Species: Yosemite Toad Bufo canorus

Family: Bufonidae Order: Anura Class: Amphibia

Species note author: Carlos Davidson
Reviewed by: Cynthia Kagarise Sherman
Date: July 1994

DISTRIBUTION, ABUNDANCE, AND SEASONALITY

The Yosemite Toad is a small montane toad, endemic to the Sierra Nevada Mountains from Ebbetts Pass, Alpine County to south of Kaiser Pass and Evolution Lake, Fresno County (Stebbins 1966, Karlstrom, 1962, 1973). Yosemite toads occur from 6,400 to 11,300 feet elevation, with the majority of sites between 8,500 and 10,000 feet (Karlstrom 1962). It is found in open montane meadows near lodgepole pine forests (Kagarise Sherman 1980 and Martin in prep.).

Yosemite toads are closely related to three other species (Black toad Bufo exsul, western toad Bufo boreas, and Amargosa toad, Bufo nelsoni) which together constitute the "boreas group" (Blair 1972). The Yosemite Toad can be distinguished from the similar looking Western Toad (Bufo boreas) by its smaller size, wide paratoid glands, the small space between paratoids, lack of vertebral stripe and high degree of sexual dichromatism (Martin in prep.). Males are olive green with varying black spotting (Kagarise Sherman 1980), females are grey or brown with dark markings. Males and females are more dimorphic and dichromatic than any other California anura.

SPECIFIC HABITAT REQUIREMENTS

Feeding: Examination of the stomach contents of Yosemite toads has found tenbronid beetles, weevils, large ants, a centipede, spiders, ladybird beetles, dragonfly nymphs, mosquitoes and lepidoptera larvae (Grinnel and Storer 1924, Mullally 1953). Wood (1977) reported that Hymenoptera were almost 80 percent of the summer diet.

Reproduction: Yosemite toads breed in shallow pools and small, slow moving, shallow streams usually in meadows (Martin in prep.). Karlstrom (1962) describes breeding sites as meadow edges without deep water or adjacent steep terrain.

Water: Yosemite toads have seldom been found more than a hundred yards from permanent water, although they spend little time actually in water (Karlstrom 1962).

Cover: When not active, Yosemite toads take cover in rodent burrows, under surface objects and in willow thickets (Karlstrom 1962). In the Tioga Pass area toads primarily utilized burrows of meadow mice (Microtus montanus) and pocket gophers (Thomomys monticola) (Karlstrom 1962).

Temperature: Karlstrom (1962) estimates critical thermal maximum of 36-38o C for larvea and 31o C as upper limiting temperature for egg development. Critical thermal maximum for adults were estimated at 38-40o C and a lower thermal limit at -1o C (Karlstrom 1962). Toads seem to exhibit little temperature preference in the field, where temperatures were reported between 2-30o C, with no sign of temperature stress (Karlstrom 1962).

SPECIES LIFE HISTORY

Hibernation: Yosemite toads enter hibernation in late September or early October, and emerge in the Spring (April-July) after five to six months. The toads utilize rodent burrows, crevices under rocks, or the base of willows for hibernation (Kagarise Sherman 1980). In the Tioga pass area rodent burrows utilized were of voles (Microtus montanus), gophers (Thomomys sp.) and Belding ground squirrels (Spermophilus beldingi) (Kagarise Sherman 1980).

Reproduction: Males emerge from hibernation for breeding as soon as snow melts from pools in meadows (Kagarise Sherman 1980 and Martin in prep.). Males arrive at breeding pools several days before females (Kagarise Sherman 1980 and Kagarise Sherman and Morton 1984). Breeding takes place from mid May to mid August (Kagarise Sherman 1980 and Martin in prep.) Both sexes are primarily active during the day (Kagarise Sherman 1980, Martin in prep., and Kagarise Sherman and Morton 1984). There may be ten times as many males as females at a breeding site (Karlstrom 1962 and Kagarise Sherman 1980). Males call diurnally and can be heard from more than 100 yards (Kagarise Sherman 1980 and Martin in prep.). Grinnel and Storer (1924) remarked on the toads call, "It's mellow notes are pleasing additions to the chorus of bird songs just after the snow leaves." Camp (1916) gave the species the name canorus which in latin means "tuneful".

Eggs are laid in single or double strands, typically in pools or streams not more than three inches deep with a loose silt substrate (Martin in prep.). A single females lays an estimated 1,500 to 2,000 eggs (Martin in prep.). In the Tioga Pass area, eggs hatched in about 10-12 days, and tadpoles metamorphose seven to nine weeks after the eggs are laid (Kagarise Sherman 1980, Kagarise Sherman and Morton, 1984). Females first breed at 4-6 years and males at 3-5 years of age (Kagarise Sherman 1980). Kagarise Sherman and Morton (1984) estimate that some females may live at least 15 years and males at least 12 years.

Kagarise Sherman (1980) and Kagarise Sherman and Morton (1984, 1993) found that desiccation was the largest cause of tadpole mortality in the Tioga Pass area. A thin snowpack or extra warm summer lead to drying of breeding ponds before tadpoles metamorphose.

Movement: Individual males only stay at breeding ponds for a week or two, and females only for only a few days (Kagarise Sherman 1980 and Kagarise Sherman and Morton 1984). After breeding both sexes move into meadow areas to feed for two to three months before hibernating (Kagarise Sherman 1980 and Kagarise Sherman and Morton 1984). From late summer on the toads are only active on warm days (Kagarise Sherman 1980). Unlike Bufo boreas which travels widely at night, Bufo canorus stays in a relative small area and is generally only active during the day (Martin in prep.). At Tioga Pass Meadow Kagarise Sherman found toads traveled 150-230 meters between hibernaculum and breeding ponds. First year juveniles hibernate near the pools from which they emerged (Kagarise Sherman 1980). Due to the patchy distribution of the montane meadow habitat used by Yosemite Toads, their populations may be isolated.

Niche: Mullally (1953) observed that Yosemite toad tadpoles are preyed upon by Foothill Yellow-legged frogs (Rana boylii) and dragon fly nymphs. Kagarise Sherman (1980) and Kagarise Sherman and Morton (1984, 1993) reported robins (Turdus migratorius) and diving beetles (Dytiscus sp.) eating tadpoles. Karlstrom (1962) reported that garter snakes, Brewers black birds and California Gulls have all been observed eating other species of tadpoles and all are present in the range of canorus, but there are no records of them eating canorus tadpoles. Kagarise Sherman (1980) and Kagarise Sherman and Morton (1984, 1993) report on adult Yosemite toads being killed by Clark's Nutcrackers (Nucifraga columbiana) and California gulls (Larus californicus). Ravens may also eat adult Yosemite toads (see below).

Status: Kagarise Sherman and Morton (1993) report sharp declines in Yosemite toads in the Eastern Sierra Nevada from 1971 to 1991. At one site at Tioga Pass the number of marked males entering breeding pools declined nine fold. They attribute the decline to drought, predation and disease. As discussed above, tadpoles are susceptible to mass desiccation if breeding ponds dry up before metamorphosis. The prolonged California drought has resulted in below average snow depths, and low or no reproductive success at many breeding sites. Kagarise Sherman and Morton (1993) report that disease caused extensive deaths of adults in 1977 and 1978. At one site it appears that predation by common ravens (Corvus corax) may have contributed to declines. According to the Breeding Bird Survey (USFWS, unpublished data) the number of common ravens have increased in the Sierra Nevada by 9.5 percent annually over the period from 1966 to 1989. Increases in ravens may be related to human activities (Kagarise Sherman and Morton 1993).

Karlstrom (1962) noticed Yosemite toads would stop calling when vehicles passed on roads as far as half a mile away. He suggested that continual daytime traffic along some roads may account for the low number of canorus populations near roads (Karlstrom 1962). To date, no research has been done to determine if traffic noise is detrimental to Yosemite toads. Kagarise Sherman and Morton (1993), however, found no correlation between extent of declines and distance to nearest roads at their study sites. Finally, livestock grazing may have detrimental impacts on Yosemite toads through trampling, alteration of meadow habitat, and possible lowered water quality (Martin, pers. com.).

Literature Cited

Blair. 1972. Evolution in the Genus Bufo. Austin: University of Texas Press.

Camp, C. L. 1916. Description of Bufo canorus: A new toad from the Yosemite National Park. University of California Publications in Zoology 17 (6): 59­62.

Grinnel, J. and T. I. Storer. 1924. Animal Life in the Yosemite. Berkeley, CA: University of California Press.

Kagarise Sherman, C. 1980. A comparison of the natural history and mating system of two anurans: Yosemite Toads (Bufo canorus) and Black Toads (Bufo exsul). PhD Thesis, University of Michigan No. 8106225:i­394. University Microfilms International.

--- and M. L. Morton. 1984. The toad that stays on its toes. Natural History 93 (3): 72­78.

--- and M. L. Morton. 1993. Population declines of Yosemite Toads in the Eastern Sierra Nevada of California. J. Herpetol. 27(2):186-198.

Karlstrom, E. L. 1962. The toad genus Bufo in the Sierra Nevada of California. University of California Publications in Zoology 62 (1): 1­104.

­­­. 1973. Bufo canorus Camp, Yosemite Toad. Catalogue of American Amphibians and Reptiles.

Martin, D. L. , in prep. Bufo canorus Camp, the Yosemite toad. Unpublished manuscript.

Mullally, D. P. 1953. Observations on the ecology of the toad Bufo canorus. Copeia 3: 182­183.

Stebbins, R. C. 1985. A field guide to western reptiles and amphibians. Houghton Mifflin. Boston, Mass.

Wood, T. S. 1977. Food habits of Bufo canorus. Unpubl. MS Thesis, Occidental College.