Cane Toad

The much maligned venomous **Cane Toad** or **Marine Toad** (*Rhinella Marina*) earned its very bad reputation shortly after being released into the Australian ecology in 1935 with the hope that they would control the destructive cane beetle population. They turned out to be failures at controlling beetles, but remarkably successful at reproducing and spreading themselves. The *Cane Toad* is an old species. A fossil toad (specimen UCMP 41159) from the La Venta fauna of the late Miocene of Colombia is indistinguishable from the modern *Cane Toad* from northern South America. It was discovered in a floodplain deposit, which suggests the *Cane Toad* habitat preferences have long been for open areas.

## Description

The cane toad is very large; the females are significantly longer than males, reaching an average length of 10–15 cm (3.9–5.9 in), with a maximum of 24 cm (9.4 in). 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. It has distinct ridges above the eyes, which 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 10 to 25 mm (0.39 to 0.98 in) in length.

## Distribution

The cane toad is native to the Americas, and its range stretches from the Rio Grande Valley in South Texas to the central Amazon and southeastern Peru, and some of the continental islands near Venezuela (such as Trinidad and Tobago). This area encompasses both tropical and semiarid environments. The density of the cane toad is significantly lower within its native distribution than in places where it has been introduced. In South America, the density was recorded to be 20 adults per 100 m (109 yd) of shoreline, 1 to 2% of the density in Australia.

### **Introduction to Australia**

About 3,000 cane toads were released in the sugarcane plantations of north Queensland in 1935. They now number well into the millions, and their still expanding range covers thousands of square miles in northeastern Australia. They are considered pests, and government eradication efforts include asking residents to help collect and dispose of them.

## Ecology and Behavior

The common name "marine toad" and the scientific name *Rhinella marina* suggest a link to marine life, but the adult cane toad is entirely terrestrial, only venturing to fresh water to breed. However, laboratory experiments suggest that tadpoles can tolerate salt concentrations equivalent to 15% of seawater (~5.4‰), 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 20 m (66 ft) in length. The black eggs are covered by a membrane and their diameter is about 1.7–2.0 mm (0.067–0.079 in). 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 10–11 mm (0.39–0.43 in) in length, and grow rapidly. While the rate of growth varies by region, time of year, and gender, an average initial growth rate of 0.647 mm (0.0255 in) per day is seen, followed by an average rate of 0.373 mm (0.0147 in) 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. Because they lack this key defence, only an estimated 0.5% of cane toads reach adulthood.

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 70 and 80 mm (2.8 and 3.1 in), while toads in Panama achieve maturity when they are between 90 and 100 mm (3.5 and 3.9 in) 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 40–42 °C (104–108 °F) and a minimum of around 10–15 °C (50–59 °F). The ranges can change due to adaptation to the local environment. 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.

### **Effect on Australian Ecology**

Their effects on Australia's ecology include the depletion of native species that die eating cane toads; the poisoning of pets and humans; depletion of native fauna preyed on by cane toads; and reduced prey populations for native insectivores, such as skinks.

### **Toxicity**

Cane toad poison is a mix of toxins that primarily affects the functioning of the heart. It is present throughout their bodies and is secreted as a milky liquid from the parotoid glands located over the toad's shoulders. Envenomation is painful, but rarely deadly to humans, although some people have died from eating cane toads and even their eggs.

### **Diet**

Most frogs identify prey by movement, and vision appears to be the primary method by which the cane toad detects prey; however, the cane toad can also locate food using its sense of smell. They eat a wide range of material; in addition to the normal prey of small rodents, reptiles, other amphibians, birds, and even bats and a range of invertebrates, they also eat plants, dog food, and household refuse.