Cane toads have acquired a bad reputation for something that wasn’t their fault – in 1935, 102 cane toads (Rhinella marinas) were brought to Queensland, Australia, and released to combat grey-backed cane beetles (Dermolepida albohirtum) destroying sugarcane plants. In 1937, 62,000 toadlets were released and quickly increased exponentially. Unfortunately, grey-backed cane beetles live at the tops of sugar cane and cane toads are not good climbers. Even worse, the hungry cane toads gorged on native predatory reptiles and severely affected Australian biodiversity. On top of that, because they’re highly toxic, they’re killing or sickening any animals that try to feed on them -- including crocodiles.
Now, new research has found that the cane toads finally have an enemy – cane toads! Specifically, hungry cane toad tadpoles that cannibalistically feed on their younger, newly hatched siblings. In fact, ‘gorge’ would be a better word. According to a new study published in the journal Proceedings of the National Academy of Sciences, lead researcher Jayna DeVore, who was then a postdoctoral research associate at the University of Sydney, and her team watched as Australia’s invasive cane toad tadpoles demonstrated a cannibalism that is unknown to native ones.
“Here, we show that cane toad tadpoles (Rhinella marina) from invasive Australian populations have evolved an increased propensity to cannibalize younger conspecifics as well as a unique adaptation to cannibalism—a strong attraction to vulnerable hatchlings—that is absent in the native range.”
This should be good news – DeVore told Live Science “When I first saw this behavior in the wild, I was amazed at how voraciously cane toad tadpoles sought out cane toad hatchlings and ate them.” Cane toads eating themselves into extinction – what could possibly go wrong? Well, two things. First, the tadpoles that survive by cannibalizing are getting nutrients and eliminating later competition for resources, so they’re growing larger and at a faster rate, resulting in super cane toads that could invade new areas. But that’s not the worst problem.
“In response, these toads have also evolved multiple strategies for reducing the duration of the vulnerable period, indicating an evolutionary arms race between the cannibalistic tadpole stage and the vulnerable egg and hatchling stages in invaded habitats.”
DeVore and her team discovered that cane toad tadpoles are themselves evolving to grow faster after they’ve hatched.
"We found that cane toad clutches from Australia developed more quickly; they reached the invulnerable tadpole stage in about four days, whereas native range clutches took about five days."
And if that’s not enough, these evolved hatchlings were "more likely to be able to smell when cannibals are around and actually accelerate their development in response." So the evolution and the battle between the young and the cannibals (soon to be a major motion picture?) is not over yet.
Is there any good news about the evolution of cannibal cane toads in Australia? Yes! Live Science reports that the researchers tested 1,190 tadpoles for survival, development, growth and plasticity, and found that those that developed faster as eggs and hatchlings to escape cannibalism fared worse and developed more slowly at the tadpole stage than native-range, non-Australian tadpoles. In other words, the survivors were weaker, potentially setting the stage for some recovery among native lizards and other cane toad prey. While she doesn’t see them cannibalizing themselves into extinction, DeVore thinks this will control their population growth and help to regulate their abundance.
While we wait for further developments, Cannibal Cane Toads would make a great band name.