Western Toad. Bufo boreas

Description: Adult: A relatively large toad, with a head and body length of 55-125 mm. The dorsal and lateral skin, and that of the upper portions of the limbs, is covered with small round or oval warts. Parotid glands are prominent, oblong and larger than the eye. The tympanum is smaller than the eye. The pupils are horizontal. Cranial crests are weakly developed. The limbs are relatively short and the hind toes partially webbed. Keratinized tubercles for digging are found on the ventral surfaces of the feet. Background colour usually green or brown, with a light vertebral stripe from the snout to the vent.

Warts may be reddish-brown and / or may be encircled by a ring of dark pigment. The venter is pale, with dark mottlings on the abdomen and black marks on the throat. Numerous scattered tubercles with black tips occur on the belly.

Larvae: Tadpoles hatch at about 10 mm, and grow to reach 25-30 mm total length. The eyes are placed high on the head. Metamorphosed toadlets are about 12 mm snout-vent length. The tail fin of the tadpole is low and rounded. Labial teeth in two rows anterior and three rows posterior to the mouth - the second anterior row has a gap in the midline. Tadpoles are blackish dorsally and somewhat lighter ventrally. The fins are pigmented, but diffusely so. Anus medial, spiracle sinistral.

Variation: Toadlets are typically dark, with little patterning. Males have nuptial pads. The dark marks on the throat are more prominent in males. This species exhibits physiological colour change, with individuals becoming lighter at higher temperatures. The dorsal colour is highly variable, but the dorsal stripe is rarely lacking. Males have relatively narrower heads and longer arms, and females are generally considerably larger than males. Males lack a vocal sac.

Natural history: In Alberta, active from April until September. Typically found around ponds, streams, rivers and lakes. Active primarily at night at lower elevations, but diurnal at higher elevations and at higher latitudes. It is a largely terrestrial species that may burrow into loose soil or seek shelter in pre-existing burrows of small rodents. It digs hibernacula up to 1.3 m deep. Western toads tend to walk rather than hop. Secretions from the parotid glands and warts are used in defence.

They generally prefer damp conditions and have a ventral pelvic patch of skin that allows them to absorb moisture from the ground. They feed on worms, slugs, insects and other arthropods. In Alberta, beetles and ants appear to predominate in the diet, although other insects and spiders are also taken. They are eaten by snakes, owls, magpies, shrikes, gulls, crows and especially ravens as well as mustelids, bears, foxes and coyotes. They have a distinctive behaviour of inflating themselves with air and standing high on all four limbs when threatened. If all else fails they may urinate on the attacker if molested.

The tadpoles exhibit synchronized behaviour and will school. This may be mediated, in part, by alarm substances released by damaged individuals. Tadpoles eat algae, seek out the warmer microhabitats in the bodies of water in which they live, and may fall prey to tiger salamander larvae. Juveniles may be found in basking aggregations at the edges of ponds. Tadpoles and metamorphosing toadlets are consumed by a wide variety of predators, including dragonfly larvae, mallards, sandpipers, American robins, and red foxes. In captivity individuals have survived for up to thirty-six years.

Reproduction: Breeds in Alberta in April to June. In spring they congregate in bodies of water, usually pools or small ponds, to breed. May breed in water below 10°C. They prefer shallow water with a sandy bottom and will breed in either permanent or temporary bodies of water. Males have a mating call. Males clasp females in pectoral amplexus. If males grasp other males, a release call is emitted. Nearly all breeding activity ceases at night. Eggs of 1.5-1.7 mm diameter are laid in long strings, typically entwined about submerged vegetation. Up to 16,500 eggs may be produced by a female in a single clutch. Hatching is rapid and takes three to twelve days.

Tadpole densities maybe extremely high. A further six to eight weeks are required for metamorphosis. By mid-September in the Rockies, all larvae have metamorphosed and have left the breeding ponds. The animals reach sexual maturity in two to three years. Egg laying at a particular site tends to be synchronized, and all females laying at that site will do so in less than a week. Certain individuals of each batch of tadpoles grow rapidly at first, and release a growth-inhibiting substance that acts upon the smaller tadpoles. This results in metamorphosis occurring in waves rather than all at once. The outcome of this is that mass mortality due to predation or unfavourable climatic conditions may be lessened.

Voice: A repeated, quiet peeping. A slightly more emphatic version of the same sound is used as a release call when molested.

Distribution: In Alberta, it occurs to at least 2,300 m north from the Montana border to at least as far as Lesser Slave Lake. Isolated records are also known from the Loon Lake and High Level areas. Distribution in the northern part of the province may be more extensive, but lack of access to most areas has made observation and verification difficult. Absent from the drier eastern regions of

the province, where it is replaced by Bufo cognatus in the extreme southeast and Bufo hemiophrys in the east. There is some range overlap with the latter. It occurs at all elevations within its range, but is found primarily in boreal forest, sub-alpine and alpine habitats. It occurs marginally in aspen parkland and short-grass prairie. Extralimitally, it is found from coastal Alaska and the southern Yukon south to northern Baja California, east as far as western Wyoming. Isolated populations occur in central Wyoming and Colorado, and northern New Mexico.

Conservation status: This species appears secure in its range in Alberta. There is no evidence of recent declines.

Remarks: Two subspecies are recognized, that occurring in Alberta is Bufo boreas boreas. This has the same range as the species as a whole, except for California and Baja California. Amplexus has been observed in Alberta between this species and Bufo hemiophrys.

Significant references: Black 1970a, 1971a, b; Black and Black 1969; Black and Brunson 1971; Burger and Bragg 1947; Clarke 1974; Cook 1977a; Eaton et al. in press; Feder 1979; Ferguson 1954; Hews 1988; Hews and Blaustein 1985; Huey 1980; Jones et al. 1999; Karlstrom 1962; Lillywhite et al. 1973; Lillywhite and Wassersug 1974; Logier 1932; Moore and Strickland 1954, 1955; Mullally 1956; O'Hara and Blaustein 1982; Rodgers and Jellison 1942; Salt 1979; Sander, 1987; Schueler 1982a.