The cane toad it may be the animal of the invasive species poster. Originally from South America, it has been introduced to many other ecosystems in hopes of quelling agricultural pests. In contrast, toads have become a pest, especially in Australia. Free from predators and parasites in their native area, the toad’s venom glands have proven to be a danger to most species trying to eat it wherever it has been introduced.
But this does not mean that it is completely free of the risk of predation. Australian reed toad has been observed feeding on its offspring. This cannibalism seems to be an evolutionary response to the lack of competing species in its invasive range, causing cane toads to activate their remaining competition: each other. And the toad has already become an additional evolutionary response to trying to limit the danger of cannibalism.
Only competing with themselves
From an evolutionary perspective, cannibalism can make sense as a way to limit the competition posed by other members of your species. But the University of Sydney research team that has tracked cane toad cannibalism suggests that the successful invasion of the species in Australia has exacerbated this evolutionary pressure, which may also occur with other predators. invaders. One of the hallmarks of an invasive species is its abundance in its new range, at which time competition for limited resources becomes more likely. Cannibalism not only limits this competition, but also provides nutritional resources.
With the Australian population reaching about ten times the population density in the native cane toad area, there are many opportunities for toad competition. And this competence has been documented in the early stages of toad development. Newly hatched toads spend several days becoming tadpoles, and during this time they are usually eaten by older, more mature tadpoles. In a heavily populated body of water, claws laid after mature tadpoles may be completely extinct before they can live beyond the breeding stage.
Tadpoles that eat tadpoles can be produced in South America. But it happens much more often in Australia. Thus, the researchers decided to see if cannibalism produced biological differences between native and invasive populations.
To do so, they obtained toads from native and invasive populations and monitored offspring behavior. To begin with, the researchers simply placed fertilized eggs in a container with a single tadpole. This showed that Australian cane toads had become aggressive cannibals, as the eggs they laid there were more than 2.5 times more likely to be cannibalized before producing a tadpole.
While many changes can make such a difference, the researchers showed that Australian tadpoles were more likely to look for freshly hatched cane toads. When it was decided to move to empty containers and one that contained cane toad creatures, invasive Australian toads were almost 30 times more likely to enter the container with creatures.
When the young reach the stage of the tadpole and are too old to eat, their fellow tadpoles lose interest. There is some indication that the above attraction is based on toxins introduced by the egg fertilized by the mother.
The best defense
High levels of predation tend to produce evolutionary responses to limit vulnerability, and cannibalism is no different. The researchers found that Australian toads were simply spending less of their development time in the vulnerable stage of hatching to avoid some of the impact of cannibalism.
This occurred through two different mechanisms. One of them depended specifically on the presence of tadpoles. In other words, when the threat was present, development accelerated. But there was an independent acceleration regardless of whether there were tadpoles. While South American toads spent about five days in total in the breeding stage, Australian populations only spent three days. Thus, the pressure of cannibalism had reduced the breeding development time by almost half.