The cane toad is a well-known invasive species and can be considered the most “aggressive” among them.
This species of toad originates from South America and has been introduced into various ecosystems with the hope of controlling agricultural pests and protecting crops. However, not only did they fail to achieve the expected results, but they also became a harmful species, most notably in Australia—where they are not preyed upon by native animals, and Australian parasites have no effect on them. Meanwhile, the toxin from the cane toad poses a danger to most animals that attempt to eat them.
This species of toad originates from South America.
The giant toad truly exists and lives in tropical regions of Central and South America, with an average length of only 10-15 cm, and the largest can reach up to 24 cm. This is the largest toad in the world – the cane toad.
However, this does not mean they are completely free from predation. Cane toad tadpoles in Australia have been observed eating their own kind. This cannibalism seems to be an evolutionary response to the lack of competing species in their invasive range, which has also led to increased competition among cane toads. Additionally, this species is evolving further to try to mitigate the risk of cannibalism.
When threatened, these glands secrete a milky substance known as toad toxin.
The surface of the cane toad’s body has venom glands, and in sufficient quantities, this toxin can cause humans to experience difficulty breathing and even blindness. The skin of the cane toad is also highly toxic, with a raised area above their eyes that extends to the nose, and behind each eye is a large venom gland hidden within these bulges. When threatened, these glands secrete a milky substance known as toad toxin. This is a mixture of 14 different compounds, many of which are harmful to various animal species, including a chemical known as Benfotiamine.
From an evolutionary perspective, cannibalism can serve as a means to limit competition from other members of the species. However, a research group at the University of Sydney has monitored the cannibalistic behavior of cane toads, showing that the successful invasion of this species into Australia has highlighted this evolutionary pressure—something that may also occur with other invasive predatory species.
One sign of an invasive species is its abundance and rapid growth in a new range, but over time, resources become limited, and intraspecific competition becomes more likely, making cannibalism seem inevitable for wild animals.
This competition has been recorded in the early stages of toad development. Newly hatched toads take several days to develop into tadpoles, during which they are often preyed upon by larger, more mature tadpoles. In a water body with many tadpoles, late-spawning eggs may be completely wiped out before they can hatch, as the tadpoles from other clutches have already hatched and matured.
Tadpoles eating tadpoles can occur in South America, but this happens much more frequently in Australia. Therefore, researchers decided to investigate whether cannibalism creates biological differences between native and invasive populations.
<pTo do this, they collected toads from both native and invasive populations and monitored their behavior. Initially, the researchers simply placed fertilized eggs into a container with a single tadpole. This showed that Australian cane toad tadpoles are more aggressive, as the eggs placed with them had a 2.5 times higher likelihood of being eaten than normal.
While many changes could account for this difference, the researchers demonstrated that Australian tadpoles are more likely to identify and seek out newly hatched cane toads to eat. When given the choice to move into empty containers or a container with newly hatched cane toads, the invasive Australian toads were nearly 30 times more likely to enter the container with the young toads.
Australian tadpoles are more likely to identify and seek out newly hatched cane toads to eat.
High levels of predation tend to create evolutionary responses to limit vulnerability, and cannibalism is not much different from this mechanism. While South American cane toads spend a total of about five days in the hatching stage, populations in Australia take only three days. Thus, the pressure from cannibalism has reduced the developmental time of the young by nearly half.
Such changes due to predator-prey interactions have been observed in many different species. The involved researchers have made a compelling case that the differing environments in which an invasive species resides help promote this type of interaction.
