Boreal toad (Anaxyrus boreas)

Also known as: California toad, western toad
Synonyms: Bufo boreas, Bufo politus
KingdomAnimalia
PhylumChordata
ClassAmphibia
OrderAnura
FamilyBufonidae
GenusAnaxyrus (1)
SizeFemale snout-vent length: c. 11 cm (2)
Male snout-vent length: c. 9.5 cm (2)
Tadpole head-body length: 1.4 - 1.7 cm (2)
Tadpole tail length: 1.6 - 2 cm (2)

The boreal toad is classified as Near Threatened (NT) on the IUCN Red List (1).

A large, robust amphibian of western North America, the boreal toad (Anaxyrus boreas) has thick, dry skin that is covered in dark or rusty blotches (1) (3). Variable in colour, the boreal toad has a mixture of green, tan, reddish-brown, dusky gray, and yellow on the upperparts (3), while the underparts tend to be pale with black mottling (4). There is also a pale stripe down the middle of the back, and the throat is pale (3) (4). 

The boreal toad has a stocky body, with short legs, and tends to walk rather than hop. The head is wide, with a prominent kidney-shaped swelling called a ‘parotoid gland’ behind the large eyes. The eyes are beautifully flecked with gold and have distinctive horizontal pupils (4). The male boreal toad is generally smaller than the female,and  has less blotched, smoother skin (3). The male also has dark pads on the thumbs that help it cling to the female during mating (4). 

Unlike many other toads, the male boreal toad has no vocal sac and, therefore, no mating call. However, males are known to emit a chirping sound when distressed, when clasped by other males, or when gathered in breeding aggregations. Females will also emit a chirping sound, but are usually silent (2). 

There are currently two recognised subspecies of the boreal toad: the California toad, Anaxyrus boreas halophilus, and the nominate, Anaxyrus boreas boreas. Anaxyrus boreas boreas generally has more dark blotches on the belly than the California toad, as well as a wider head, larger eyes and smaller feet (3).

The boreal toad occurs throughout much of the mountainous areas and along the Pacific coastline of the western United States (2). It ranges from southern Alaska south to Baja California, and eastward to the Rocky Mountains in west-central Alberta, Montana, Wyoming, Utah and Colorado. It formerly occurred in northern New Mexico, but is now thought to be extinct at this location (1). 

The California toad ranges throughout most of California, from the northern forests, east into west-central Nevada, and south through most of the state, east of the deserts, into northern Baja California (3).

Occupying a great variety of habitats, the boreal toad may be found around desert springs, meadows, woodlands and mountain wetlands, as well as ponds, lakes, reservoirs, and slow-moving rivers and streams (1) (3). During the breeding season the boreal toad is most commonly found near permanent or temporary water bodies that have shallow sandy bottoms, while after breeding it disperses into terrestrial habitats, such as forests and grasslands. Even when it roams far from water, the boreal toad prefers moist conditions (4). This species is found at elevations between sea level and around 3,640 metres (1).

Active by night and day, the boreal toad seeks out its invertebrate prey by moving slowly across the ground (3). The boreal toad locates its prey using its excellent vision, and then lunges its large sticky tongue to catch the target and bring it to its mouth to eat. Boreal toad tadpoles consume algae and detritus, including fish carrion (3). 

For defence, the boreal toad relies on its paratoid glands and warts, which secrete a poison that deters predators. As a result, many predators have learned to avoid the boreal toad to prevent them ingesting this poison. However, some other predators have learnt to only consume edible body parts; ravens, for example, only eat the internal organs (3). 

The boreal toad hibernates through winter months. In mountainous regions, it emerges from its burrow refuge when the snow has melted and the temperature during the day remains above freezing. Breeding begins soon after it emerges, occurring in July or August at higher altitudes, but as early as May at lower altitudes (2). The male boreal toad arrives at the breeding sites first, using scent cues to find its way (3). It soon initiates breeding by vigorously pursuing and clasping receptive females (2). Mating takes place in water and during amplexus (the mating embrace), the mating male will kick away other males, occasionally even engaging in brief fights (3). The female boreal toad lays 2 strings of up to 16,500 eggs in shallow water (2). The eggs contain some of the adult boreal toad’s toxins, which offers some protection from predation, and the eggs hatch after three to ten days (3). 

Tadpole development is highly dependent on water temperature, and the tadpoles may enter metamorphosis as early as 30 days after hatching in warm water (3), but may take as long as 75 days in cold water (2). During metamorphosis, the tadpoles aggregate in shallow water. Newly-transformed boreal toads often spend the winter at the border of the wetland in which they hatched, although they may disperse to nearby sites. The boreal toad may live for up to nine years (3).

The boreal toad population has undergone a drastic decline, disappearing completely from many parts of its range. A number of threats to the boreal toad have been identified, yet the exact cause of this decline, as well as the significance of each threat, is currently poorly understood. The Rocky Mountain populations in Colorado and Wyoming have undergone a drastic decline since the 1970s. Populations of the boreal toad have also declined greatly in the Yosemite area of the Sierra Nevada, while this species is also undergoing localised declines in Yellowstone National Park, Montana (1). 

One of the greatest threats to the boreal toad is thought to be habitat destruction and degradation (1). Development in and around wetlands can destroy or isolate populations, while traffic on roads can kill migrating toads (3). The pollution and introduction of aquatic predators, such as fish stocking in lakes, also degrades this species’ habitat, as well as causing increased mortality and the greater likelihood of the spread of disease (4). In the Cascade Range of Oregon, predation by ravens during the toad breeding season appears to have contributed significantly to some population declines, while significant predation by birds has also been observed in Colorado and Idaho (1). 

Another threat to the boreal toad is the chytrid fungus, which may have been the cause of mass deaths in the southern Rockies. This threat is likely to increase in significance with global climate change, which will create the ideal conditions for the spread of this deadly disease (1) (4).

The boreal toad is afforded sanctuary in a number of national parks, wildlife refuges, and wilderness areas in the U.S, where its habitat is protected. In Mexico, its habitat is also protected by the Parque Nacional Sierra de San Pedro Martir (1). 

The alarming decline of the boreal toad has prompted a large amount of conservation action. In some areas, monitoring programmes have been implemented, while surveys have been conducted to identify breeding sites and assess the habitat requirements of this species (4). Other research has focused on developing methodologies for reintroducing the boreal toad to areas where it formerly occurred, as well as refining husbandry practices for captive toads and ways to detect the chytrid fungus in the wild (5). 

For more information on amphibian conservation:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
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  1. IUCN Red List (May, 2011)
    http://www.iucnredlist.org/
  2. U.S. Fish & Wildlife Service (2004) Species Assessment and Listing Priority Form. U.S. Fish & Wildlife Service, U.S.
  3. CaliforniaHerps.com - Anaxyrus boreas halophilus (May, 2011)
    http://www.californiaherps.com/frogs/pages/b.b.halophilus.html
  4. Government of British Columbia - Western toad (May, 2011)
    http://www.env.gov.bc.ca/wld/frogwatch/publications/factsheets/frogs/western-toad.htm
  5. Colorado Division of Wildlife - boreal toad (May, 2011)
    http://wildlife.state.co.us/WildlifeSpecies/SpeciesOfConcern/Amphibians/BorealToad.htm