Cane toad It may be a poster animal of an invasive species. It is native to South America and has been introduced into many other ecosystems in the hope that it will eliminate agricultural pests. On the contrary, the toad itself has become a pest, especially in Australia. The toad’s venom gland does not have predators and parasites in its native range, which turns out to be a hazard for most species that try to eat it where it was introduced.
But this does not mean that it has no risk of predation. Tadpoles of Australian sugar cane toads have been observed to feed on the offspring of their sibling sugar cane toads. This cannibalism seems to be an evolutionary response to the lack of competing species within its invasion range, causing the cane toads to start their remaining competition: each other. Toad has turned to an additional evolutionary response, trying to limit the danger of cannibalism.
Only compete with yourself
From an evolutionary perspective, cannibalism can be used as a way to limit competition among other members of your species.But the research team at the University of Sydney has Tracking cane toad cannibalism Shows that the species’ successful invasion of Australia has exacerbated this evolutionary pressure-this may also happen to other invasive predators.One of the signs Invasive species It is its richness in the new scope, at which point competition for limited resources becomes more likely. Cannibalism not only limits this competition, but also provides nutritional resources.
Since the population density of Australia has reached about 10 times the population density of the origin of sugarcane toads, there are many opportunities for competition among toads. This kind of competition is documented in the early stages of toad development. Recently hatched toads take a few days to develop into tadpoles, during which time they are often eaten by older and more mature tadpoles. In densely populated waters, a litter of eggs laid by mature tadpoles may disappear completely before they survive the hatching stage.
Tadpoles that eat tadpoles may occur in South America. But it happens more often in Australia. Therefore, the researchers decided to see if cannibalism would create a biological difference between the local population and the invasive population.
To this end, they obtained toads from local and invasive populations and tracked the behavior of offspring. First, the researchers simply put the fertilized egg into a container containing a tadpole. This indicates that the Australian cane toads have become aggressive carnivores, because the eggs placed with them are more than 2.5 times more likely to be cannibalized before they produce tadpoles.
Although many changes can produce this difference, researchers have shown that Australian tadpoles are more likely to look for sugarcane toads that have recently hatched. When you can choose to move into an empty container and a container with larvae of sugar cane toads, the probability of invading Australian toads entering the container with larvae is nearly 30 times higher.
When the young turtles reach the tadpole stage and are too large to eat, their tadpole companions lose interest. There are indications that the earlier attraction is based on the toxins put into the fertilized egg by the mother.
Best defense
High predation tends to produce evolutionary responses to limit vulnerability, and cannibalism is no exception. Researchers found that Australian toads only spend less development time in the fragile hatching stage to avoid some of the effects of cannibalism.
This happens through two different mechanisms. One of them is particularly dependent on the presence of tadpoles. In other words, when threats exist, development accelerates. However, whether or not tadpoles are present, there is a separate acceleration. South American cane toads spent a total of about five days in the incubation phase, while the Australian population only took three days. As a result, the pressure of cannibalism has cut the development time of young turtles by nearly half.
