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Rana boyliiFoothill Yellow-legged Frog

By Samantha Aliah

Geographic Range

Foothill yellow-legged frogs were historically found in the majority of Pacific drainages west of the Cascade Mountain crest from Marion County, Oregon to Los Angeles County, California, and was considered one of the most abundant amphibians in the area. Recently, however, the abundance and distribution status of this species has significantly declined. Today, these frogs are unevenly distributed in the Pacific northwest. While still ranging from western Oregon to southern California (near Los Angeles County), they are present in fewer drainages than before. Populations that were previously found in the San Gabriel Mountains or along the south coast of Monterey County, for example, no longer exist, and there is only a limited number of individuals remaining in the foothills of the southern Sierra Nevada Mountains. ("AmphibiaWeb", 2010; "California Reptiles and Amphibians", 2010; "Encyclopedia of Life", 2010; Ashton, 1997; Leonard, et al., 1993; Stebbins, 2003)

Habitat

Foothill yellow-legged frogs are highly aquatic and almost always found in, or within a short distance from, water. The most frequently used habitats are streams, springs, and freshwater lakes, with a preference for rocky-bottomed creeks; it usually occurs in gently flowing water. Slow-flowing creeks or streams with cobble-sized pebbles are preferred egg laying sites for foothill yellow-legged frogs. Adults often spend the majority of their time sitting on rocks in the stream or nearby on the banks. If startled, however, they will immediately leap into the water and swim swiftly to the bottom. In clear waters, they will take cover under overhanging rocks; in streams with muddy bottoms, they will stir up the silt and hide in the mud. They may occur at any elevation from sea level up to 2,040 m. ("Amphibians of Western North America", 1951; "Handbook of Frogs and Toads of the United States and Canada", 1995; Leonard, et al., 1993)

  • Aquatic Biomes
  • rivers and streams
  • temporary pools
  • Range elevation
    sea level to 2,040 m
    to ft

Physical Description

Foothill yellow-legged frogs range from 3.8 to 8.1 centimeters (1.5 to 3.2 inches) in size. Dorsal colors often resemble those of the ground, with variations including blackish, dark brown, reddish brown, gray, olive-like, or greenish with varying amounts and strengths of spots and speckles. Some individuals may possess a light spot within a dark area on the upper eyelid. Ventral coloration is typically whitish to yellowish, with a gradient towards yellow at the posterior end of the body and hind limbs. The throat and anterior surface of the femurs often display the most mottling. ("Amphibians of Western North America", 1951; "Handbook of Frogs and Toads of the United States and Canada", 1995; Ashton, 1997; Leonard, et al., 1993; Stebbins, 2003)

These frogs typically have a broad and pointed head. The tympanum is small and not very evident, usually covered with small tubercles, as is the rest of the dorsal body skin. The tibia is elongated and extends more than half the length of the body, with a shorter hind foot that barely reaches half of the tibia length. The hind feet are entirely webbed and there is a slight expansion of the toe tips. Dorso-lateral folds in this species are obscure and vomerine teeth are sometimes not apparent. However, a sacral-hump is rather conspicuous. ("Amphibians of Western North America", 1951; "Handbook of Frogs and Toads of the United States and Canada", 1995; Ashton, 1997; Leonard, et al., 1993; Stebbins, 2003)

Sexual differences include a bulbular swelling covered in minute papillae at the base of the male's first finger's dorsomedial surface. The females lack this rough, swollen area, and have a longer first finger than the males do. Adult females are larger than males, with snout-vent lengths 20 to 25 mm greater. During the mating season, males may be identified by swollen arms and enlarged nuptual pads on their thumbs for use in gripping females during amplexus. ("Amphibians of Western North America", 1951; Stebbins, 2003)

The larger tadpoles are usually olive to olive-gray colored above, with dark brown spots on the tail and fins. There are six or seven labial tooth rows above the mouth and five or six tooth rows below the mouth. May reach 5 cm (2 in) prior to metamorphosis. (Leonard, et al., 1993; Stebbins, 2003)

  • Sexual Dimorphism
  • female larger
  • Range length
    5 to 9 cm
    1.97 to 3.54 in

Development

After the 5 to 37 day incubation period, foothill yellow-legged frog tadpoles hatch out and will remain around the egg mass for about a week, possibly aiding in species identification. Larvae can be up to 18 mm in length for the head and body, and 29 mm in length for the tail. Larval growth rate is dependent on water temperature and food availability, but tadpoles normally undergo metamorphosis within three to four months. Similar to other frogs, typical metamorphic features include absorption of the tail, emergence of front limbs, and reorganization of the digestive tract. ("AmphibiaWeb", 2010; "Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; Ashton, 1997; Leonard, et al., 1993)

Frog maturity often occurs upon reaching a size of 40 mm snout-vent length (SVL). The reproductive organs become functional in the first summer after metamorphosis, but the first breeding activity often occurs in the second post-metamorphic year. Some individuals, however, have been known to reproduce as early as six months after metamorphosis. ("AmphibiaWeb", 2010; "Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; Ashton, 1997; Leonard, et al., 1993)

Reproduction

During the breeding season, adult male foothill yellow-legged frogs congregate along gravel and cobble bars of the river and establish calling sites. Male movements occur after river flow decreases following the snow thaw. Thus, the breeding season usually begins in April or March. Females arrive later, asynchronously, after the air and water temperatures have warmed further. Breeding migration by adult frogs appears to be limited to small movements along stream corridors. Individuals exhibit site-fidelity to breeding locations that retain optimal breeding conditions. ("AmphibiaWeb", 2010; "Amphibians of Western North America", 1951; "Handbook of Frogs and Toads of the United States and Canada", 1995; "Mating strategy and breeding patterns of the foothill yellow-legged frog (Rana boylii)", 2008; Ashton, 1997; Leonard, et al., 1993)

Males attract females by calling from their specific sites, which later become oviposition sites for laying females. Foothill yellow-legged frogs are polygynous. Mate calling normally occurs underwater, but males will also call above water. The call is described as several short grating notes followed by a rattling sound. These calls, however, are faint and don't carry very far. ("AmphibiaWeb", 2010; "Amphibians of Western North America", 1951; "Handbook of Frogs and Toads of the United States and Canada", 1995; "Mating strategy and breeding patterns of the foothill yellow-legged frog (Rana boylii)", 2008; Ashton, 1997; Leonard, et al., 1993)

At lower elevations, foothill yellow-legged frogs breed from late March to early May, after the high-water stages of streams subside. At higher elevations, breeding usually takes place from June to August, after the ice and snow has melted from the high-mountain lakes. Reproduction is aquatic, with external fertilization. Unlike other Ranidae frogs of the area, mating and egg-laying occurs exclusively in the slow-flowing, shallow water of streams and rivers, but not in ponds or lakes. ("AmphibiaWeb", 2010; "Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; "Encyclopedia of Life", 2010; "Handbook of Frogs and Toads of the United States and Canada", 1995; Ashton, 1997)

Eggs are laid in compact, grape-like clusters that are normally attached to the downstream sides of submerged stones, and occasionally vegetation. The eggs have three gelatinous envelopes present, all of which are firm and distinct, and are often black above, and white or light gray tan below. Anywhere from 100 to over 1,000 eggs may be laid per mass, although the average amount is 900. Egg masses normally have size dimension ranges of 2x2x1.5 to 2x4x2.4 inches. In order to hide them from predators, these masses are often covered with a layer of silt for camouflage. ("AmphibiaWeb", 2010; "Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; "Encyclopedia of Life", 2010; "Handbook of Frogs and Toads of the United States and Canada", 1995; Ashton, 1997)

Depending on the water temperature, the eggs may hatch within 5 to 37 days. This development is probably accelerated in warmer temperatures, and slowed in colder ones. Metamorphosis occurs when tadpoles are 3 to 4 months of age, but they do not reach reproductive maturity until 1 to 2 years of age for males and 2 years for females. ("AmphibiaWeb", 2010; "Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; "Encyclopedia of Life", 2010; "Handbook of Frogs and Toads of the United States and Canada", 1995; Ashton, 1997)

  • Breeding interval
    Foothill yellow-legged frogs breed once yearly.
  • Breeding season
    Foothill yellow-legged frogs breed from late March to early May at lower elevations, and from June to August at higher elevations.
  • Range number of offspring
    100 to 1000+
  • Average number of offspring
    900
  • Range time to hatching
    5 to 37 days
  • Range time to independence
    3 to 4 months
  • Range age at sexual or reproductive maturity (female)
    1 to 2 years
  • Average age at sexual or reproductive maturity (male)
    2 years

Foothill yellow-legged frogs have little parental investment beyond sperm and egg development and mating. Females ensure attachment of egg masses to the downstream side of a substrate in slow-moving water. They will also cover the masses with a layer of silt to help hide them from predators. ("California Reptiles and Amphibians", 2010; "Encyclopedia of Life", 2010)

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female

Lifespan/Longevity

The lifespan of foothill yellow-legged frogs is currently unknown. One recaptured female was aged at three years, however, longevity may potentially reach 12 years or more based on studies of closely related species. ("AmphibiaWeb", 2010; Ashton, 1997)

The individual lifespan of foothill yellow-bellied frogs is often limited by many threats, such as predation and parasites, especially helminth worms. Drought leads to congregation of frogs on land and therefore increased risk of predation, while it also makes eggs more susceptible to desiccation. Floods that occur soon after oviposition may detach egg masses from their substrate, decreasing the chances of survival. Loss of habitat, use of pesticides, and introduced fish species also have negative effects on this species. The nonnative American bullfrog (Lithobates catesbeianus), has caused declines of foothill yellow-legged frog populations as well, due to competition for resources and direct predation. ("AmphibiaWeb", 2010; "California Reptiles and Amphibians", 2010; Ashton, 1997)

Increased exposure to UV-B is believed to have a negative impact on egg hatching success in other ranids, however this has not yet been determined in this particular species. Chytrid fungus has also been found on foothill yellow-legged frogs, but the effect on this frog is not yet known. ("AmphibiaWeb", 2010; "California Reptiles and Amphibians", 2010; Ashton, 1997)

  • Range lifespan
    Status: wild
    3 (high) years

Behavior

Foothill yellow-legged frogs are typically active during the daytime, though they are very shy. They can be found basking on rocks or along the shore, but will quickly jump into the water when frightened. If their hiding place is not accessible, they will leap great distances. Adults have been observed to move hundreds of meters to gather at optimal breeding sites, but this species is generally sedentary. In warmer climates, foothill yellow-legged frogs may remain active year-round but those in colder climates are known to hibernate in cold winters. ("Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; "The Frog Book", 1913; Leonard, et al., 1993)

When handled, this species is also known to emit a distinct, oily odor, similar to that of California toads (Anaxyrus boreas halophilus); this odor is said to resemble mink musk or garlic. Foothill yellow-legged frogs are also able to change from a dark to lighter color, within about half an hour. ("Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; "The Frog Book", 1913; Leonard, et al., 1993)

Home Range

The home range size of Rana boylii is unknown. ("AmphibiaWeb", 2010)

Communication and Perception

Like most other Ranid frogs, this species has small, paired vocal sacs, however their call is faint and rarely heard. Most of its calls during mating season occur underwater due to the difficulty of hearing in the air within its noisy stream habitat. This frog's call generally consists of low-pitched and raspy croaks, grunts, or oinks given in a series of 4 to 6 notes per second. Other forms of communication are unknown for this species. ("Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010)

Food Habits

Foothill yellow-legged frogs eat mostly insects. Arthropods (Arthropoda), both aquatic and terrestrial, serve as its main food source. Its most common prey items are various types of spiders (Araneae), beetles (Coleoptera), "true" bugs (Hemiptera), and flies (Diptera). However, specimens have been found with other sources of food within their stomachs, such as grasshoppers (Caelifera), hornets (Vespa), carpenter ants (Camponotus), water snails, small moths, water striders (Gerridae), and stoneflies (Plecoptera). ("Amphibians of Western North America", 1951; "California Reptiles and Amphibians", 2010; Wegner and Crayon, 2009)

Food is often captured by use of the frog's large, sticky tongue to bring the prey towards its mouth, and is generally located by sight. Tadpoles are herbivorous during early stages of development and consume algae, diatoms, and debris by grazing the surface of vegetation and rocks. ("AmphibiaWeb", 2010)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • Plant Foods
  • algae

Predation

Foothill yellow-legged frogs often fall prey to various species at all stages of life. Rough skinned newts and Centrarchid fishes, such as green sunfish, are common predators of egg masses and larvae. Sacramento squawfish eat egg masses, tadpoles, and adult frogs. Tadpoles are also food sources for predaceous insects such as diving beetles, water bugs, and water scorpions. Herons, some passerine birds, and raccoons will eat tadpoles and adult frogs. Garter snakes (Thamnophis) feed on both tadpoles and post-metamorphic stages. Species such as common garter snakes, terrestrial garter snakes, and Sierra garter snakes are known to eat mainly juvenile frogs, while Oregon aquatic garter snakes have a preference for tadpoles. American bullfrogs have contributed greatly to population declines of foothill yellow-legged frogs due to direct predation along with competition for resources. ("AmphibiaWeb", 2010; Ashton, 1997; Leonard, et al., 1993)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Like other frogs, foothill yellow-legged frogs are an important facet of their ecosystem. They consume a great number of insects and may help keep the populations of its prey in check. They also serve as a suitable source of food to many predators, and as a host to various endoparasites. Tadpoles contribute by aiding in the control of algal growth. (Ashton, 1997; Chanson and Boucher, 2004; Leonard, et al., 1993)

Amphibians in general are also excellent indicators of environmental quality. Having permeable skin allows waterborne contaminants to enter the body readily and display the effect on the environment much quicker than could be done by other animals, providing managers with more time for change and repair before it is too late. (Chanson and Boucher, 2004)

Commensal/Parasitic Species
  • helminth woms

Economic Importance for Humans: Positive

Like other amphibians, foothill yellow-legged frogs are an exceptional indicator of environmental health. Due to its permeable skin, waterborne contaminants can be detected early in this frog by ecosystem managers, allowing time for the necessary action to take place. They also consume a lot of insects, perhaps helping to control pest populations. (Ashton, 1997; Chanson and Boucher, 2004)

Economic Importance for Humans: Negative

There are no known adverse effects of Rana boylii on humans.

Conservation Status

Rana boylii is currently listed as a California Species of Special Concern, according to the California Department of Fish and Game, and the IUCN Red List labels it as Near Threatened. Leading causes of the decline of this species include stream scouring, introduction of nonnative species (such as American bullfrogs) habitat loss, the use of pesticides, logging and mining. Any activity that alters stream flow, water temperatures, or stream bed siltation may harm this species. ("AmphibiaWeb", 2010; Fellers, 2010)

Some populations of Rana boylii occur in national forests of California and Oregon, a few national, regional, and state parks, as well as on properties owned by The Nature Conservancy. However, these do not necessarily contribute an adequate amount of support for the declining species. ("AmphibiaWeb", 2010; Fellers, 2010)

Contributors

Samantha Aliah (author), Michigan State University, James Harding (editor), Michigan State University, Rachelle Sterling (editor), Special Projects, Catherine Kent (editor), Special Projects.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

detritus

particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

freshwater

mainly lives in water that is not salty.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

polygynous

having more than one female as a mate at one time

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltatorial

specialized for leaping or bounding locomotion; jumps or hops.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

visual

uses sight to communicate

References

1951. Amphibians of Western North America. Berkeley and Los Angeles: University of California Press.

1995. Handbook of Frogs and Toads of the United States and Canada. Ithaca and London: Comstock Publishing Associates.

1913. The Frog Book. Garden City New York: Doubleday, Page & Company.

2010. "AmphibiaWeb" (On-line). AmphibiaWeb.org. Accessed November 19, 2010 at http://amphibiaweb.org/cgi/amphib_query?where-genus=Rana&where-species=boylii&account=amphibiaweb.

2010. "California Reptiles and Amphibians" (On-line). CaliforniaHerps.com. Accessed November 19, 2010 at http://www.californiaherps.com/frogs/pages/r.boylii.html#description.

2010. "Encyclopedia of Life" (On-line). www.eol.org. Accessed November 20, 2010 at http://www.eol.org/pages/1019448.

2008. Mating strategy and breeding patterns of the foothill yellow-legged frog (Rana boylii). Herpetological Conservation and Biology, 3(2): 128-142.

Ashton, D. 1997. "FOOTHILL YELLOW-LEGGED FROG (Rana boylii) Natural History" (On-line). Accessed November 20, 2010 at http://www.krisweb.com/biblio/gen_usfs_ashtonetal_1997_frog.pdf.

Chanson, J., T. Boucher. 2004. Disappearing Jewels: The Status of New World Amphibians. Arlington, Virginia: NatureServe.

Fellers, G. 2010. "IUCN Red List of Threatened Species: Rana Boylii" (On-line). IUCN Red List of Threatened Species. Accessed November 22, 2010 at http://www.iucnredlist.org/apps/redlist/details/19175/0.

Leonard, W., H. Brown, L. Jones, K. McAllister, R. Storm. 1993. Amphibians of Washington and Oregon. Seattle, WA: Seattle Audubon Society.

Stebbins, R. 2003. A Field Guide to Western Reptiles and Amphibians. New York, NY: Houghton Mifflin.

Wegner, K., J. Crayon. 2009. Diets of Three Species of Anurans from the Cache Creek Watershed, California, USA. Journal of Herpetology, 43(2): 275-283.

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