Description
A small frog (2.5 to about 5 cm). Dorsal coloration variable; ground color may be cream, grey, red, or black. Dorsum may show a variable pattern of dark streaks and blotches. Head lacks tympanum, pupil is vertical, and a light streak may extend from tip of snout to above the eyes. Whitish-yellow venter, more yellow in femoral region. Males have a distinctive tail-like extension of the cloaca, and during breeding season show greatly enlarged forearms. (Stebbins, 2003)

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Canada, United States

U.S. state distribution from AmphibiaWeb's database: California, Oregon, Washington

Canadian province distribution from AmphibiaWeb's database: British Columbia

	View distribution map in BerkeleyMapper.
	View Bd and Bsal data (22 records).

From sea level to timberline, ranging along Pacific coast of North America from northern California to southern British Columbia. Also includes a disjunct distribution in northern Idaho and western Montana (Nussbaum et al 1983). Associated with cold, fast-moving streams with cobbled bottoms and emergent boulders (Bull and Carter 1996).

Life History, Abundance, Activity, and Special Behaviors
Ascaphus is non-vocal. Its internal fertilization (an adaption to ensure fertilization in fast moving streams) is unique among frogs. Breeding season lasts from May through September. Eggs are deposited in strings under rocks in fast-moving streams. Tailed frogs are primarily aquatic; adults may emerge to forage on land in cool, wet conditions, especially at night. Larvae take between 1 to 4 years to metamorphose, depending on drainage (Bull and Carter 1996; Wallace and Diller 1998).

Juveniles are sexually mature in 7-8 years. Maximum average Ascaphusdensity per linear meter of stream was 0.162 larvae and 0.035 adults in western Oregon (Bull and Carter 1996). Nussbaum et al (1983) cite densities as high as 1 frog per meter of stream in eastern Washington.

Larva
Larvae have a distinctively large, round mouth, modified for suction, which they use to cling to streamside rocks in fastmoving streams (modified from Stebbins 1951).

Trends and Threats
Not federally listed, but a California Species of Special concern. Habitat modification due to timber harvesting may be a major cause of declines in Ascaphus. Bull and Carter (1996) found that the presence of a tree-lined buffer zone was significantly correlated with Tailed frog abundance, while overall logging level within the watershed showed only a non-significant trend. In British Columbia, conservation plans are difficult to implement, since Tailed Frogs (along with Pacific Giant Salamanders, Dicamptodon tenebrosus) avoid areas with predatory game fish; it is the riparian areas with those game fish that are protected (Orchard 1992).

Possible reasons for amphibian decline

Habitat modification from deforestation, or logging related activities

Comments
Ascaphus species (A. truei and A. montanus) are the only living representative of a basal lineage of anurans, and may retain some relatively primitive traits of early frogs (e.g. Ford and Cannatella 1992).

See another account at californiaherps.com.

This species was featured in the News of the Week 14 April 2025:

Predicting how populations will respond to environmental change is an inherently complex problem often simplified due to the limitations of particular research approaches. For example, conservation genomics studies often lack insight into phenotypic traits important to environmental tolerance, while physiological studies are often limited to individuals from a single population and lack insight into population variation and the capacity for traits to evolve. Forester et al. (2025) combine conservation genomics and physiological ecology to gain important insights into the climate change vulnerability of two cold water stream frogs (Ascaphus montanus and Ascaphus truei). They developed the first annotated reference genome for A. truei and took advantage of a large data set quantifying thermal tolerance across populations of both species from a diversity of thermal environments. The authors first showed evidence for local adaptation to temperature in both species and a genetic basis to critical thermal maximum temperature (CTmax), a common metric to assess thermal tolerance. Importantly, by examining population variation, they were able to conclude that Ascaphus montanus populations appear to exhibit adaptive divergence in CTmax, such that populations have similar vulnerability to future warming. In contrast, Ascaphus truei showed only genomic evidence for local adaptation and no divergence in CTmax. A similar sensitivity to high temperatures across populations means that A. truei populations occupying warmer streams (e.g., low elevation) are closer to their upper tolerance limits and thus more vulnerable to future warming compared to populations occurring in colder streams (e.g., high elevation). The real-world implications of these results mean that management decisions used to mitigate the impacts of future warming on Ascaphus montanus may differ for Ascaphus truei, although both species appear to have high levels of evolutionary potential based on the genomic and physiological data. (Written by Chris Funk)

References

Bull, E. L. and Carter, B. E. (1996). ''Tailed Frogs: Distribution, ecology, and association with timber harvest in Northeastern Oregon.'' United States Forest Service Research Paper, (497), 1-12.

Ford, L.S., and Cannatella, D.C. (1993). "The major clades of frogs." Herpetological Monographs, 7, 94-117. [link]

Nussbaum, R. A., Brodie, E. D., Jr., and Storm, R. M. (1983). Amphibians and Reptiles of the Pacific Northwest. University of Idaho Press, Moscow, Idaho.

Orchard, S.A. (1992). ''Amphibian population declines in British Columbia.'' Declines in Canadian amphibian populations: designing a national monitoring strategy. C. A. Bishop nd K.E. Petit, eds., Canadian Wildlife Service, 10-13.

Stebbins, R. C. (1985). A Field Guide to Western Reptiles and Amphibians. Houghton Mifflin, Boston.

Stebbins, R. C. (2003). Western Reptiles and Amphibians, Third Edition. Houghton Mifflin, Boston.

Stebbins, R.C. (1951). Amphibians of Western North America. University of California Press, Berkeley.

Wallace, R. L. and Diller, L. V. (1998). ''Length of the larval cycle of Ascaphus truei in coastal streams of the redwood region, northern California.'' Journal of Herpetology, 32(3), 404-409.

Species Account Citation: AmphibiaWeb 2025 Ascaphus truei: Pacific Tailed Frog <https://amphibiaweb.org/species/2049> University of California, Berkeley, CA, USA. Accessed May 1, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 1 May 2025.

AmphibiaWeb's policy on data use.