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Rhinella marina, the cane toad, is a large, heavy-bodied true toad of the family Bufonidae, native to much of South America east of the Andes. Most people know it from somewhere else. Cane toads were released to control insect pests, mainly in sugar cane fields, and the species is now considered the most widely introduced amphibian in the world, with established populations across the Pacific and the Caribbean, Hawaii and Australia included.
The pest-control plan did not deliver: there is no evidence that it has controlled any pest in Australia, where it is now counted as a pest itself.
Identification
A cane toad is a big, warty, brown or dark-mottled toad that can weigh up to 1.5 kg. The back is grey-olive-brown with warts tipped by dark brown caps, over a whitish-yellow underside with dark brown speckles. The pupils are horizontal, the iris is golden, and bony ridges run from above the eyes down the snout. Males have sturdier forelimbs than females, used to grip the female while mating. The call is a slow, low-pitched trill often compared to a distant tractor.
On each side of the body sits one large parotoid gland, running from just behind the eye far down the flank. These glands are the toad’s defence. Under attack they release a white, viscous bufotoxin that can cause profuse salivation, twitching, vomiting and collapse of the hind limbs in an animal that swallows it. Where the toad was introduced, native predators have no natural immunity to the toxin and have declined; in Australia the toxin often kills native predators that try to eat the toad.
Most adults measure 150–175 mm, with a recorded maximum of 238 mm; the average mass is in the facts box above. It sits fairly upright and moves in short, fast hops.
Habitat and range
Rhinella marina favours tropical forest and rainforest with semi-permanent water nearby. It is terrestrial and nocturnal, and forages mainly at night in mature forest and along roads. Open ground made by people suits it: in the Amazon, forest cleared for farming and roads is quickly invaded, and in Australia toads use sparsely vegetated roadside clearings as corridors. In the dry parts of Hawaii they turn up mostly around golf courses, homes, resorts and other irrigated ground.
The native range is smaller than older accounts give. Cane toads were long treated as one species from Texas to the Amazon, but studies from 2016 onward split them in two: R. marina in South America east of the Andes, and Rhinella horribilis from South America west of the Andes through Central America to Texas. The native toads of southern Texas now count as R. horribilis. Which species each introduced population belongs to is still being worked out: three cane toads from Polk County, Florida, were confirmed as R. horribilis, and no Florida toad has been confirmed as R. marina.
Australia’s toads arrived in Queensland in 1935. They have since spread through most of Queensland, west across the north of the Northern Territory to the Kimberley in Western Australia, and south to the mid-north coast of New South Wales, and they are still moving west.
Behaviour and breeding
The cane toad is an opportunistic carnivore that eats a wide range of invertebrates, vertebrates including other frogs and smaller cane toads, and carrion. Near houses it will take pet food left outdoors and household scraps. In cold or dry seasons it stays inactive in shallow scrapes beneath ground cover.
Males gather in still or slow-moving water and call for mates, and the mating system is promiscuous: more than one male may fertilise a single female’s eggs. A female lays 5,000 to 32,000 eggs in gelatinous strings, draped over rocks, debris or emergent plants, in almost any temporary or permanent water, brackish water included. That tolerance of salty water may explain the name marina. The eggs hatch in two to seven days, and the tadpoles become toadlets in roughly forty-five to fifty-five days.
There is no fixed season. The toad breeds at almost any time of year unless it is too cold, and the Australian Museum likewise records breeding at any time of the year in Australia.
Conservation
The IUCN category is in the facts box above. In its native range the cane toad is common and not in need of special conservation efforts. Where it was introduced it is a pest and a target of extermination efforts. Australia has no native toads of the family Bufonidae, which is thought to leave many of its animals more susceptible to the toxin, and the toad has been linked to severe population impacts on freshwater crocodiles, northern quolls, monitor lizards and other lizards and snakes. Research into how to reduce its growing population is under way.
Similar species
The closest look-alike is the Mesoamerican cane toad, Rhinella horribilis, the species split from R. marina. The two look identical from the outside; telling them apart takes an X-ray of the skull or a genetic test, so in the field the range is the guide: R. marina east of the Andes, R. horribilis west and north of it.
In the United States, cane toads differ from the native Southern Toad and American Toad in their parotoid glands, which are large and triangular where the native toads’ are small and oval, and in the ridges on top of the head between the eyes. In Australia, adults look like none of the native species. Young toads are easier to confuse: they resemble many Uperoleia and Crinia species but lack the colourful markings on the groin and the backs of the thighs.