Recently, I was minding my business, trying to read about apex predator loss in Australia, and I stumbled into the captivating, surprisingly many-sided story of the cane toad…
Cane toads are a notorious invasive species capable of decimating entire populations of predators that naively eat these poisonous pests. But cane toads are also an incredibly interesting species that’s played a role in human affairs for so long that they’ve become one of the most ecologically and economically relevant (for better or worse) species of amphibian. Covering the topic completely would require a book rather than a blog post, so here’s just a few aspects of cane toad history and biology:
-When agitated, cane toads secrete bufotoxins from glands on their backs powerful enough to stop an adult human’s heart. Cane toads were one of the several frog species used by the Emberá tribes of Colombia to coat their dart tips.
-Cane toads are voracious. In addition to any living creature that they can fit into their mouths, they will also eat garbage scraps, carrion, and steal dog and cat food from pets’ bowls.
-From sometime in the 1930s until the 1960s, cane toads were widely used in the U.S., Australia, and other countries for pregnancy testing. The patient’s urine was injected into the toad, and if human chorionic gonadotropin (the pregnancy hormone) was present in the urine, a male toad would produce spermatozoa and a female toad would ovulate. The test was quite accurate and only took a few hours.
-The easiest way to tell a male from females is to pick them up. The males make a special ‘release call’, a unique sound that informs other male cane toads that they’ve grabbed an incorrect mate.
-Cane toad races are popular in Port Douglas (Australia) pubs, and on Australia Day.
-Dogs often pick up cane toads with their mouths or lick them, resulting in potentially fatal poisoning. Some dog owners in Australia report that their dogs have become ‘addicted’ to licking cane toads. The dogs seemingly seek out cane toads in order to ingest just enough toxin to experience its effects without dying.
-It’s not just dogs. People have tried to alter their mental state by licking toads, boiling them and drinking the water, or drying and smoking the toxic slime from their backs. It’s unclear how widespread this practice is, but it’s enough of an issue that it is illegal to possess, buy, or sell bufotoxin/toad slime in the United States or Australia. The toxins do induce powerful hallucinations, but often with the inconvenient side effects of vomiting, seizures, and cardiac arrest.
-Their poison has shown promise as an anti-viral, anti-cancer, and anti-fungal agent. Australia is considering exporting cane toads to China in order to make a medicinal product called chan su, which is typically made from the now-overharvested Asiatic toad.
-The toads can be made into various leather products. As a wedding gift, Princess Diana and Prince Charles received a book bound in cane toad leather from the Queensland Government. If you’re looking for a present for an upcoming wedding, here’s where you can buy a gift worthy of royalty:
-A female cane toad lays 8,000 – 25,000 eggs at a time, in a gooey string that can be as long as 66 ft. Cane toad tadpoles will eat the eggs and tadpoles of other frogs and other cane toads, but not their own siblings.
-People had been introducing cane toads to sugarcane plantations to attempt to control various pests since the 1840s, but it was the “successful” example of Puerto Rico that spurred introductions across the world. In Puerto Rico in the 1920s, white grubs (Phyllophaga spp. beetle larvae) were a huge problem for sugarcane growers. One possible explanation for the white grub problem was that mongoose had been introduced earlier to control rats, but also wiped out a local whiptail lizard that specialized in eating white grubs. After they had tried parasitic wasps and flies for biological control of the beetles, cane toads were brought from South America to Puerto Rico. Beetle populations soon plummeted, though it could have been that year’s weather anomalies that reduced their numbers, rather than the cane toads.
–Cane toads were introduced to Hawaii after nobody challenged the unsubstantiated claims of prominent scientists that it was the “harmless” toads that had successfully controlled the beetles in Puerto Rico. Cane toads were collected from Puerto Rico and released near the Lyon Arbortoreum in Manoa Valley and in a taro patch in Waipio in 1932. Toad populations in Hawaii exploded, and the myth of the miracle toad spread; cane toads were sent from Hawaii to dozens of other countries, including Australia in 1935.
-Right before cane toads were released in Australia, the toad-advocates in Hawaii finally learned of their toxicity; sadly, a two year old child of Filipino sugarcane workers in Honolulu died after a guest brought over a large cane toad for dinner. The advocates worried for the toads’ popularity, and decided to keep the death a secret until after the toads had been established in Australia, at which point they would issue an announcement about the toad’s inedibility. The warning was eventually published in an obscure journal.
-Fortunately, while Australians are encouraged to kill cane toads, it is illegal to do so in a cruel manner. No matter how loathed an invasive species is, the animal itself is not guilty of any wrongdoing, so humane methods of euthanasia are encouraged. There is even a commercially available toadicide called “HopStop®”.
The cane toad invasion in Australia
Since their introduction in 1935, cane toads have spread from Queensland to the northernmost parts of Australia. Their spread has been likened to an army of stormtroopers. As they advance, they leave a trail of dead predators behind. Upon the toads’ arrival, monitor lizards, freshwater crocodiles, quolls, tiger snakes, red-bellied black snakes, death adders, and dingoes are some of the predators that die en masse after consuming the poisonous toads. Yellow-spotted monitor populations are estimated to have declined by 90% in the wild. Invaded areas show higher populations of mid-level predators after the top predators have been extirpated, as well as rapid adaptations, such as snakes with mouths too small to eat the toads. At first, the cane toad horde spread at a rate of ~10 km a year, but now that rate has increased to 40 – 60 km a year. Evolutionary selective pressures have caused a spatial sorting of different toad phenotypes within the toad army. That is, the toads at the advancing frontline are physically differently from those toward the back – they have longer legs, hop more often, and travel in straighter lines than their slower, more sedentary counterparts. These quicker toads also commonly suffer from arthritis, which is just sad.
Scientists are developing innovative, clever methods to control cane toad populations and slow their spread:
-Tadpole traps baited with cane toad pheromones – cane toad tadpoles are attracted to the trap, while native tadpoles are indifferent.
–Releasing juvenile cane toads into the wild in front of the advancing toad frontline, in addition to dropping tainted cane toad sausages from helicopters – Scientists hope that unpleasant, sublethal experiences with smaller, less toxic cane toads will teach native predators to avoid eating adult cane toads when they arrive. Preliminary testing showed that about half of wild goannas that had been fed juvenile toads (by hanging them from fishing poles) survived subsequent encounters with adult cane toads; all of the “untrained” goannas died. Further, they intend to choose genetically “inferior” juvenile cane toads with short legs, so that those juveniles that survive and mature will mate with the long-legged arriving cane toads to produce slower offspring.
–Transferring toad-savvy, prudent quolls from Queensland to areas invaded by toads. The hope is that if the prudency trait is genetic, then it will be incorporated into the naive quoll populations.
-Protection of native predators capable of preying on the toads – Some spiders and ants are immune to the poison, and will prey on the toads. The Australian crow has figured out how to safely eat the toads by flipping them over and avoiding the poison on their backs.
-Building a better toad trap that keeps its captives alive long enough to sort native frogs from cane toads
-Scientists have sought a pathogen that would only affect cane toads, but to no avail so far.
–Putting cane toads on the menu – While many parts of the toads can be toxic, their legs can be safely prepared for human consumption. They supposedly taste like gamey chicken. Enticing people to eat invasive species has been somewhat successfully attempted for lionfish in the Atlantic, but unsuccessfully for the nutria in Louisiana.