Critter Of The Week #5 : Cane Toad

Cane toads are quite impressive. They are the largest toads on earth reaching up to 3 pounds and are as long as 15 inches. The world record holder was a pet toad kept in Sweden that weighed 5.86 pounds! That is as large as your average chicken! Compare this to the average American toad, which weighs in at just 1 ounce, and you really get an idea how impressive these guys are (see the video below). They go by several different names:  the giant neotropical toad and the marine toad are just a couple.
They are native to open grasslands and forests in Central and South America. They also like to hang around human settlements for cover and shelter. 
Cane toads have a legendary appetite, which is one reason they are one of the most successful amphibians on earth. They will eat anything they can overpower and fit into their mouth.
They also have a rather peculiar habit rarely seen among frogs and toads; they will scavenge non-living food. It is a well-known fact and common to see cane toads hanging around pet food dishes gobbling down kibble. Another unique characteristic they have is resistance to the detrimental chytrid fungus. The chytrid fungus was believed to be brought from released pet frogs from Africa. It is now the single biggest threat to all amphibians in Central and South America and has lead to near extinction of several species. Interestingly, the cane toad is immune to the infection and lives on.
   
Cane toads are true toads, which means they have large glands on their back called parotid glands that secrete a poison that protects them against being eaten by predators. When they are scared, they puff up their body and secrete a milky poison from their glands. If a predator comes into contact with them, it will cause foaming at the mouth, hallucinations and in rare cases, death. It is an incredibly effective defense mechanism for a fragile amphibian with no teeth, claws, or speed to fight or run away from danger.
They are also ectothermic, which does not mean they are cold-blooded, it means their body temperature is the same as the environment they live in. If it is 80 degrees outside, they are around the same temperature. 
  

The video below really puts into perspective just how large cane toads are. The American Toad is common throughout the United States and averages a few ounces in size. Compare that to the cane toad and the difference is staggering! 

    

Scientific Classification :

Kingdom : Animalia

Phylum : Chordata

Class : Amphibia

Order : Anura

Family : Bufonidae

Genus: Rhinella

Conservation Status : 

Colugo Conservation Status

"Least concern" means their population has been counted, but they are not in imminent danger of extinction.

 

Number of Different Species : 1

Cane Toad : Rhinella marina

Image by Perigrijn

Population & Range :

It is hard to estimate the population of cane toads in their native range. But in countries where they have been introduced, the global population is likely in the millions. It is estimated in Australia that there are 200 million cane toads!

Cane toad's natural range extends from South America through Central America into Southern Texas.

They have been introduced to Southern Florida, Puerto Rico, New Guinea, Jamaica, Cuba, Fiji, Australia, and Hawaii, among several other Pacific Islands. 

Habitat :

Cane toads prefer open tropical grasslands and some woodlands for cover. They are also found in and around human settlements. Below is an image of the Costa Rican rainforest they can be found in.  

Diet :

Cane toads will eat anything they can overpower and fit into their mouth including each other! When they are young, small insects and spiders are the normal fair. But when they grow into adulthood, they regularly eat rodents, snakes, frogs, lizards, and have even been seen eating bats. 

Perhaps their most peculiar habit is the fact they will also scavenge. This is rarely seen among any other species of frog or toad. They will use their sense of smell to locate and feed on roadkill. As can be seen in the picture below, they will even eat cat or dog food left outside!

This adaptability is one of the reasons the cane toad has become so successful throughout their range.

 

Image by Kristy

Reproduction :

In their native range, cane toads will mate throughout the year. In other areas where it is drier, they will mate in synchronization with the wet season.

Male cane toads are smaller than females and are usually a more yellow coloration. They will hang around ponds and bodies of water calling for females. Once they find one, the two engage in amplexus where the male will ride on top of the female and he will fertilize her eggs as she lays them.

She can lay anywhere from 8,000 to 25,000 eggs. In hotter climates, they will hatch within 48 hours and the tadpoles will form schools together. Even at this stage, the tadpoles are toxic, so they have little fear of predators.

In about a month, the tadpoles become toadlets, and the cane toads will emerge from the water. They will grow rapidly until they reach sexual maturity.

In the video below, you can hear the call cane toads use to find each other. 

Two cane toads engaged in aplexus.

 

 Female laying eggs.

 

Tadpoles.

 

Cane toad toadlet by Andrew James Ward 

Relationship With Humans :

Cane toads have a long and interesting relationship with humans. In their native range, the only major threats they face from humans are road collisions and persecution. They are not harvested commercially or on a subsistence level, and are immune to other amphibian diseases. Some are collected for the pet trade but are in sustainable levels that do not threaten their populations. They do well around human settlements so habitat destruction is not a major threat.

 

Because of their voracious appetite, they have been used as a biological control agent for the sugarcane industry. Many major growers of sugarcane had problems with native insects eating their crops. As early as the 1840's, cane toads were introduced to a number of Caribbean islands to reduce rat populations. They did not succeed.

 

However, in the 1930's, they were introduced to Puerto Rico to control a beetle infestation. On the surface it seemed to work. At that time, the cane toads became popular as a viable biological control method for insects ravaging sugarcane crops. 

 

It was then they were introduced to Florida, Australia, Fiji, the Philippines, and more.

 

They were unsuccessful at controlling insect infestations, but were very good at reproducing. Especially in Australia where there are few predators that will consume cane toads. This, combined with a small human population, made conditions perfect for a toad invasion. There are many plans to control them since they have caused considerable damage to native predators that try to eat them, including freshwater crocodiles, a small carnivorous marsupial related to the tasmanian devil called the spotted quoll, and many large lizard species. 

 

Surprisingly, the cane toads at the leading edge of the invasion have developed longer legs and can hop over large distances. In fact, scientists discovered these toads move so much, that they develop osteoarthritis as seen in some marathon runners!  

 

The cane toad is perhaps the most well-studied and famous example of what an invasive, non-native species can do to the environment, and how misguided our ancestors were. 

 

Although they have caused much damage in Australia, the future is promising. There are new programs in place that are using sausages baited with small amounts of toad toxin to teach local wildlife not to eat cane toads. This works by making the predators that eat the sausage sick but they don't die like they would if they ate the toads. The program is currently showing success. This way, future generations will avoid the toads and many of Australia's iconic species found nowhere else on earth can bounce back.