Great Basin Spadefoot
Great Basin Spadefoot
Other names: Scaphiopus intermontanus
The Great Basin Spadefoot is a secretive species that is primarily nocturnal. They are generally small, growing up to 5-6.5 cm in length. They are distinctive from other frogs and toads in having teeth in their upper jaw, vertical pupils, relatively smooth skin, a blunt snout, and no .They are characterized by a sharp, wedge-shaped, keratinized or “spade” on the hind foot. They have upturned eyes, small tubercles covering their smooth skin, a glandular hump or “boss” between the eyes, and short legs with webbed feet. The is small and discrete. They range in colour from gray-green, to olive, to tan and often the colouration matches the local substrate. The belly is white and they may display light stripes on the flanks. The eyes are golden yellow with distinctive vertical pupils. Great Basin Spadefoot tadpoles have long tails with a large tail fin. The head is triangular with upturned eyes that are high on the head. They are gray, brown, or black with brassy speckling and the belly is golden. Larvae may grow up to 7 cm before undergoing metamorphosis.
Great Basin Spadefoot Call
The Great Basin Spadefoot call is a series or rapid, low-pitched, guttural noises that sounds like “gwaa, gwaa”. Males may form a chorus that can be heard hundreds of meters away.
The Great Basin Spadefoot may be confused with the Western Toad, the only other toad species in British Columbia. However, the Western Toad has bumpier skin, horizontal pupils, large parotid glands behind the eyes, and lacks the ‘spade’ on the hindfeet that is characteristic of the Great Basin Spadefoot.
The Coastal and Rocky Mountain Tailed Frogs also have vertical pupils but they do not have a tympanum and occupy very different habitats.
Credit: Charles Peterson
Great Basin Spadefoot
Credit: Joe Crowley
Credit: Dave Huth
Coastal Tailed Frog
Credit: Charles Peterson
Rocky Mountain Tailed Frog
The Great Basin Spadefoot ranges from southern British Columbia to eastern California along the eastern side of the Coast Mountains and Arizona, and east to Colorado and parts of New Mexico and Wyoming. In Canada, it is restricted to the Okanagan, Thompson/Nicola, Similkameen, Kettle-Granby, and Fraser Valleys in the arid south-central region.
The Great Basin Spadefoot is found in drier habitats than most toads and prefers semi-arid habitats like dry grasslands, open woodlands, and sagebrush plains. In British Columbia, they breed in vernal ponds and semi-permanent alkali lakes. These toads are primarily found below 1600 m elevation, in areas with neutral to basic soils. They hibernate through the winter below the frost line in burrows they excavate themselves, or by reusing old mammal burrows. They are well adapted to drought and requires loose, sandy soils that they can burrow in to seek shelter during hot and dry periods. They are most active after heavy rains, when they emerge from their burrows to feed and breed. Great Basin Spadefoot are born in aquatic habitats, but quickly migrate to nearby terrestrial habitats. Metamorphosed spadefoots spend the majority of their time on land.
Breeding in Great Basin Spadefoot is triggered by substantial rainfall that fills breeding sites. After emerging from hibernation in early April, the toads become active on the surface during warm spring evenings and move from their overwintering habitats to a variety of different aquatic breeding areas. Great Basin Spadefoot prefer ephemeral pools that fill and dry each year, but may be found breeding in slow-moving streams, irrigated depressions, ditches, and springs. Males will gather and call at breeding sites to attract females. Males grasp the females in amplexus with fertilization occurring externally. The female will lay up to 1000 eggs in small clusters of 20-40 eggs at a time that are typically attached to vegetation in shallow water. Eggs are black on top with creamy undersides. Egg development is rapid, completing within a week but as quickly as two days if conditions are very warm, and tadpoles metamorphose after 4-8 weeks. Sexual maturity is reached within 2-3 years of age with a lifespan that may be over 10 years.
Adults and toadlets generally forage nocturnally and eat a variety of invertebrates including worms, beetles, flies, ants, and grasshoppers. Tadpoles may be omnivorous or carnivorous and the mouth morphology of the individual will shift depending on their diet. For example, carnivorous tadpoles have a sharp “beak” to assist in hunting fish or conspecifics. Adults may be able to gather enough food to survive for a year in dormancy in only a few feedings.
The Great Basin Spadefoot appears to have declined throughout its range in British Columbia and is particularly vulnerable to habitat destruction by cattle-grazing and recreational vehicle use. The grassland ecosystems that they occupy are among the rarest habitats in B.C. and Great Basin Spadefoot are restricted within these habitats to areas with suitable breeding ponds. Trampling of soils by cattle may compact earth to the point that the toads may not be able to effectively burrow. Cattle presence may also decrease water quality in breeding areas through siltation and increased nutrient loading. Agricultural practices that reduce the water table in grasslands can result in the complete loss of breeding ponds. However, the creation of reservoirs can create new habitat and be beneficial to Spadefoots. The introduction of predatory species of fish to some areas may also be contributing to declining populations. Road mortality can cause high levels of mortality, particularly during mass spring migrations as these toads typically move at night and are quite small, making it difficult for drivers to avoid them. Pollution from pesticides and herbicides, road salt, and sediment from forestry can cause mortality and developmental defects. They may also be susceptible to pathogens such as Chytrid fungus and Ranavirus. Climate change is a threat to all frog and toad species as it may increase the frequency and severity of drought and flood events.
Did You Know?
Great Basin Spadefoots are nocturnal, avoiding the hot and dry parts of the day by burrowing underground with their spades. On cool, damp evenings they emerge on the surface to forage and breed. They may be coaxed to the surface artificially, by stomping on the ground or driving over their burrowing site, which is believed to be mistaken for the vibrations of heavy rain falling.
Great Basin Spadefoots are well adapted to drought and dry conditions. They are able to lose almost 50% of their body moisture without any negative effects.
They may spend prolonged periods underground during drought, where they are able to absorb water directly through their skin from the soil in their burrows.
When threatened by predators, Spadefoots secrete a noxious substance from their skin as a deterrent.