Adaptations of Australian animals to cane toads facts for kids
Cane toads are originally from South America and Central America. They were brought to Australia in the 1930s. Since then, they have become an invasive species. This means they are a big threat to Australia's native animals.
Cane toads harm Australian ecosystems mainly by poisoning other animals. They have special parotoid glands behind their heads. These glands produce a very toxic liquid. This liquid contains chemicals called bufadienolides. No native Australian frogs or toads have this kind of defense. Because of this, many native animals that eat frogs and toads suffered greatly. Their numbers dropped quickly after cane toads arrived.
Even though cane toads have caused big problems, they probably haven't caused any native species to become extinct. Also, many animal populations that first declined have started to recover. This suggests that native animals are adapting to the cane toad. They might be learning to avoid the toads. Or, they might be changing over many generations due to natural selection.
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How Animals Learn to Avoid Cane Toads
One way Australian animals have adjusted is by learning new behaviors. This often happens through something called conditioned taste aversion.
This type of learning occurs when an animal eats a cane toad and gets sick. After that bad experience, the animal learns to avoid eating cane toads again. This works best if the animal gets very sick. But often, just tasting the toad's toxins is enough. The bad taste makes the animal avoid anything that looks or smells like a cane toad later on.
Some animals have learned different ways to eat cane toads. These animals are usually more resistant to the toad's toxins. This includes some birds and rodents.
Learning in Small Marsupials
The planigale is a tiny marsupial that eats meat. Like other marsupials such as quolls, planigales are very sensitive to cane toad toxins. They have not evolved with toxic toads. Studies show that planigales that have never met a cane toad will attack them. But these small marsupials often survive. They are very precise hunters. Planigales usually drop a cane toad quickly after catching it. Or, they might only eat the toad's snout, avoiding the poisonous glands.
Tests in labs show that planigales learn to avoid cane toads after only one or two tries. They even avoided other non-toxic frogs after meeting a cane toad. They also avoided any prey that smelled like a cane toad. This shows how strong their taste aversion learning is.
Some planigales in northern Queensland have lived with cane toads for over 60 years. They might have evolved to handle the toxins better.
Fish Avoiding Toad Tadpoles
Cane toad tadpoles are also very toxic. They are a danger to native fish that eat them. But some Australian fish have changed their hunting habits. They have adapted to the presence of cane toad tadpoles. Fish like Barramundi and the northern gudgeon trout can choose their prey carefully. They can tell the difference between toxic cane toad tadpoles and safe tadpoles.
The toxins in cane toad tadpoles are mostly in their skin. So, just mouthing a tadpole is enough for most fish to taste the poison. In lab tests, Barramundi and northern gudgeon trout quickly spat out cane toad tadpoles. After spitting them out, the fish shook their heads strongly. They did not do this with other food. This shows that the bad taste makes them reject the tadpoles. These fish learned not to eat cane toad tadpoles after just a few tries. Most fish could recognize and avoid the toxic tadpoles days later. They used either sight or smell.
Frogs Learning to Avoid Toads
Native Australian frogs are very sensitive to cane toad toxins. They are too small to eat adult cane toads. But most will eat small, recently changed toads. These young toads have the lowest amount of toxins. Still, eating them can often kill a native frog. So, cane toads are a big threat to native frog populations. Especially ground-dwelling frogs that are more likely to meet them.
Australian marbled frogs learn very quickly. They avoid eating small cane toads after a bad experience. Frogs that have met toads likely use both sight and smell to avoid them.
Native Australian frogs also avoid the smell of adult cane toad urine. This behavior might help them avoid dangerous meetings with large toads. However, frogs that have never met cane toads also show this avoidance. This suggests they might just be looking for places free of strange smells. They might not be learning to avoid cane toads directly from their smell.
Birds and Their Eating Habits
Birds seem to be more resistant to cane toad toxins than reptiles or amphibians. So, birds are less likely to die from eating cane toads. For some birds, cane toads might even be a new food source.
However, native raptors and some crow species have learned special ways to eat cane toads. They only eat the parts of the toad that are less toxic. This behavior is probably because the toad's toxins taste bad to them.
A good example is how black kites and whistling kites eat dead cane toads found on roads. These birds have learned to eat only the toad's tongue. They leave the rest of the body behind. This way, they eat less poison. This behavior is more common during the dry season. That's when food is harder to find. Studies showed kites were twice as likely to eat dead cane toads in the dry season. They also preferred frogs over cane toads. And they liked smaller, young toads more than larger adult toads.
There is also some evidence that Torresian crows teach each other how to eat cane toads. These crows seem to have a trick. They flip the toads onto their backs. Then they eat only the internal organs and part of the legs. It's possible that young crows learn this from older crows.
Wading birds, like the Nankeen night heron and purple swamphen, usually avoid eating cane toads. They probably find them unappetizing at all stages of life.
Rodents Eating Cane Toads
Rodents are naturally able to handle many plant and animal toxins. They can learn to avoid bad-tasting food. But studies show that the introduced black rat and native rodents like the dusky rat and water rat can handle cane toad toxins. So, they often eat these invasive toads.
Lab tests showed that dusky rats and grassland melomys will eat cane toads. They do this even when there are other non-toxic foods available. However, these rodents prefer to eat small, recently changed toads. These young toads have the lowest amount of toxins.
How Animals Evolve to Handle Cane Toads
Cane toads were brought to northern Queensland between 1935 and 1937. Their population grew fast. They spread west at about 15 kilometers per year. Today, cane toads are still spreading across northern Australia. They are moving even faster, at about 55 kilometers per year.
At the edge of where the toads are spreading, native animals meet them for the first time. These animals have never evolved with toxic toads. So, they are more likely to react in ways that hurt them. This lack of experience makes them very vulnerable to the toads.
But over time, the presence of cane toads can cause animals to evolve. This means their genes can change over many generations. Native species might evolve to handle more toad toxins. Or they might develop a natural ability to avoid eating them. Some snakes have even changed their bodies to eat less poison.
Evolutionary changes are more likely to happen in animals that have lived with cane toads for a long time. This time must also be many generations for the animal. Animals that reproduce quickly and have shorter lives are more likely to evolve faster.
Snake Adaptations
Cane toads are very poisonous to snakes. Since cane toads arrived, many native snake populations have dropped a lot. For some snakes, attacking a cane toad can be deadly in half of all encounters.
Snakes with larger bodies tend to have smaller heads. The size of a snake's head limits how big a prey it can swallow. So, larger snakes with smaller heads are less likely to eat big cane toads. Big toads have more deadly toxins.
Because cane toads are so toxic, eating them is very dangerous for snakes. This creates a strong pressure for snakes with smaller heads. These snakes are less likely to eat a deadly dose of toxin.
Data collected over 80 years shows that some toad-vulnerable snakes, like red-bellied black snakes, have changed. Their body length has increased over time since they met the toads. Their head size has gotten smaller as their bodies got longer. Snakes that are less at risk from cane toads, like the marsh snake, have not changed much.
Cane toads have also caused changes in how snakes eat and how well they handle toxins. Red-bellied black snakes from areas where toads have been for a long time are more resistant to toad toxins. They also have a natural dislike for cane toads as food. These changes probably happened in less than 23 generations of black snakes. This shows how strong the pressure from cane toads is.
Planigales and Toxin Tolerance
Studies have shown that planigales can learn to avoid cane toads. But over many generations, planigale populations exposed to cane toads have also evolved. They can now handle the toad's toxins better.
Cane Toads as Food for Arthropods
Native frogs have evolved with predators for millions of years. But cane toads are new to Australia. Some of their traits are not good for avoiding new predators.
Cane toads are especially bad at avoiding native arthropods. Arthropods don't seem to be harmed by cane toad toxins. Young cane toads are very vulnerable to meat ants. Meat ants have been seen killing many small toads near water in northern Australia. Toads are more likely to meet meat ants than native frogs. This is because toads are active during the day, not at night. They also prefer open areas where meat ants are common.
Also, toads don't escape meat ants well. When attacked, toads often stay still. This doesn't work against meat ants, which eat the toads alive. A study found that meat ants could seriously hurt young toads in 5 seconds. More than 80% of these attacks ended in death for the toad.
Many other native arthropods also eat young cane toads. These include fishing spiders, water beetles, water scorpions, and dragonfly nymphs. Dragonfly nymphs and fishing spiders specifically choose to eat toads over native frog species.
See Also
Images for kids
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Cane toad parotoid gland.jpg
A cane toad's parotoid gland, which produces toxins.
