Pacific Treefrog Silhouette Natural History of the Pacific Chorus Frog
Pseudacris regilla

All text and photographs © Michael F. Benard

Introduction and Identification
Identification
Breeding season
Life as a tadpole
Metamorphosis and beyond
Conservation
Hitchhiking to Alaska in Christmas Trees
A note about names
About Me
Photo Gallery

  
floating pacific chorus frog The Pacific Chorus Frog
The Pacific Chorus Frog (also known as the Pacific Treefrog) is perhaps the most abundant amphibian on the west coast of North America. The geographic range of these little frogs extends from British Columbia down to Baja California, and from the Pacific Ocean to Nevada and Montana. Within this huge area, Pacific Chorus Frogs can be found from sea level to over 10,000 feet, in habitats that include deserts and redwood forests. If you live within the range of this frog, you have probably heard them calling, or seen them hopping about on the ground. The goal of this page is to give you some insight into the lives of these abundant frogs; if you have any questions about their natural history, feel free to send me an email:
email

Identification

The Pacific Chorus Frog can be distinguished from all other frogs within its geographic range by two characteristics: 1) the presence of toepads, and 2) a dark stripe that extends from just before the nostril, through the eye, and past the tympanum (ear). No other frogs found within the geographic range of the Pacific Chorus Frog have both of these characters (Stebbins 2003). eyestripe
toepads
eyestripe
eyestripe
Adult pacific treefrogs are generally 3.0 to 4.5 cm (1 to 2 inches) long. On average, females are larger than males. During the breeding season, males can be distinguished from females by the color of their throats. Females have a smooth, white throat, while males have a dark brown or yellow throat with wrinkly skin. Male Head
Male
Female Head
Female
Sounds of love: Pacific Treefrog breeding season
breeding male pacific chorus frog breeding male pacific chorus frog breeding male pacific chorus frog
Pacific Treefrog Sounds

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Slow series of advertisement calls.

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Rapid series of advertisement calls.

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Encounter call.

These calls were recorded in 2003 at the UC Quail Ridge Reserve.

Pacific Chorus Frogs have a prolonged breeding season that can last for several months. In Napa County, I have observed them breeding from January until May, although the breeding peaks in February and March. Most breeding occurs at night. Males sit near the water's edge, and use different types of vocalizations to communicate with other males and females. Many people are familiar with the Pacific Chorus Frog's 'advertisement' call, which consists of a two-part 'Kreek-eeck' sound. During the peak of the breeding season, huge numbers of males may gather on the same nights, creating an incredible din. Because Pacific Chorus Frogs are common in southern California, the sound of these choruses has been recorded and in the background of many Hollywood movies. While we find their vocalizations to be pleasant, the primary function of the advertisement call is to attract mates. As male Pacific Chorus Frogs make advertisement calls, female Pacific Chorus Frogs approach and select their mate. Mate selection is not a random process; female Pacific Chorus Frogs prefer to mate with males that make advertisement calls more frequently than the other males (Whitney and Krebs 1975). Additionally, larger male frogs are more likely to mate with female frogs than smaller males ). Why larger males are more likely to mate than smaller males is unknown. It may be because they produce more attractive calls; alternatively, larger males may be more successful in fights with other males.
Aggressive encounters between adult male Pacific Chorus Frogs are common. During these encounters, males often use a second type of call: The 'encounter' call. The encounter call, which is a trilling sound, serves as a warning when another male gets too close. The intruder will often retreat in response to the encounter call. Sometimes both males will make encounter calls until one retreats. Occasionally, there is a brief physical altercation before one frog retreats. While the encounter call can serve as a useful signal,there are drawbacks to using it; female Pacific Chorus Frogs are not attracted to the encounter call (Brenowitz and Rose 1999).

Pacific Treefrog breeding groups can be quite chaotic. On warm (~50 F or 10 C), humid nights, many males can be calling. As the breeding area becomes more crowded, encounter calls begin to get mixed in with advertisement calls. The males will often attempt to mate with any non-vocalizing amphibian that gets too close. This includes not only male Pacific Treefrogs, but other species of frogs, and even salamanders!

amplexing pacific treefrogs
A mating pair of pacific treefrogs in amplexus. The male is clinging to the back of the female.
In the midst of all of the chaos at the breeding site, females find their way to males. When a female approaches a male, the male will move onto her back, and grasp her under her forelimbs with his forelimbs in a position known as "amplexus". The female will then swim out into the water to lay eggs on vegetation, debris and other structures. As the female lays eggs, the male will release sperm to fertilize the eggs. A single female frog can lay 500 to 1,250 eggs in a year (Wright and Wright 1947). The eggs are generally laid in multiple small clusters, each containing from one to over 100 eggs.
amplexing pacific treefrogs frog amplex newt pacific treefrog eggs
puddle Pond

A shallow puddle in Napa County, CA, where Pacific Chorus Frogs bred. In February this puddle was full of eggs. However, it dried long before any of the tadpoles could escape the puddle by metamorphosing.

Temporary pond supporting pacific chorus frogs and California newts

An ephemeral pond in Napa County, CA, where Pacific treefrogs breed. This photo was taken in February when the pond was at its deepest. Predators of Pacific Chorus Frogs at this pond include predaceous diving beetles, giant water bugs, and garter snakes.

Fish Pond

A permanent pond in Napa County, CA, where Pacific treefrogs breed. This pond contains predatory invertebrates, such as odonate nymphs, as well as large centrachid fish like bluegill sunfish.

Pacific Chorus Frog Habitat
LEFT: Small ephemeral stream in California's coast range where Pacific Treefrogs breed. This habitat is also used by California Newts (Taricha torosa) and Foothil Yellow-Legged Frogs (Rana boylii).

RIGHT: Male pacific treefrog in a small ephemeral breeding pool.

Pacific Chorus Frog Habitat
wounded adult Pacific Chorus Frog
A male pacific treefrog missing its rear right foot. Despite its wound, this male was seen frequently throughout the 2005 breeding season. I observe a few frogs each year with similar wounds. Many more frogs have scars on their back in the shape of the jaws of snakes.
The breeding season is not just a time of fun for pacific tree frogs. A variety of predators will capture adult Pacific Chorus Frogs, including snakes and raccons. One common predator I have observed hunting breeding adults is the giant water bug, Lethocerus americanus (Benard 2007). These enormous insects capture the frogs with their raptorial forelimbs, and use their piercing mouthparts to inject digestive toxins into the frogs. It turns out that the water bugs are more likely to catch big frogs than little frogs.

After the breeding season is over, the Pacific Chorus Frog disperse back into the habitat surrounding their pond. There is little information on how far from their breeding pond the adults will move. However, I have observed a few individually-marked adults up to 300 meters from the breeding pond. Both male and female pacific treefrogs will return to the same pond to breed. Some may live through as many as four breeding seasons. However, only 10-20 percent of the adults will survive from one breeding season to the next (Benard, unpublished data, Jameson 1956).

Giant Water Bug eating treefrog
Life as a tadpole
Giant water bug nymph
Giant water bug nymph, one of many predators on pacific treefrog tadpoles.
Pacific treefrog eggs may take several days to hatch. Upon hatching, the tadpoles are at risk from a host of predators that view pacific treefrog tadpoles as a tasty meal. These predators include birds, garter snakes, fish and predatory insects, among others. However, generations of evolution by natural selection has prepared the tadpoles for these threats. Tadpoles can detect chemical cues from these different predators, and initiate defensive responses that are specific to each kind of predator.

For example, pacific treefrog tadpoles develop enlarged tails when they smell predaceous diving beetles; these enlarged tails serve as a lure to distract strikes from the body of the tadpole toward the tail (Benard 2006). However, when tadpoles smell bluegill, they develop a shape that improves their swimming speed: shallow tails and deep tail muscles. These distinct responses to each type of predator represent adaptations to each predator's foraging strategy. Beetles are sit-and-wait predators, while fish can chase their prey for short distances.

Bluegill Sunfish
Bluegill sunfish, another predator.
tadpole phenotypic plasticity
In the presence of predatory insects, tadpoles develop deep tails (top tadpole). In the presence of predatory fish, tadpoles have shallow tails (bottom tadpole). Tadpoles that are not exposed to cues from predators are intermediate in shape.
Dytiscid larvae eating tadpole
Predaceous diving beetle larvae eating a tadpole.
dry frog pond Predators are not the only threat tadpoles face. Because Pacific Treefrogs commonly breed in emphemeral ponds, a major source of mortality for tadpoles is pond drying. When these ponds dry early in the year, any Pacific Treefrog tadpole that cannot metamorphose into a small froglet will be killed.

However, tadpoles have an adaptation to improve their chance of surviving in these ephemeral habitats as well. When Pacific Treefrog tadpoles detect that their pond is drying, they can accelerate their development rate so that they metamorphose earlier in the year. While this does not always save the tadpoles, it can improve their chances of survival in some ponds.

LEFT: A recently dried pond.

pacific chorus frog tadpole
Pacific Treefrog tadpoles in a drying pond. Many of these tadpoles will not be able to metamorphose before the pond dries
Metamorphosis and Beyond
metamorphosing pacific chorus frog
Metamorphosing pacific treefrog.
metamorphosing pacific chorus frog
Metamorphosing pacific treefrog.
snake eating tadpoles
Mountain garter snake (Thamnophis elegans), looking at a group of pacific treefrog tadpoles and metamorphs.
Conservation Issues
Over the last two decades, scientists and the general public have become alarmed by the rapid pace of extinctions of amphibians, called 'global amphibian declines'. While the pacific treefrog has remained abundant over most of its range, population-level declines have been observed in some areas. These localized declines are due to the introduction of new species, such as fish and bullfrogs, as well habitat destruction. If you would like to help protect amphibians like the pacific treefrog, one of the best ways to do this is through protecting habitat. Two simple ways to do this are to support legislation such as the Endangered Species Act, and to contribute to organizations such as land trusts (e.g.,The Land Trust Alliance or The Nature Conservancy) that protect habitat.

Hitchhiking to Alaska in Christmas Trees
A recent news story has brought some attention to the potential of Pacific Chorus Frogs being introduced outside of their native range. In short, Pacific Chorus Frogs were found to be hitchhiking from Oregon to Alaska by hiding inside of Christmas trees . This is not the first time that Pacific Chorus Frogs and other amphibians have been found to be transported in Christmas Trees and other horticultural products; for instance, Christy et. al (2007) report on Pacific Chorus frogs being brought to Guam in Christmas Trees. However, the observation of pacific chorus frogs hitchhiking from Oregon to Alaska raises the question of whether there is real potential for Pacific Chorus Frogs to become established in Alaska. There are already several cases of pacific treefrogs being established out of their native range. For instance, a population of pacific chorus frogs established in the Alexander Archipelago of Alaska in the 1960s was extant at least until 2002. However, it did not appear to have spread beyond a single lake, according to the S.O. MacDonald's "Amphibians and Reptiles of Alaska". In contrast, Reimchen (1991, Canadian Field Naturalist) reported on his studies of Pacific Chorus Frogs introduced to the Queen Charlotte Islands, British Columbia. The Pacific Chorus Frogs on the Queen Charlotte Islands spread at a rate of approximately 2 km per year. While the Queen Charlotte Islands are considerably south of Juneau, the islands are also north of the natural geographic range of these frogs. This suggests that Pacific Chorus Frog populations introduced to Alaska may be capable of increasing in size and spreading.

More of my thoughts on Pacific Chorus Frogs in Christmas Trees can be found at my blog.

A note about names

The most frequently used common name for these little frogs, and the one that many books use (e.g. Stebbin's Western Reptiles and Amphibians 3rd edition) is the Pacific Treefrog. However, this name is misleading; while pacific treefrogs can climb, they do not spend much time in trees. They are much more likely to be found on or under the ground. To more accurately reflect the natural history and evolutionary relationships of these frogs, some people prefer to call these frogs Pacific Chorus Frogs.

About me
I have been studying Pacific Chorus Frogs since I started graduate school at UC Davis in 1999. I am currently an assistant professor in the Biology Department at Case Western Reserve University. My work focues on the ecology and evolution of amphibians. More information on my research can be found at the Benard Lab website. Some musings on natural history and conservation can be found at my blog. I am always happy to answer questions about Pacific Chorus frogs and other amphibians. I can be contacted by email at:

email

Links
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Insect and Invertebrate Information

Photo Gallery

Pacific Chorus Frog
Pseudacris regilla

Pacific Chorus Frog
Pseudacris regilla

Pacific Chorus Frog
Pseudacris regilla

Pacific Chorus Frog
Pseudacris regilla

Pacific Chorus Frog
Pseudacris regilla

Water Bug eating
Pacific Chorus Frog
Pseudacris regilla

Diving Beetle eating
Pacific Chorus Frog
tadpole

Pacific Chorus Frog
Pseudacris regilla

Literature Cited

Croes, SA, and RE Thomas. 2000. Freeze tolerance and cryoprotectant synthesis of the pacific treefrog, Hyla regilla. Copeia. 2000:863-868.

Benard, MF 2006. Survival trade-offs between two predator-induced phenotypes in Pacific Treefrogs (Pseudacris regilla). Ecology 87:340-346.

Benard, MF 2007. Predators and Mates: Conflicting selection on the size of male Pacific Treefrogs (Pseudacris regilla). Journal of Herpetology 41:317-320.

Brenowitz, EA and GJ Rose. 1999. Female choice and plasticity of male calling behaviour in the Pacific treefrog. Animal Behaviour. 57:1337-1342

Christy, MT, JA Savidge, and GH Rodda. 2007. Multiple pathways for invasion of anurans on a Pacific island. Diversity and Distributions. 13(5):598-607.

Fisher, RN, and HB Shaffer. 1996. The decline of amphibians in California's great central valley. Conservation Biology. 10:1387-1397.

Jameson, DL. 1956. Growth, dispersal and survival in the pacific treefrog. Copeia. 1956:25-29.

Jameson, DL, JP Mackey, M Anderson. 1973 Weather, climate, and external morphology of pacific tree toads. Evolution 27:285-302.

Jameson, DL and S Pequegnat.. 1971. Estimation of relative viability and fecundity of color polymorphisms in anurans. Evolution 25:180-194.

Matthews, KR, KL Pope, HK Preisler, RA Knapp. Effects of nonnative trout on Pacific treefrogs (Hyla regilla) in the Sierra Nevada. Copeia 2001:1130-1137.

Reimchen, TE. 1991. Introduction and dispersal of the Pacific Treefrog, Hyla regilla on the Queen Charlotte Islands, British Columbia. The Canadian Field-Naturalist. 105:288-290.

Stebbins, RC 2003. A Field Guide to Western Reptiles and Amphibians. 3rd Edition. Houghton Mifflin Company, 2003.

Wente, WH and JB Phillips. 2003. Fixed green and brown color morphs and a novel color-changing morph of the pacific treefrog, Hyla regilla. American Naturalist. 162:461-473.

Wente, WH and JB Phillips. 2005a. Microhabitat selection by the Pacific treefrog, Hyla regilla. Animal Behaviour. 70:279-287.

Wente, WH and JB Phillips. 2005b. Seasonal color change in a population of pacific treefrogs (Pseudacris regilla). Journal of Herpetology. 39:161-165.

Wright, AH and AA Wright. 1947. Handbook of Frogs and Toads of the United States and Canada. Comstock Publishing Associates, Ithaca, NY, USA