General Information and Natural History of the Pacific Treefrog (Hyla regilla)

Susan DuBay

Introduction

The Pacific Treefrog is possibly one of the most commonly noticed amphibians in its range which extends from southern British Columbia to southern Baja California. The goals of this paper are to provide general information to people interested in learning more about the Pacific Treefrog and to supply a resource for people wishing to pursue a more in depth understanding.

Identification

Nussbaum et al. (1983) state that the Pacific Treefrog is a small frog of 50 mm or less from snout to vent, usually with a black stripe on each side of the head which passes across each eye and reaches the forelimb. The basic body color can be bright green, brown, red or gray. Green and brown are the most common colors on the coast. Black spots are often present and vary in size and pattern among individuals.

Coloration

Some have speculated that the Pacific Treefrog has the ability to change color, for example, a green frog changing its ground color to red. However, Brattstrom and Warren (1955) performed experiments which suggested otherwise. They covered half of an aquarium with brown paper and the other half with green paper. On the brown side they added brown sticks and on the green side they added green grass. Then they put brown and green Pacific Treefrogs into the tank. None of the frogs changed color or moved to the part of the tank which matched their skin color. Brattstrom and Warren then went to a reservoir that on one side contained green alga and on the other side contained brown boards and sticks. They observed that brown Pacific Treefrogs were just as likely to be found on the green side as they were to be found on the brown side of the reservoir. The same observation applied to the green Pacific Treefrogs in the reservoir. Nussbaum et al. (1983) states that the ground color of a Pacific Treefrog is under genetic control and does not change.

Although Pacific Treefrogs can not change ground color, Brattstrom and Warren (1955) note that if Pacific Treefrogs are exposed to bright light and high temperatures, the frogs will become much lighter and appear bright yellow. However, when heat and light are returned to normal levels, each frog will return to its original color. The lightening to bright yellow is caused by the contraction of melanophores which are pigment cells that contain dark pigment. Contraction of melanophores can also causes a spotted frog to lose its spots. Pacific Treefrogs seem to have some ability to regulate melanophore contraction. Brattstrom and Warren (1955) put spotted Pacific Treefrogs two aquariums; one aquarium had a solid background while the other had a patterned background. Frogs in the solid background lost their spots while frogs placed on the patterned background retained their spots.

Vocalization

Allan (1973) suggests that male Pacific Treefrogs have four different types of calls and that each call is used to produce a different result. The four calls are referred to as diphasic, monophasic, trill, and release call. The call most often heard by people is the diphasic call which functions to attracts female frogs to male frogs. The diphasic also attracts male frogs to other male frogs. If two male frogs become engaged in amplexus, a male frog will begin the release call. If a male frog senses another frog moving nearby, the male will switch from diphasic to produce monophasic calls. The trill is used by male frogs to keep other male frogs from approaching. It is possible that having a variety of different calls allows the frogs to save energy by allowing them to communicate through vocalization rather than through direct physical contact. Additionally, it allows females to find males in order to mate. The diphasic call of the Pacific Treefrog is distinctive enough to allow females to distinguish males of their species. Calling also provides a mechanism for males to space themselves out from other males in the pond.

Brattstrom and Warren (1955) suggest that Pacific Treefrog chorusing depends on temperature. They state that if it is warmer than 10°C outside, frogs will be warm enough to leave their terrestrial refuges and move to the pond. Frogs will enter the water if it is warmer than 8°C. They thermoregulate by moving in or out of the water. When their body temperature is between 10 and 20°C, they begin to call. However, Schaub and Larsen (1978) show that air temperature and water temperature are not always reliable indications of the body temperatures of amphibians. They observed Pacific Treefrogs chorusing at water temperatures of 2°C and air temperatures of .5°C.

Schaub and Larsen (1978) record that Pacific Treefrogs vocalize minimally during the day. They begin to chorus as evening approaches. After dark they chorus in full for a couple of hours, after which, the chorusing gradually decreases. A couple of hours after sunrise, they start to chorus again until approximately four hours after sunrise. Brattstrom and Warren (1955) noticed that chorusing will stop if frogs sense movement. The researchers stood close to a reservoir containing frogs. When the researches moved their heads, even a couple of inches, the chorusing halted abruptly. However, the frogs did not discontinue vocalization when vibrations were created by stomping or when the researchers made noise by yelling.

Growth and Development

According to Brattstrom and Warren (1955), the time at which Pacific Treefrogs begin breeding depends primarily upon water temperature. Females will lay eggs in water that ranges between 12°C and 15°C. If the temperature of the water changes after eggs are deposited, the embryos can between 34°C and -7°C but only for a maximum of two hours. Larvae can tolerate greater temperature variations than embryos. Larvae that develop in warmer water will grow more quickly than larvae in cool water.

Predation

Jameson (1956) reported that Pacific Treefrogs are preyed upon by the red-sided garter snake (Thamnophis sirtalis concinnus), the Pacific garter snake (Thamnophis ordinoides), and the bullfrog (Rana catesbeiana). The bullfrog is an introduced species and a voracious predator. Jameson did not find Pacific Treefrogs in ponds containing bullfrogs, although others have found Pacific Treefrogs and bullfrogs in the same pond (Titus, pers. comm., pers. obs.). Pacific Treefrogs and bullfrogs differ in the amount of time required to mature (Titus, pers. comm.). Pacific Treefrogs only needs 2-3 months to complete metamorphosis while bullfrogs needs 2-3 years to mature. Pacific Treefrogs can use temporary water sources for breeding while bullfrogs require permanent water.

Conclusion

To the casual observer the Pacific Treefrog may just be another frog. However, it has a variety of characteristics which make it individual from all other species. Further research might address whether there is any significance to the gain and loss of spots in terms of communication. Also, do frogs that have spots, and therefore the ability to lose and gain spots, have any advantage over individuals that do not have any spots, perhaps in thermoregulation and in escaping predation? It is an interesting organism, and its abundance and wide range of habitat make it an excellent organism to study.

References:

ALLAN, D. M. 1973. Some relationships of vocalization to behavior in the Pacific treefrog, Hyla regilla. Herpetologica 29:366-371.

BRATTSTROM, B. H., AND J.W. WARREN. 1955. Observations on the ecology and behavior of the Pacific treefrog, Hyla regilla. Copeia 3:181-191.

BRATTSTROM, B. H. 1963. A preliminary review of the thermal requirements of amphibians. Ecology 44:238-253.

FARRELL, M. P., AND J. A. MACMAHON. 1969. An ecological study of water economy in eight species of treefrogs (Hylidae). Herpetologica 25:279-294.

AMESON, D. L. 1956. Growth, dispersal and survival of the Pacific treefrog. Copeia 1:25-29.

JAMESON, D. L., W. TAYLOR, AND J. MOUNTJOY. 1970. Metabolic and morphological adaptation to heterogenous environments by the Pacific tree toad, Hyla regilla. Evolution 24:75-89.

NUSSBAUM, R. A., E. D. BRODIE, AND R. M. STORM. 1983. Amphibians and Reptiles of Oregon. University of Idaho Press, Idaho.

SCHAUB, D. L., AND J. H. LARSEN. 1978. The reproductive ecology of the Pacific treefrog (Hyla regilla). Herpetologica 34:409-416.