Because Stillwaters has been tracking non-native Northern green frogs locally, we are often asked about another native frog species that is green: the Pacific tree frog.

Some people are surprised to learn that these little frogs can be green or brown. Others, who’ve had resident frogs in their yards for weeks or months at a time, have suspected that not only are these green and brown frogs the same species, but they may even be the same individuals.

Science has confirmed that suspicion – Pacific tree frogs can change color. Since the 1850s, biologists have known that the Pacific tree frog has green and brown color morphs and that the numbers of green vs. brown frogs differ from place to place and from season to season. Green and brown frogs were assumed to be camouflaged in different habitats and at different times of the year. However, individual frogs were also reported to have changed color between multiple captures.

In the early 2000s, biologists studying tree frog populations in California and Washington did experiments to determine whether and under what circumstances individual frogs change color. A study of California tree frogs identified three different morphs: green frogs and brown frogs that did not change color and a “color-changing” morph that changed gradually over a matter of days or weeks.

Given a choice, the non-changing frogs would choose backgrounds that matched their colors, attempting to camouflage themselves. In contrast, the color-changing frogs did not prefer matching backgrounds, but, in field studies, they changed from green in spring to brown in summer/fall, apparently tracking color changes of their environments.

While two-thirds of the California frogs did not change color, in a separate study, all Pacific tree frogs from a Washington population could change color. The Washington frogs also changed color more quickly, in as little as one hour. Color change occurred faster from brown to green than from green to brown, although the frogs were ultimately better able to match brown than green backgrounds.

Color change was also faster at lower temperatures and light levels, although again the frogs matched their backgrounds better when they changed more slowly under higher temperatures and light levels. In addition to providing camouflage, color change may help regulate body moisture and temperature.

These studies are far from the last word in explaining why Pacific tree frogs change color – or not. Do Washington and California frogs really differ so much in their color-changing abilities or were the observed differences in the prevalence of color-changing and its speed due to differences in the experiments themselves? We don’t yet know, but the tiny, common Pacific tree frog will provide rewarding opportunities for future studies.

Melissa Fleming recently retired as program director at Stillwaters Environmental Center, which has a monthly column in this newspaper.