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Frogs of the Willamette Valley, Oregon

 --by Karen Wegner


There are approximately 5,000 described species of amphibians (Class Amphibia) worldwide.  Frogs and toads, Order Anura, make up the majority of these species.  The Order Caudata, otherwise known as salamanders, is the next most common amphibian group. The lesser-known caecilians, a wormlike amphibian known only in tropical environments, make up the Order Gymnophiona.

During recent decades, much has been published regarding declines in amphibian populations.  Declines are better documented in some species and regions than in others, but concern for these trends has increased in many areas of the globe. Many potential causes are currently being studied, including: disease, environmental factors, introduced species, habitat loss including degradation, modification, and the conversion of temporary water to permanent water.  All probably have important roles in the population declines depending on species and location.

In the Willamette Valley, habitat loss through development, and the conversion of temporary water to permanent water are probably the strongest stressors on native amphibian populations.  Temporary water, also known as ephemeral water, seasonal ponds, or vernal pools, is usually a shallow depression, or pond where water collects during the rainy season and persists for a short period of time.  Most temporary ponds are dry by the middle of summer.  The benefit of temporary water to Willamette Valley amphibians is the impermanence, as it prevents fish from coexisting within the same pond as amphibians.  Willamette Valley amphibians evolved in an environment generally free from predatory fish.  When the depth of a temporary pond is increased it can become habitat to non-native fishes such as bluegill, sunfish, bass, and carp.  Non-native fish have been shown to reduce native amphibian populations through direct predatory actions, or indirectly by increasing the survival rates of bullfrog tadpoles which may out-compete native tadpoles.

Only 3% of amphibian species live in North America.  In Oregon, we have 12 native species of frogs and toads, and one introduced species.  The Willamette Valley is currently the home for two native species of frogs, and the one introduced species.   Although the amphibians of Oregon occur in a wide variety of habitats including cold, torrential streams, warm ponds, ephemeral pools, and high mountain lakes, Willamette Valley frogs have similar habitat requirements and often co-occur. There are historical records of a third native frog (Oregon Spotted Frogs, Rana pretiosa) in the Willamette Valley, but this species appears to be lost from the region. 

Amphibian Life History

The word amphibian is derived from the Greek work, amphibious, meaning living a double life.  Frogs lead a “double life” since adults may spend most of their time on land, but their eggs must be laid in water.   Newly hatched frogs, commonly called tadpoles or pollywogs, remain in the water until metamorphosis occurs.

Egg masses laid by Willamette Valley frog species differ from each other greatly and are easily distinguishable (see descriptions below in species accounts).  Commonly, egg masses include a varying number of dark embryos individually encased in a clear capsule.  All the individual embryos are then encased as a whole within a clear gelatinous mass. 


Stage 1–Egg

Tiny frog eggs are laid in masses in the water by a female frog. The eggs hatch into tadpoles.

Stage 2–Tadpole

Tadpoles can also be called pollywogs. This stage hatches from the egg. The tadpole spends its time swimming in the water, eating, and growing. Tadpoles breathe using gills and have a tail.

Stage 3–Tadpole with Legs

In this stage, the tadpole sprouts legs (and then arms), has a longer body, and has a more distinct head. It still breathes using gills and has a tail.

Stage 4–Froglet

In this stage, the almost mature frog breathes with lungs and still has some of its tail.

Stage 5–Adult Frog

The adult frog breathes with lungs and has no tail because the body has absorbed it.

Copyright ©2001 EnchantedLearning.com  Permission to reproduce granted for non-commercial use.

Species Accounts

Pacific Treefrog (Hyla regilla)  

Photo by Christopher Pearl

Pacific treefrogs are the most common frog species found in the Willamette Valley and throughout most of the west coast. There are two calls commonly associated with male treefrogs, one heard during the breeding season while males are in water, and another often referred to as a “dry-land” call that comes from males while some distance from water.  Their diet consists mostly of small, terrestrial invertebrates.


Treefrogs are small frogs often under 50mm in length.  They have three distinctive features: a dark stripe across each eye extending to the shoulder, a “Y” shaped mark on the top of the head, and rounded toe pads.  Treefrogs occur in a variety of colorations, including bright green, brown, gray, and red.   Coloration generally relates to geographic area, and the frogs in the Willamette Valley are most often green or brown.


Treefrogs are found in both permanent and temporary ponds during the breeding season.  The rest of the year they occupy upland areas, and in moist areas they may be found on vegetation.


In the Willamette Valley, male treefrogs move to breeding ponds and begin to vocalize in January.  Females lay a small egg mass about half the size of a tennis ball, which averages 25 eggs attached to submerged aquatic vegetation.  The eggs hatch in 3-5 weeks, and metamorphosis follows in 8-10 weeks.


Northern Red-Legged Frog (Rana aurora aurora) 

Photo by Christopher Pearl 

The Northern Red-legged frog ranges from Mendocino County, California, to British Columbia, Canada.  The closely related California Red-legged Frog (Rana aurora draytonii) occurs farther south in California, and is listed as threatened under the Endangered Species Act.  Red-legged frog diets consist primarily of aquatic and terrestrial invertebrates, and occasionally, other amphibians.             


Red-legged frogs are 30-100mm in length.  They are generally brown or reddish brown with black flecks, with a light lip line on the upper jaw, and may have a dark facemask.  There are strong dorsolateral folds extending down the sides of the back. The underside of the hind legs is red, the groin is mottled, and the belly is cream to gray.  The red may continue onto the sides and belly where it will appear blotchy with the predominant belly color.


Red-legged frogs may be found in cool, shady coniferous or deciduous upland areas some distance away from water in the non-breeding season.  Breeding may occur in permanent, or temporary ponds. On warm rainy nights, adults may be seen on roads. Aquatic vegetation is needed to anchor the egg mass.


Red-legged frogs usually begin breeding in February. Males vocalize under water. Females deposit 500 -1,000 eggs in an egg mass approximately the size of a cantaloupe on submerged aquatic vegetation.  Time to hatching depends on water temperature, but is about three weeks. There are documented hatch times of 83 days in very cool water, and 6.6 days in very warm water.  Time to metamorphosis is around 14 weeks.


Forest Service Sensitive


Bullfrog (Rana catesbeiana)

Photo from USFWS

Bullfrogs are not native to Oregon.   Their native range, east of the Mississippi, has been extended to include most the entire Untied States except high elevations, and parts of the northern plains.  Bullfrogs appear to have been introduced in Oregon in the early 1900s, and were probably brought here as a food item.  Males have a booming call; juveniles give a chirping call when startled—often as they retreat into water.  Because of their size, bullfrogs can ingest a remarkable variety of prey.  Turtle hatchlings, rodents, other amphibians, and birds have all been documented as prey.  These types of prey items are likely the exception with aquatic and terrestrial invertebrates forming the basic diet.  The dietary habits of the bullfrog may make them a threat to native amphibians, although recent research suggests that introduced warm water fish may pose an even greater threat. 


Bullfrogs are the largest North American frogs. They range in color from light green to deep olive, and often have black splotches.  Dorsolateral folds bend around the tympanum down to the shoulder, instead of running down the back as in other ranid species in which folds are present.   Females may be as large as 200mm, and males may achieve 180mm.   Males have a very large tympanum that is equivalent in size to twice the diameter of the eye, a yellow throat, and during breeding season the nuptial pads on the back of their thumbs will be enlarged and dark.


Bullfrogs are usually pond dwelling amphibians, although they may be found at least temporarily in streams and temporary ponds.  In the West, bullfrogs usually require at least one winter before metamorphosis can occur, so breeding is concentrated in permanent ponds. 


Breeding occurs in the Willamette Valley from June to August.  Female bullfrogs can lay more than 10,000 at a time.  The egg masses may be 2ft. in diameter.  Clutch size is correlated to female size and age, the larger the female the larger the clutch size.  Tadpoles hatch within a week, depending on latitude; metamorphosis usually begins to occur after one winter.


Adams, MJ., Pearl, C.A., and R.B. Bury. 2003.  Indirect facilitation of an anuran invasion by non-native fishes.  Ecology Letters (6):  343-351.

Corkran. C.C, and C.T. Thoms.  1996.  Amphibians of Oregon, Washington and British Columbia.  Lone Pine Publishing. Renton, Washington.

Duellman, W.E., and L. Trueb, 1994.  Biology of Amphibians.  Johns Hopkins University Press. Baltimore, Maryland.

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

Leonard, W.P., Brown, H.A, Jones, L. L. C., McAllister, K.R., Storm, R. M. 1993.  Amphibians of Washington and Oregon.  Seattle Audubon Society.  Seattle, Washington.


Amplexus---Breeding in frogs.

Class/Order---The Swedish scientist Carolus Linnaeus (1707-1778) developed the two-name, binomial nomenclature, (Genus and species) system for identifying and classifying living things. This system is based on a simple hierarchical structure in which organisms are sorted using the nomenclature listed below. Organisms belonging to the same kingdom are not necessarily very similar; organisms belonging to the same species are very similar and can produce offspring.  For example, the Northern Red-legged Frog would be:









Dorsolateral Folds—Glandular ridges that usually run down both sides of a frog's back.  Not all frogs have them.

Metamorphosis—A physical, biochemical, and behavioral change that usually differentiates the larval and adult stages in amphibians.

Nuptial Pads—A thickening of tissue on the thumbs of some male frogs.  Aids in helping the male hold onto the female during Amplexus.

Tympanum—A circular membrane, which functions as an eardrum, located externally behind the eyes. 

Herpetology—The study of amphibians and reptiles.


Thanks to… Susan Brodeur, Cedric Cooney, Audrey Hatch, Steve Morey for their comments, and to Christopher Pearl for his comments and photos.

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