Cane Toad

The Cane Toad (Bufo marinus), also known as the Giant Neotropical Toad or Marine Toad, is a large, terrestrial true toad native to Central and South America. A member of the genus Bufo, which includes hundreds of different true toad species in different habitats throughout the world, the Cane Toad is a prolific breeder — females lay single-clump spawns with large numbers of eggs. Its reproductive success is partly due to opportunistic feeding, and it has a diet, unusual among frogs, of both dead and living matter. Adults average 10 to 15 centimetres (4–6 in) in length; the largest recorded specimen weighed 2.65 kilograms (5.84 lb) and measured 38 centimetres (15 in) from snout to vent.

The Cane Toad has large poison glands, and adults and tadpoles are highly toxic to most animals if ingested. Because of its voracious appetite, the Cane Toad has been introduced to many regions of the Pacific as a method of agricultural pest control, notably in the case of Australia in 1935, and derives its common name from its use against sugar cane pests.

Taxonomy

The common name of "Cane Toad" is derived from the original purpose of using it to eradicate pests in sugar cane crops. The Cane Toad has many other common names, including "Giant Toad" and "Marine Toad"; the former refers to their size, and the latter to the binomial name, Bufo marinus. The specific epithet, marinus, was chosen by Carolus Linnaeus, and was based on an illustration by Albertus Seba. Seba 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 Australia, the adults may be confused with species of the Limnodynastes, Neobatrachus, Mixophyes, and Notaden genera. These species can be readily distinguished from the Cane Toad by the lack of large parotoid glands behind their eyes. Cane toads have been confused with the Giant Burrowing Frog (Heleioporus australiacus), because both are large and warty in appearance; however, the Giant Burrowing Frog can be readily distinguished from the Cane Toad by its vertical pupils. Juvenile Cane Toads may be confused with species of the Uperoleia genus, because they all have large parotoid glands; juvenile Cane Toads can be distinguished from these species by the ridging around their eyes and the lack of bright colouring on their thighs.

In the United States, the Cane Toad is morphologically similar to many species in the Bufo genus. In particular, it could be confused with the Southern toad (Bufo terrestris) and Fowler's toad (Bufo fowleri). The Southern Toad can be distinguished by the presence of two bulbs in front of the parotoid glands, and the Fowler's Toad has a pale, cream-white stripe that runs down the dorsal surface; the Cane Toad lacks this stripe.

It is possible to confuse the Cane Toad with the Rococo Toad (Bufo schneideri), sometimes referred to as Schneider's Toad, whose range overlaps that of the Cane Toad. The Rococo Toad grows to nearly the same size, but has additional poison glands on its back legs which can be used to reliably identify it. Within its native range, the Cane Toad can be distinguished from the other true toads by the shape of its paratoid glands, and the arrangement of the ridges on its head.

Physical description

The Cane Toad is a very large frog, reaching an average length of 10–15 centimetres (4–6 in), and much longer in some cases. "Prinsen", a specimen kept as a pet in Sweden, is listed by the Guinness Book of Records as the largest recorded specimen, which weighed 2.65 kilograms (5.84 lb) and measured 38 centimetres (15 in) from snout to vent, or 54 centimetres (21 in) when fully extended. A preserved specimen in the museum of Queensland was 24.1 centimetres (9.4 in) long 16.5 centimetres (6.5 in) wide and weighed 1.36 kg (3 lb). They have a life expectancy of 10 to 15 years in the wild, and as long as 20 years in captivity.

The skin of the Cane Toad is dry and warty. It has distinct ridges above the eyes, which run down the snout. Cane Toads can be grey, brown, red-brown or olive in colour, with varying patterns. A large parotoid gland lies behind each eye. The ventral surface is cream 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.

Juvenile Cane Toad are much smaller than adults—only 5 to 10 millimetres (0.2–0.4 in) long. Typically, they have smooth, dark skin, although some specimens have a red wash. Juveniles lack the adults' large parotoid glands, so they are usually less poisonous. Because they lack this key defence, it is estimated that only 0.5% of metamorph Cane Toads reach adulthood. The tadpoles are small and uniformly black. They are bottom-dwellers and congregate around plants forming schools. Tadpoles reach 27 millimetres (1 in) in length, but are smaller, up to 22 millimetres (0.9 in), under overcrowded conditions.

Ecology and behaviour

Adult Cane Toads possess enlarged parotoid glands behind the eyes, and other glands across the back. When threatened, the Cane Toad secretes a milky-white fluid known as bufotoxin from these glands. Bufotoxin contains components that are toxic to many animals. There are many reported deaths of animals, and even reported deaths of humans, after consumption of Cane Toads. A Cane Toad is capable of inflating its lungs, puffing up and lifting its body off the ground to appear taller and larger to a predator.

Most frogs identify prey by their movements; Cane Toads can also locate food using their sense of smell. They are therefore not limited to living prey and can eat plants, carrion, dog food and household refuse apart from the normal frog prey of small vertebrates and a range of invertebrates. They are active primarily at night, and can range far from water.

The common name of "Marine Toad", and the scientific name Bufo marinus, suggest a link to marine life; however, there is no such link. Adult Cane Toads are entirely terrestrial, venturing to fresh water to breed, and tadpoles have been found to only tolerate salt concentrations equivalent to that of 15% of sea water. Both eggs and tadpoles are toxic to many animals. This toxic protection is lost for a period after metamorphosis until the parotoid glands develop. Cane Toads inhabit open grassland, generally avoiding forested areas; this inhibits their spread in many of the regions in which they were introduced.

Many species prey on the Cane Toad in its native distribution. These include the Broad-snouted Caiman (Caiman latirostris), the Banded Cat-eyed Snake (Leptodeira annulata), the eel (family: Anguillidae), various species of killifish, the Jungle Perch (Kuhila rubestris), some species of catfish (order: Siluriformes) and some species of ibis (subfamily: Threskiornithinae). Predators outside the Cane Toad's native range include the Whistling Kite (Haliastur sphenurus), the Water Rat (Hydromys chrysogaster), the Black Rat (Rattus rattus) and the Water Monitor (Varanus salvator). Occasional cases of the Tawny Frogmouth (Podargus strigoides) and the Papuan Frogmouth (Podargus papuensis) feeding on Cane Toads have been reported. These predators require either a tolerance to the Cane Toad's toxins, or behavioural adaptations that allow them to avoid the most poisonous areas of the frog as they hunt and consume it.

Reproduction

The males of the Cane Toad will call near a still water source to attract females. The call is a loud, long trill; likened to the sound of a small engine or "purr". As with all true toads, the male and female undergo inguinal amplexus, in which the male is behind the female, and grips her around the waist. The female will then release her eggs, which the male covers with sperm.

Females lay from 4,000 to 36,000 eggs per clutch, and breed at least twice per year. They are opportunistic breeders, breeding whenever enough water is available. As a result, they have no breeding season. The eggs are laid in still or slow flowing water, where the males congregate and call in a chorus. Eggs are laid in long strands, usually tangled around plants or submerged objects. The eggs are black, and surrounded in a clear jelly 4-5mm in diameter. The duration until the eggs hatch is dependent upon the water temperature — lasting from 25 hours at 34 degrees Celsius (93.2 Fahrenheit) to 155 hours at 14 degrees Celsius (57.2 Fahrenheit). The development of tadpoles also varies, from 12 to 60 days.

Distribution

The Cane Toad is native to the Americas, from the Rio Grande Valley in southern Texas to central Amazon and south-eastern Peru. This range includes both tropical and semi-arid environments. The density of the Cane Toad within its native distribution is significantly lower than that of its introduced distribution. In South America, it is recorded at densities of 20 adults per 100 square metres, but in Australia the density reaches 1000-2000 adults over the same area.

Introductions

The Cane Toad has been introduced to many regions of the world, particularly the Pacific, for the biological control of agricultural pests. Up until 1844, Cane Toads had been introduced into Martinique, Barbados and Jamaica. They were unsuccessfully introduced into Jamaica to control the rat population.

In 1920, Cane Toads were introduced into Puerto Rico to control the populations of white-grub (Phyllophaga spp.), a pest of sugar cane. By 1932, the Cane Toad was well established on the island, and the populations of white-grubs were dramatically decreased. The decrease in the white-grub population was attributed to the Cane Toad, however there may have been other factors. The six year period after 1931 (when the Cane Toad was most prolific, and the white-grub saw dramatic decline) recorded the highest ever rainfall for Puerto Rico. However, the assumption that the Cane Toad controlled the white-grub led to the large scale introductions throughout many parts of the Pacific.

There are introduced populations in Australia, Florida, Papua New Guinea, the Philippines, the Ogasawara and Ryukyu Islands of Japan, most Caribbean islands and many Pacific islands, including Hawaii and Fiji. The introductions generally failed to control the targeted pests, most of which were later controlled by the use of insecticides. Since then, the Cane Toad has itself become a pest in the host countries, posing a serious threat to native animals. The introduction of the Cane Toad has had a particularly great impact on Australian biodiversity. This is probably due to the large number of species that the Cane Toad successfully competes with, and the large areas of open grassland.

The Cane Toad was successfully introduced into New Guinea to control the hawk moth larvae, which were eating the sweet potato crops. Cane Toads have since become abundant in rural and urban regions, and have not penetrated the forested areas; because most of the native wildlife is restricted to forested areas, the Cane Toad has not caused significant environmental problems.

Introduction to Australia

In attempts to control the native Cane Beetle, 102 Cane Toads were introduced to Australia from Hawaii in June 1935. They bred immediately in captivity and by August 1935 more than 3000 young toads were released in areas around Cairns, Gordonvale and Innisfail in northern Queensland. More Toads were released around Ingham, Ayr, Mackay and Bundaberg. Releases were temporarily limited due to environmental concerns, but resumed in other areas after September 1936. Since their release, toads have rapidly multiplied in population and now number over 100 million. The population is increasing at a rate of about 25% a year. The toads have steadily expanded their range through Queensland, reaching the border with New South Wales in 1978 and the Northern Territory in 1984. The toads on the western frontier of their advance have evolved larger legs; this is thought to be related to their ability to travel further. It is estimated that Cane Toads migrate at an average of 40 kilometres (25 mi) a year.

The long-term impact of toads on the Australian environment is difficult to determine. Precipitous declines in populations of the Northern Quoll have been observed after toads have invaded an area. There are a number of reports of declines in goanna and snake populations after the arrival of toads. The preliminary risk assessment of Cane Toads in Kakadu National Park stated that the predation of the Cane Toad by native wildlife is the greatest risk to biodiversity. Other factors, such as competition with native wildlife for resources, and the predation of the Cane Toad on native wildlife, were considered much lower risk factors.

A number of native species have been reported as successfully preying on toads. Some birds, such as the Black Kite (Milvus migrans), have learned to attack the toad's belly, avoiding the poison producing glands on the back of the head. Reports in the Northern Territory suggest that a native frog, Dahl's Tree Frog (Litoria dahlii), is able to eat the tadpoles and live young of the toad without being affected by the poison that often kills other predators. This is believed to account for slower than expected infestations of toads in certain areas of the Northern Territory. Some snakes have been reported to have evolved smaller jaws so that they are unable to swallow large Cane Toads which have large quantities of poison.

As of 2005, ultraviolet light has been used to lure and capture Cane Toads for extermination. In June 2006, the University of Queensland announced research into a gene to reverse the sex of female Cane Toads. This would lead to a population of males, and thus eliminate the population. However, this would risk a reverse introduction that would wipe out Cane Toads, and possibly other species, in their native range.

Cane Toads in popular culture

The introduction and migration of the Cane Toad in Australia was popularised by the film An Unnatural History (1988), which tells the tale with a humourous edge and is often shown in Environmental Science courses. It was referred to in Simpsons episode Bart vs. Australia.

The children's novel, Toad Rage, by Morris Gleitzman is set in Australia; it is about a Cane Toad named Limpy who goes on an adventure to find out why humans go out of their way to run over Cane Toads. The novel has two sequels, called Toad Heaven and Toad Away.

In Australian Rugby League, the Queensland State of Origin Team are informally known as the Cane Toads, while their opponents, the New South Wales State of Origin Team, are known as the cockroaches.

Bufotenin, one of the chemicals excreted by the Cane Toad, is classified as a Class 1 drug under Australian drug laws. This is the same classification as heroin and cocaine. It is thought that the effects of bufotenin are similar to that of mild poisoning; the stimulating effect, which includes mild hallucinations, lasts for less than one hour. As the Cane Toad excretes bufotenin in very small amounts, and other toxins in relatively large amounts, toad licking could result in serious illness or death.

In Australia, attempts have been made to make use of dead Cane Toads, which can number in the thousands and cause hygiene problems. This includes processing the carcasses into liquid fertilisers. Their skin can also be used as a leather for clothing and accessories.

In the Australian states where the Cane Toad is common, cruelty towards the toad has become popular - which includes injuring the animals with golf clubs or cricket bats. However, this is not effective, as the force exerted by a golf club or cricket bat is not sufficient to kill the animals immediately, and their poison remains toxic after their death. In April, 2005, Dave Tollner, a Northern Territory Member of Parliament, called for the legalisation of attacking the Cane Toad, which was criticised by many animal and conservation groups. The short film "Cane Toad - What happened to Baz?" diplays the Australian attitude towards the Cane Toad. This film won the "Best Comedy" award at the 2003 St. Kilda Film Festival. . However, the short relies on humour which Australians connect with, and screenings in overseas festivals have been more reserved.

The Cane Toad is being considered as a candidate by the National Trust of Queensland for being listed as a state icon of Queensland, alongside the Royal Flying Doctor Service and the mango tree.

Notes

References

Toads | Argentine fauna | Fauna of Guyana | Fauna of Trinidad and Tobago | Invasive species

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