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Canadian Herpetological Society Société d'Herpétologie du Canada |
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Western Toad
Anaxyrus boreas
Family: Bufonidae
Until recently, the Western Toad was in the genus Bufo, but it is now in the genus Anaxyrus.
COSEWIC status:- Special Concern
- Special Concern
- Least Concern
Description
The Western Toad is a large toad and females can grow to 12.5 cm in body length, but males are smaller and do not exceed 11 cm. Individuals are typically brownish grey or brownish green, but colouration can vary from green to reddish brown to almost black. There is usually a white or cream-coloured vertebral stripe that runs from the snout down the length of the back, though it can be faded on some individuals. The belly is dull white with dark mottling. Western Toads have granular skin with large wart-like bumps and a large, kidney-shaped parotoid gland behind each eye, but they lack cranial crests that are typical of other toads in Canada. The warts and parotoid glands are often reddish brown and may be encircled by black rings. The legs are short and the hind feet have moderate webbing between the toes. Males develop enlarged black nuptial pads on their thumbs during the breeding season. Larvae (tadpoles) have long tails with a large fin and lack front legs (newly hatched tadpoles are legless). Young larvae are black with a clouded tail fin that lacks patterning, but they become less dark as they age. The larvae can grow up to 4 cm in total length before metamorphosis.
Western Toads in Alberta, southeastern BC and possibly at some sites in the northwestern U.S. have vocal sacs and produce a long, high-pitched trill that is characteristic of other toads. Throughout the rest of the species’ range, however, individuals lack vocal sacs and the breeding call is limited to a series of soft chirps.
Similar Species
The Western Toad can be confused with the Canadian Toad and Great Basin Spadefoot. The Canadian Toad has prominent cranial crests (elevated ridges between the eyes), which the Western Toad lacks. The Great Basin Spadefoot lacks cranial crests and parotoid glands and has vertical pupils.
Distribution
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In Canada, the Western Toad occurs throughout most of BC and Alberta, the southeastern Yukon and in the extreme southwestern Northwest Territories; it is absent from extreme northeastern BC and southeastern Alberta, and its distribution is poorly understood in northern Alberta. This species occurs throughout the Western U.S. from the Pacific Coast east to western Montana and northern New Mexico, though it is absent from Arizona.
Habitat
Western Toads typically breed in shallow aquatic habitats with sandy substrates, such as the edges of lakes, rivers, ponds, streams and ditches. Beaver ponds are commonly used for breeding in the northern portion of their range. Outside of the breeding season, Western Toads can be found in a variety of habitats, such as wetlands, the shorelines of lakes and rivers, forests, meadows, scrublands and subalpine and alpine meadows. Individuals hibernate underground below the frost line in mammal burrows, rock crevices, cavities in peat hummocks and other underground cavities.
Biology
In Canada, Western Toads hibernate during the winter and are active from March or April until late September or October, depending on latitude and elevation. Males call to attract females during the breeding season, which occurs over a short period between April and early June in Canada. During breeding, the male grasps the female (amplexus) and fertilization occurs externally in aquatic habitats as the female lays eggs. The female lays 3,000–12,000 eggs in two long strings that can be over 20 m in length. The eggs are black and are surrounded by two clear jelly envelopes. The eggs develop rapidly and hatch in 3–12 days and the tadpoles transform into juvenile toads after 4–12 weeks, depending on water temperature. Males reach sexual maturity at 3–4 years of age and females mature at 4–6 years of age. Western Toads can live for more than 10 years. Individuals forage, primarily at night, for a wide variety of invertebrates, such as insects, spiders, slugs, and worms. Toxins that deter predators are produced by glands in the toad’s skin, particularly in the parotoid glands and the wart-like bumps. Western Toads can tolerate dry conditions and are often found considerable distances from water. Individuals often return to the same breeding site in successive years, and they can travel several kilometers between breeding sites, summer habitats and hibernation sites.
Threats
Habitat loss resulting from urban development, large scale oil and gas extraction and intensive agricultural has resulted in population declines and local extirpation. The extensive road network associated with these activities also poses a significant threat to Western Toads throughout much of their range, resulting in particularly high rates of road mortality when roads bisect breeding sites or travel corridors. Pesticides/herbicides, road salt, industrial contamination and other environmental pollutants can be detrimental to frog and toad populations by causing direct mortality as well as developmental deformities. Heavy metals and airborne pollution from oil and gas extraction are of particular concern in Western Canada. Pathogens, such as chytrid fungus and Ranavirus, can cause mass mortality of frog and toad populations. Although these pathogens are present in Western Toad populations in Canada, population decline and extirpation from disease outbreaks appears to be rare. Climate change may also pose future threats to Canada’s frog and toad populations, particularly by increasing the frequency and severity of droughts.