotted, and finally dark and unspotted.
Any given collection of these tree frogs taken from the pond will contain spotted specimens, unspotted dark and unspotted light ones, and many in which the pattern shows obscurely. But all will develop the typical pattern vividly if they are put under the right conditions. Of the one hundred fifty specimens examined, no frog was found in which the pattern was wholly lacking, though three out of the number had the pattern so reduced that it consisted of a very small triangular patch between the eyes and mere dashes of colour on the back. (See frogs in lower right-hand corner of Colour Plate VIII.)
The dark band through the ear and its continuation in dark spots on the sides are constant factors in the colouration. The only time that they are not visible is when the background is as dark as the markings.
Hyla regilla lives low, hopping about on the ground among the blackberry vines and other vegetation, as does Hyla pickeringii of eastern North America. It is found especially about springs, ponds, and moist places of all sorts. It calls from some place of concealment in these situations on dark or rainy days and at dusk. It may be heard occasionally throughout the winter, even. In spring, it is to be found in great numbers in shallow pools. The frogs then sing in chorus (especially at night), and a small number of them can produce a noise so great that it sounds as though made by thousands.