Pseudacris regilla
All text and photographs © Michael F. Benard
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Natural History of the Pacific Treefrog Pseudacris regilla All text and photographs © Michael F. Benard Introduction and Identification
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| Sounds of love: Pacific Treefrog breeding season |
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Pacific Treefrogs 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 Treefrog'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 Treefrogs 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 Treefrogs make advertisement calls, female Pacific Treefrogs approach and select their mate. Mate selection is not a random process; female Pacific Treefrogs 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 (Benard 2007). 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. | |
Click on the frog head for a Quicktime Video (1150 kb) of a calling male pacific treefrog. |
Aggressive encounters between adult male Pacific Treefrogs 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 Treefrogs 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! | |
 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. |
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A shallow puddle in Napa County, CA, where Pacific treefrogs bred. In February this puddle was full of eggs. However, it dried long before any of the treefrog tadpoles could escape the puddle by metamorphosing. |
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 treefrogs at this pond include predaceous diving beetles, giant water bugs, and garter snakes. |
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. |
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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.
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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 treefrogs, 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.
After the breeding season is over, the pacific treefrogs 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). |
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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. 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, another predator. |
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. |
Predaceous diving beetle larvae eating a tadpole. |
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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 Treefrog tadpoles in a drying pond. Many of these tadpoles will not be able to metamorphose before the pond dries |
Metamorphosing pacific treefrog. |
Metamorphosing pacific treefrog. |
Mountain garter snake (Thamnophis elegans), looking at a group of pacific treefrog tadpoles and metamorphs. |
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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.
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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 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. 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 | ||||
| Attention All text and photograhs on this website are copyright Michael Benard. If you have any inquiries about using any of this material, or for any other questions, I can be contacted by email at mfbenard 'at' gmail dot com. |
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