Cane toad
The cane toad, also known as the giant neotropical toad or marine toad, is a large, terrestrial true toad native to South and mainland Central America, but which has been introduced to various islands throughout Oceania and the Caribbean, as well as Northern Australia. It is a member of the genus Rhinella, which includes many true toad species found throughout Central and South America, but it was formerly assigned to the genus Bufo.
A fossil toad from the La Venta fauna of the late Miocene in Colombia is morphologically indistinguishable from modern cane toads from northern South America. It was discovered in a floodplain deposit, which suggests the R. marina habitat preferences have long been for open areas. The cane toad is a prolific breeder; females lay single-clump spawns with thousands of eggs. Its reproductive success is partly because of opportunistic feeding: it has a diet, unusual among anurans, of both dead and living matter. Adults average in length; the largest recorded specimen had a snout-vent length of.
The cane toad has poison glands, and the tadpoles are highly toxic to most animals if ingested. Its toxic skin can kill many animals, both wild and domesticated, and cane toads are particularly dangerous to dogs. Because of its voracious appetite, the cane toad has been introduced to many regions of the Pacific and the Caribbean islands as a method of agricultural pest control. The common name of the species is derived from its use against the cane beetle, which damages sugar cane. The cane toad is now considered a pest and an invasive species in many of its introduced regions. The 1988 film Cane Toads: An Unnatural History documented the trials and tribulations of the introduction of cane toads in Australia.
Taxonomy
Historically, the cane toad was used to eradicate pests from sugarcane, giving rise to its common name. The cane toad has many other common names, including "giant toad" and "marine toad"; the former refers to its size, and the latter to the binomial name, R. marina. It was one of many species described by Carl Linnaeus in his 18th-century work Systema Naturae. Linnaeus based the specific epithet marina on an illustration by Dutch zoologist Albertus Seba, who mistakenly believed the cane toad to inhabit both terrestrial and marine environments. Other common names include "giant neotropical toad", "Dominican toad", "giant marine toad", and "South American cane toad". In Trinidadian English, they are commonly called crapaud, the French word for toad.The genus Rhinella is considered to constitute a distinct genus of its own, thus changing the scientific name of the cane toad. In this case, the specific name marinus changes to marina to conform with the rules of gender agreement as set out by the International Code of Zoological Nomenclature, changing the binomial name from Bufo marinus to Rhinella marina; the binomial Rhinella marinus was subsequently introduced as a synonym through misspelling by Pramuk, Robertson, Sites, and Noonan. Though controversial the binomial Rhinella marina is gaining in acceptance with such bodies as the IUCN, Encyclopaedia of Life, Amphibian Species of the World and increasing numbers of scientific publications adopting its usage.
Since 2016, cane toad populations native to Mesoamerica and northwestern South America are sometimes considered to be a separate species, Rhinella horribilis.
In Australia, the adults may be confused with large native frogs from the genera Limnodynastes, Cyclorana, and Mixophyes. These species can be distinguished from the cane toad by the absence of large parotoid glands behind their eyes and the lack of a ridge between the nostril and the eye. Cane toads have been confused with the giant burrowing frog, because both are large and warty in appearance; however, the latter can be readily distinguished from the former by its vertical pupils and its silver-grey irises. Juvenile cane toads may be confused with species of the genus Uperoleia, but their adult colleagues can be distinguished by the lack of bright colouring on the groin and thighs.
In the United States, the cane toad closely resembles many bufonid species. In particular, it could be confused with the southern toad, which can be distinguished by the presence of two bulbs in front of the parotoid glands.
Taxonomy and evolution
The cane toad genome has been sequenced and certain Australian academics believe this will help in understanding how the toad can quickly evolve to adapt to new environments, the workings of its infamous toxin, and hopefully provide new options for halting this species' march across Australia and other places it has spread as an invasive pest.Studies of the genome confirm its evolutionary origins in northern part of South America and its close genetic relation to Rhinella diptycha and other similar species of the genus. Recent studies suggest that R. marina diverged between 2.75 and 9.40 million years ago.
A recent split in the species into further subspecies may have occurred approximately 2.7 million years ago following the isolation of population groups by the rising Venezuelan Andes.
Description
Considered the largest species in the Bufonidae, the cane toad is very large; the females are significantly longer than males, reaching a typical length of, with a maximum of. Larger toads tend to be found in areas of lower population density. They have a life expectancy of 10 to 15 years in the wild, and can live considerably longer in captivity, with one specimen reportedly surviving for 35 years.The skin of the cane toad is dry and warty. Distinct ridges above the eyes run down the snout. Individual cane toads can be grey, yellowish, red-brown, or olive-brown, with varying patterns. A large parotoid gland lies behind each eye. The ventral surface is cream-coloured and may have blotches in shades of black or brown. The pupils are horizontal and the irises golden. The toes have a fleshy webbing at their base, and the fingers are free of webbing.
Typically, juvenile cane toads have smooth, dark skin, although some specimens have a red wash. Juveniles lack the adults' large parotoid glands, so they are usually less poisonous. The tadpoles are small and uniformly black, and are bottom-dwellers, tending to form schools. Tadpoles range from in length.
Ecology, behaviour and life history
The common name "marine toad" and the scientific name Rhinella marina suggest a link to marine life, but cane toads do not live in the sea. However, laboratory experiments suggest that tadpoles can tolerate salt concentrations equivalent to 15% of seawater, and recent field observations found living tadpoles and toadlets at salinities of 27.5‰ on Coiba Island, Panama. The cane toad inhabits open grassland and woodland, and has displayed a "distinct preference" for areas modified by humans, such as gardens and drainage ditches. In their native habitats, the toads can be found in subtropical forests, although dense foliage tends to limit their dispersal.The cane toad begins life as an egg, which is laid as part of long strings of jelly in water. A female lays 8,000–25,000 eggs at once and the strings can stretch up to in length. The black eggs are covered by a membrane and their diameter is about. The rate at which an egg grows into a tadpole increases with temperature. Tadpoles typically hatch within 48 hours, but the period can vary from 14 hours to almost a week. This process usually involves thousands of tadpoles—which are small, black, and have short tails—forming into groups. Between 12 and 60 days are needed for the tadpoles to develop into juveniles, with four weeks being typical. Similarly to their adult counterparts, eggs and tadpoles are toxic to many animals.
When they emerge, toadlets typically are about in length, and grow rapidly. While the rate of growth varies by region, time of year, and sex, an average initial growth rate of per day is seen, followed by an average rate of per day. Growth typically slows once the toads reach sexual maturity. This rapid growth is important for their survival; in the period between metamorphosis and subadulthood, the young toads lose the toxicity that protected them as eggs and tadpoles, but have yet to fully develop the parotoid glands that produce bufotoxin. Only an estimated 0.5% of cane toads reach adulthood, in part because they lack this key defense—but also due to tadpole cannibalism. Although cannibalism does occur in the native population in South America, the rapid evolution occurring in the unnaturally large population in Australia has produced tadpoles 30x more likely to be interested in cannibalising their siblings, and 2.6x more likely to actually do so. They have also evolved to shorten their tadpole phase in response to the presence of older tadpoles. These changes are likely genetic, although no genetic basis has been determined.
As with rates of growth, the point at which the toads become sexually mature varies across different regions. In New Guinea, sexual maturity is reached by female toads with a snout–vent length between, while toads in Panama achieve maturity when they are between in length. In tropical regions, such as their native habitats, breeding occurs throughout the year, but in subtropical areas, breeding occurs only during warmer periods that coincide with the onset of the wet season.
The cane toad is estimated to have a critical thermal maximum of and a minimum of around. The ranges can change due to adaptation to the local environment. Cane toads from some populations can adjust their thermal tolerance within a few hours of encountering low temperatures. The toad is able to rapidly acclimate to the cold using physiological plasticity, though there is also evidence that more northerly populations of cane toads in the United States are better cold-adapted than more southerly populations. These adaptations have allowed the cane toad to establish invasive populations across the world. The toad's ability to rapidly acclimate to thermal changes suggests that current models may underestimate the potential range of habitats that the toad can populate. The cane toad has a high tolerance to water loss; some can withstand a 52.6% loss of body water, allowing them to survive outside tropical environments.