Bufo boreas

Western Toad

Figure 1: Western Toad

Photo by Melanie Murphy

 

Taxonomy:

Kingdom: Animalia

Phylum: Chordata

Superclass: Tetrapoda

Class: Amphibia

Subclass: Lissamphibia

Order: Anura

Suborder: Neobatrachia

Family: Bufonidae

Genus: Bufo

 

Recognized Subspecies:

Boreal toad (Bufo boreas boreas)

California toad (Bufo boreas halophilus)

 

Identification:

Western Toads range in size from 5.1-12.7 cm. They can be tan, gray, greenish, yellowish, or brown on top, with a thin white or cream dorsal stripe. Dorsal warts are set in dark blotches sometimes tinged with a rusty color. Ventrally Western Toads have dark blotches on a light cream background. Parotoid glands are oval but not overly elongated, well separated, and slightly larger than upper eyelids when viewed from above. They have well developed tarsal fold and no cranial crests. Other identifying features include no teeth in the upper jaw, a horizontal pupil, and two tubercles on each hind foot. Males are less blotched and have smoother skin than females. Both males and females have pale throats.

Figure 2: Adult Western Toad

Photo by Melanie Murphy

 

How to distinguish from similar species: Black Toad is more heavily mottled both above and below with black and is smaller. Woodhouse's Toad has conspicuous cranial crests and the male has a dark throat.

 

Young: Newly transformed Western Toads are around .6 cm. Dorsal stripe can be weak or absent in young.

Figure 3: Metamorph at Timpanogas Lake, Oregon

Photo by Catherine Searle, Blaustein Lab, Oregon State University

 

Tadpoles: 5 cm or less in length. Dark coloration (dark brown, dark gray, charcoal or black) including in tail muscle. Dorsal fin may be speckled. Eyes don't extend past margin of the head. Tadpoles often aggregate in large groups along the shallows in lakes and ponds.

                                        

Figure 4: Tadpole aggregation at Todd Lake, Oregon                                                    Figure 5: Western Toad Tadpoles

Photo by Catherine Searle, Blaustein Lab, Oregon State University                             Photo by Melanie Murphy

 

 

Eggs: Laid in 1-3 tangled strings, strings usually contain a zigzag double row of eggs. The strings have two gel layers (this helps to distinguish them from the eggs of Woodhouse's Toad which only have a single layer). Up to 16,500 eggs per clutch, usually around 12,000 per clutch.  Strings are often entwined in vegetation near the shore of ponds, streams, or reservoirs.

Figure 6: Western Toad egg strings at Lost Lake, Oregon

Photo by Catherine Searle, Blaustein Lab, Oregon State University

 

Distribution:

Western Toads range from southern Alaska to northern Baja California, from the Pacific Coast to the Rocky Mountains. They are mostly absent in the southwestern United States. Elevation ranges from sea level to over 3,600 meters. This species has experienced range reduction in the Pacific Northwest, and in the southern Rocky Mountain region. Western Toad is severely imperiled in parts of Wyoming and may be extinct in New Mexico.

 

Distribution within Idaho:

Western Toads can be found throughout Idaho in appropriate habitats.

 

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Map accessed at http:www.natureserve.org/explorer on September 25, 2008

 

Habitat:

Found in a wide variety of habitats including grassland, woodland, desert streams and springs, mountain meadows, and montane forests. Live in and near ponds, reservoirs, rivers, lakes, and streams but are largely terrestrial. Adults are often found near the edge of the water or basking on partially submerged logs in the spring and early summer.  Later in the year during colder weather they are found in more terrestrial habitats but usually still in moist areas or near water.

 

Behavior:

Western Toads usually walk rather than hop. They tend to burrow in loose soil or use the burrows of other animals. Activity differs seasonally and geographically. At low elevations Western Toads are nocturnal in summer and diurnal in late winter/spring. In Mountain regions they are active day or night in the summer depending on weather conditions.  In cold climates they may hibernate during winter.

 

Reproduction:

Western Toads become sexually mature at 4-6 years old. Breeding occurs from late January to through July depending on conditions. Adults emerge from hibernation and congregate at breeding sites. Breeding sites are typically still or barely moving water such as streams, ponds, drainage dishes, and rain pools.  Males often outnumber females by as much as 20:1 at breeding sites. Females deposit as many as 16,500 eggs per clutch (12,000 on average) near the edges of still ponds, streams, and reservoirs. Females may skip one or more years between breeding events. Tadpoles metamorphose during the first summer at lower elevations, and during the second summer in the mountains.

 

                  

Figure 7: Breeding pair of Western Toads                                                             Figure 8: Breeding pair of Western Toads

Photo by Catherine Searle, Blaustein Lab, Oregon State University                    Photo by Melanie Murphy

 

Males do not have typical mating calls, but they do vocalize and their chorus may be attractive to females when they come together in breeding aggregations. An alternate proposed function for the call of the Western Toad is that males do not discriminate between sexes when trying to mate, some studies have shown that the call may be a release call triggered when a non discriminating male tries to initiate amplexus with another male. The call of the Western Toad is not a typical anuran advertisement call and its function is unresolved.

 

Voice:

Western Toads have no vocal sac and do not produce standard anuran mating calls. They can vocalize though and produce a high-pitched chirping sound that sounds similar to a chick peeping.  When vocalizing in chorus they sound like a flock of geese in the distance.

 

Diet:

Larvae are filter feeders that eat suspended plant material and detritus.  Adults eat spiders, crayfish, earthworms, sowbugs, and flying insects. The toads use their vision to locate prey then catch the prey with a quick extension of their large sticky tongue  (called a "zot") and bring it to their mouth.

 

Predators:

Birds, garter snakes, coyotes, raccoons, and badgers feed on adult Western Toads. Tadpoles are vulnerable to predatory insect larvae, ducks, spotted sandpipers, and garter snakes. Western toads can secrete a mild toxin from their parotoid glands that some predators find unpalatable.  This toxin may also be deposited in the ova and provide protection to the newly laid eggs.

 

Scientific study:

Interspecific Variation in Susceptibility of Frog Tadpoles to the Pathogenic Fungus Batrachochytrium dendrobatidis.

Blaustein et al. 2005. Conservation Biology.

 

Premise: Batrachochytrium dendrobatidis (Bd) is a fungus that causes chytridiomycosis, a disease that is fatal to newly transformed and adult forms of many amphibian species. Bd affects the keratinized skin of metamorph and adult amphibians. Bd has been implicated in global epizootics and population declines in several amphibian species (Daszak et al. 2003); therefore, understanding more about this fungus is a key issue in amphibian conservation. This study set out to determine if Bd can also cause mortality in amphibians at the larval stage (by infecting the keratinized tissue found in the jaw sheaths and tooth rows of larvae), and to determine if there are differences in susceptibility to Bd among amphibian species at the larval stage.

 

 

Methods:

 

Results: Survival was significantly less among B. boreas larvae exposed to Bd when compared to B. boreas larvae from the control treatment, however, those tadpoles that died did not show histological signs of infection.  Survival was not significantly different in the Bd treatments vs. control treatments for any of the other three species. In addition, B. boreas exposed to Bd showed a marked lethargy in the days following exposure, none of the other three species exhibited this effect or any other difference in behavior between treatments. Only R. cascadae showed a significant difference in mouthpart abnormalities in the Bd treated larvae when compared to control larvae, the other three species showed no statistically significant difference. Finally, histology showed the incidence of infection to be high in B. boreas, H. regilla, and R. cascadae exposed to Bd, and found no infection in the control treatments.  For R. cateseiana, on the other hand, histology showed high levels of infection in both the Bd and control treatments, suggesting that these larvae were infected in the field prior to being included in the experiment. The results from experiment 2 supported those from experiment 1, more of the B. boreas larvae in the Bd treatment than the control treatment died during the 48 hour exposure period.

 

 

Discussion: This research suggests that B. boreas larvae may be especially susceptible to Bd when compared to the other three species in this study.  Not only did B. boreas exhibit higher mortality than the other species included in the study, they also exhibited behavioral signs of sickness which none of the other species showed.  In addition, because the B. boreas larvae that died in experiment 1 did not show histological signs of infection this study suggests that some factor related to Bd exposure other than chytridiomycosis caused the tadpole mortality. The researchers hypothesize that this factor may be a toxin produced by the fungus. Finally, this study indicates that larvae (not just recently transformed and adult frogs as has been previously suggested) can be killed by exposure to Bd.

 

References:

AmphibiaWeb: Information on amphibian biology and conservation. [web application]. 2008. Berkeley, California: AmphibiaWeb. Available: http://amphibiaweb.org/ (Accessed: Sep. 25, 2008)

 

Blaustein et al. 2005. Interspecific Variation in Susceptibility of Frog Tadpoles to the Pathogenic Fungus Batrachochytrium dendrobatidis. Conservation Biology. 19:5 1460-1468

 

Daszak et al. 2003. Infectious disease and amphibian population declines. Diversity and Distributions. 9 141-150

 

Digital Atlas of Idaho: Bufo boreas. Idaho State University. 2000. Available: http://inmh.isu.edu/digitalatlas/bio/amph/main/amphmnfr.htm (Accessed: Sep. 25, 2008)

 

Marco et al. 1998. Sex recognition and mate choice by male western toads, Bufo boreas. Animal Behavior. 55:6 1631-1635

 

NatureServe. 2008. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.0. NatureServe, Arlington, Virginia. Available: http://www.natureserve.org/explorer (Accessed: Sep. 25, 2008)

 

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

 

Thank you to Melanie Murphy (Figures 1, 2, 5, and 8) and to Catherine Searle (Figures 3, 4, 6, and 7) for graciously providing me with the photos of Western Toads found on this webpage.

 

Created by Ashley McCally