Western toad (Bufo boreas) COSEWIC assessment and status report: chapter 3


Species description

The Western Toad, Bufo boreas, can be identified by its dry, bumpy skin, conspicuous, oval-shaped parotoid glands, horizontal pupils, and cream-coloured or white dorsal stripe (Fig. 1). It varies in colour from olive-green, to reddish-brown, to almost black. The parotoid glands are large, well separated, and generally larger than the upper eyelids (Blaustein et al. 1995). These large glands, along with those of the dorsum and the dorsal surface of the upper legs, house a mild, white poison that is secreted when individuals are threatened by a predator (Leonard et al. 1993, Blaustein et al. 1995). The ventral surface is pale and coarsely mottled, and the hind feet are equipped with two horny tubercles, which assist it in burrowing (Leonard et al. 1993). These tubercles are yellow in juvenile toads. Males are 60 to 110 mm long. They possess longer arms and narrower heads than females and develop dark nuptial pads on their thumbs during the breeding season. Females are larger, attaining a total length of 125 mm. They are always silent whereas males will chirp their release call when grasped under the arms (Green and Campbell 1984).

Figure 1. Adult Bufo boreas. (Pamela Hengeveld and R. Chris Brodie, photos).

Figure 1.  Adult Bufo boreas. (Pamela Hengeveld and R. Chris Brodie, photos).

Western toad eggs are laid in long, loosely intertwined strings in the shallow margins of lakes and ponds. The eggs are black above and white below and contain a double jelly layer (Corkran and Thoms 1996). The tadpoles are small and black, roughly 12 to 13 mm snout-vent length (SVL), or 25 to 30 mm total length, prior to metamorphosis (Green and Campbell 1984, Blaustein et al. 1995). They have a square snout that juts forward from their round body outline (Corkran and Thoms 1996). Their eyes are set in from the margin of the head when viewed from above (Leonard et al. 1993). They have relatively narrow tail fins, with the dorsal fin about as tall as the greatest height of the tail musculature (Nussbaum et al. 1983). The tail fin does not extend beyond the tail as in other frog tadpoles (Corkran and Thoms 1996). Newly emerged toadlets are approximately 11 mm in snout-vent length (SVL), and possess the dry bumpy skin characteristic of adults (Corkran and Thoms 1996).

Taxonomic status

The Western Toad is also known as the northwestern or boreal toad. The species belongs to the family Bufonidae, suborder Neobatrachia (Ford and Cannatella 1993), characterized by short legs, warty skin, and large parotoid glands (Stebbins 1966).

There are two subspecies of B. boreas: B. b. halophilus in California, western Nevada, and Baja California, and B. b. boreas throughout its remaining range (Stebbins 1985). The southern Rocky Mountain B. b. boreas population (i.e., Colorado, southeast Wyoming, and north-central New Mexico) is geographically isolated from populations to the north and west by dry, non-forested intermountain valleys. It is genetically differentiated and probably represents an independently evolving lineage or species (Ireland 1997, Goebel 1999). Other subspecies may exist. For example, there is an isolated population near the Atlin region, in northern British Columbia, which is closely tied to tufa springs on the Meister and Coal Rivers. Unlike other northern populations, this one breeds in February when water temperatures range from roughly 23 to 26 °C. From this northern latitude site, toadlets can be seen emerging as early as the third week of March (B. Slough, pers. com.). In Alberta, where the range of Bufo boreas overlaps with that of the Canadian toad (Bufo hemiophrys), mismatched amplexed individuals have been recorded (Cook 1983, Eaton et al. 1999). Cook (1983) found an obvious hybrid individual of these two species, but it is not known if this hybrid was fertile.

History of Study within Canada

Only one study in Canada to date, by Davis (2000), has specifically focussed on B. boreas. Numerous amphibian inventories and studies investigating impacts of forest practices on amphibians include information on toads (e.g., Gyug 1996, Ward and Chapman 1995, Hengeveld 2000, Kinsey and Law 1998, Beasley et al. 2000, Haycock and Knopp 1998, Mennell and Slough 1998, C. Paszkowski and B. Eaton, unpublished data). Long-term data sets, extensive inventories, and intensive population studies are lacking, so the distribution and population dynamics (e.g., survivorship, annual population fluctuation, etc.) of this species within Canada remains unclear.

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