Anaxyrus exsul

Geographic Range

Black toads ( Anaxyrus exsul ) are limited to the eastern border of California. The entire population is patchily distributed within an area of 15 ha. They are found as far north as Antelope Spring and as far south as Deep Springs Lake. There is also a disjunct population near Salt Lake City, Utah.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Black toads are limited to less than a dozen inland freshwater springs within a single desert basin. Their habitats consist of marshy wetlands formed by rainwater in the desert biome. They are more commonly found in areas with shallow water and an abundance of vegetation. When breeding, they choose areas such as marshes and bogs. These water sources have an elevation of 1,450 to 1,700 m. When black toads are still young, they are found in shallower waters where they can mature completely. Vegetation surrounding water sources consist of beard grass ( Polypogon ), common rush ( Juncus ), buttercups ( Ranunculus ), sedges ( Carex ), duckweeds ( Lemna ), and more plants. In Anaxyrus , black toads are most tied to aquatic habitats. These toads have never been found more than 12 meters from the water.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial
• freshwater freshwater

• Terrestrial Biomes
• desert or dune desert or dune

• Aquatic Biomes
• rivers and streams

• Wetlands
• marsh marsh
• swamp swamp
• bog bog

Physical Description

Black toads are primarily black with distinguishable features including light stripes and dots throughout their bodies. Males and females have differing physical features. Males have lighter colored throats and smaller body masses, while females are slightly larger with more spots. The average snout-vent length (SVL) of a male A. exsul is 50 mm, while the average SVL for a female is 52 mm. The average snout-vent length as fully matured adults ranges from 44 mm to 76 mm.

When the eggs are laid, it takes 4 to 5 days from them to hatch. They remain as tadpoles for 3 to 5 weeks. Tadpoles can reach up to 35 mm long. Throughout their physical development, black toads turn from an olive color to an ebony or brownish black.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry
• poisonous poisonous

• Sexual Dimorphism
• female larger
• sexes colored or patterned differently

Development

Once black toad eggs are fertilized, they hatch in the next four to five days. They remain in the larval stage for three to five weeks until they metamorphose into adults. These toads have indeterminate growth and take less than a year to fully mature. Tadpoles can grow to be 35 mm. The average length of newly metamorphized black toads ranges from 14 to 19 mm, and increases to 22 to 33 mm SVL by the end of their first year. Adult female black toads have an average SVL of 52 mm, while adult males have an average SVL of 50 mm. Sex is evident only after full development.

• Development - Life Cycle
• metamorphosis metamorphosis
• indeterminate growth indeterminate growth

Reproduction

Breeding occurs in water sources such as marshy areas and ponds. Black toads are polygynandrous. Due to their lack of vocal sacs, male black toads have a weaker, high pitch release call but do not have a specific advertisement call. Females can also produce release calls. During breeding, black toads join in amplexus where a male attaches to a female’s back. Males externally fertilize the female’s eggs. Once complete, the eggs are left hidden in vegetation in the water.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Black toad mating season takes place in the spring from March to April. Females are oviparous and iteroparous, meaning that they lay multiple clutches of eggs across multiple seasons. They can ovulate in November, but lay their eggs in the spring. Once external fertilization occurs, eggs are linked together and commonly dropped in close proximity to each other in shallow vegetated areas. Approximately 16,000 eggs are laid at one time, and they take three to five days to hatch. Birth mass has not been reported.

Adults leave their eggs after breeding, young are fully independent upon hatching. There is no reported age at sexual maturity. Once fully developed, the average female snout-vent length is 52 mm while the average male SVL is 50 mm.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • external external
• oviparous oviparous

Black toads do not have any type of parental investment. Once mating occurs, black toad parents leave and do not return to the mating site. Therefore, there is no parental investment beyond the act of mating.

• Parental Investment
• no parental involvement

Lifespan/Longevity

There are no known records of the lifespan of black toads. Another species within the genus, western toads ( Anaxyrus boreas ), have a lifespan of 6 years in the wild. It is suspected that black toads have a similar lifespan.

Behavior

Black toads are an active species. Adults are diurnal but become nocturnal during hotter months. They are slow and inactive at temperatures about 25˚C and generally more active in temperatures ranging between 17 to 22˚C. They are active from May to September and are a non-migratory species.

Black toads are normally found in clusters resting. Males were commonly found closer to breeding sites than females. During hibernation, they group together in small holes made by rodents. These burrows are commonly found above water level, between 25 and 50 cm away from a water source. The exact months of hibernation have not been reported, but black toads are active from May to September. They are generally aquatic and swim similarly to frogs. When on land, black toads commonly walk slowly or hop in short bursts.

• Key Behaviors
• saltatorial saltatorial
• diurnal diurnal
• nocturnal nocturnal
• motile motile
• sedentary sedentary
• hibernation hibernation
• social social

Home Range

The home range for black toads is not reported, but these aquatic toads are never more than 12 m away from water. They are not reported to defend a territory.

Communication and Perception

Black toads are mainly aquatic. They inhabit marshes and other water sources near the surface. Black toads have color vision. they are capable of communicating audibly with each other, but male toads do not have vocal sacs. Male intraspecific communication includes low-pitch chirps. Females can emit release calls when they are disturbed.

Before mating occurs, male black toads are typically motionless and do not approach the females first. Tactile communication is necessary, as toads mate while in amplexus. Males fertilize the eggs that females externally release into the water.

Due to summer heat, toads will be more active at night or early in the morning. When toads hibernate, they crowd together in rodent holes near water sources. Another toad species within the family Bufonidae can emit alarm pheromones as tadpoles. It is possible that black toads may also use pheromones as tadpoles.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

The diet of black toads consists mainly of insects. These insectivores eat smaller insects such as ants and beetles. They hunt mainly in the early mornings and late evenings to avoid mid-day heat. The main difference in eating habits between adults and developing young is the size of their prey. Tadpoles feed on organic material and minerals suspended in the water. Younger toads eat ants, wasps, and mites, while fully mature toads eat larger insects such as beetles and Lepidoptera larvae. Diptera , Coleoptera , and Hymenoptera species were present most frequently in adult and juvenile black toad stomachs. Mites and mollusks were found most frequently in juveniles.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore

• Animal Foods
• insects
• terrestrial non-insect arthropods
• mollusks

• Other Foods
• microbes

Predation

Avian and mammalian species have been found to eat black toads. Common ravens ( Corvus corax ), Virginia opossums ( Didelphis virginiana ), and common carp ( Cyprinus carpio ) have been reported hunting black toads from their breeding sites. The toads can emit a toxic secretion from their parotid glands for defense.

Ecosystem Roles

Black toads’ predators include larger mammals, birds, and fish. Black toads prey on insects such as ants (family Formicidae ) and beetles (order Coleoptera ). Unknown intestinal roundworms (order Nematoda ) have been found in black toads, as well as leeches (subclass Hirudinea ) found visibly on toads.

Economic Importance for Humans: Positive

There are no positive economic impacts of black toads on humans.

Economic Importance for Humans: Negative

There are no known adverse economic effects of black toads on humans.

Conservation Status

The IUCN Red List has listed black toads as a “Vulnerable” species with a stable population trend. The ECOS has no listing status for the black toad. There is no special status on CITES or the State of Michigan List. The California Fish and Game Commission has listed them as state threatened in 1984.

There are no current threats to the species but future actions can threaten the population. Habitat alterations and individuals collecting this species from its habitat can lead to a population decline. Road systems, human activity, natural alterations due to weather, and predation all threaten black toads.

There are conservation measures in place to protect black toad populations. Deep Springs College bought nearly the entire habitat in which this species resides – around 291 hectares. They fenced out livestock to minimize grazing, allowing for a recovery of wetland vegetation in the habitat. The college stopped burning marshy areas in the habitat. They also altered their irrigation systems and stopped taking water directly from the springs, protecting breeding routines and developing toads. There are also black toads on land owned federally by the Bureau of Land Management.

Additional Links

AmphibiaWeb

Encyclopedia of Life

Contributors

Aron Hedberg (author), Radford University, Lauren Burroughs (editor), Radford University, Logan Platt (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor), Special Projects.

References

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