The skin of the common Asian toad contains a poison that can kill nearly anything that tries to eat the amphibian. Eight to 10 years ago, the toad arrived in Madagascar. That’s bad news for the unique wildlife that have evolved on this island nation off the coast of East Africa. A new genetic study shows that the toad’s toxin could likely kill nearly any of the island’s predators.
Scientists had suspected this for years. However, they had lacked data to confirm just how dangerous the toads could be. Now, they have it.
For tens of millions of years, Madagascar’s wildlife has been isolated from the rest of the world. “It’s never had toads,” notes Wolfgang Wüster. He’s a herpetologist at Bangor University in Wales. So why, he asks, “would the local animals be resistant to toads? We always expected them to be sensitive.” As a result of the new study, he says, “we have much better evidence that that’s really the case.”
Duttaphrynus melanostictus (Dut-tah-FRY-nus Mel-ah-noh-STRICT-us) is one of many toad species that secrete potent toxins. These poisons can disrupt the flow of sodium and potassium across cell membranes. That flow is critical for the proper function of muscles, especially in the heart.
If they are not resistant to the toxins, “[predators] that take a mouthful of toad can die extremely quickly from heart failure,” notes Wüster.
The common Asian toad likely stowed away on some ship in Asia. It may have been leaving Vietnam’s capital, Ho Chi Minh City. From there, the toads hitched a ride to Madagascar. They now are spreading slowly across the large island.
Some species in parts of the world where toxic toads are found — including certain reptiles and mammals — have evolved resistance to toad toxin. In 2015, Wüster was part of a research team that showed resistant animals shared something in common. All had specific mutations — changes — in a gene for their cells’ sodium-potassium pump. “That’s a universal mechanism for being able to consume toads — and particularly to deal with [their] toxins,” says Wüster.
Toxic toads have become invasive species in several parts of the world. Cane toads, for instance, have been spreading across northern Australia for decades. That has led to declines of native species, such as quolls and snakes. That invasion is well-studied. Scientists there know a lot about the species affected by it. That’s why there’s little need there to start looking for species that are immune to the toxin. More recently, though, toxic toads have invaded places such as Indonesia and Madagascar. There, such data might be helpful.
So Wüster and his colleagues looked at the gene for the sodium-potassium pump in 77 species from Madagascar. Their list included 28 birds, eight mammals and 27 snakes. Only one species, a rodent known as the white-tailed antsangy, had the mutations that would make it immune to the toad toxin. The toxin would likely be able to kill all 76 other species. The researchers reported their finding June 4 in Current Biology.
Madagascar’s wildlife is likely not entirely doomed. In Australia, scientists have documented behavioral changes in some predators that have allowed them to safely eat toads. Some birds, for instance, have learned that toad tongues are safe to eat. Some Madagascar species might similarly figure out how to safely make a meal of the toads. Or they could learn to simply not eat the toads.
People might help, too. It’s probably too late to remove all of the toxic toads from Madagascar. There are already too many of them. What’s more, there’s no good way to easily find and kill them all. But this research may help scientists in Madagascar take measures to protect certain species or sites from toads, Wüster says.
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biology The study of living things. The scientists who study them are known as biologists.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
cell membrane A structure that separates the inside of a cell from the outside of it. Some particles are permitted to pass through the membrane.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
colleague Someone who works with another; a co-worker or team member.
disrupt (n. disruption) To break apart something; interrupt the normal operation of something; or to throw the normal organization (or order) of something into disorder.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
heart failure A weakening of the heart that leads to its inability to pump enough blood to meet the needs of its tissues. It does not mean the heart has stopped. But if left untreated, heart failure can lead to death.
herpetology The biology of reptiles and amphibians. Scientists who work in this field are known as herpetologists.
immune (adj.) Having to do with the immunity. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process.
invasive species (also known as aliens) A species that is found living, and often thriving, in an ecosystem other than the one in which it evolved. Some invasive species were deliberately introduced to an environment, such as a prized flower, tree or shrub. Some entered an environment unintentionally, such as a fungus whose spores traveled between continents on the winds. Still others may have escaped from a controlled environment, such as an aquarium or laboratory, and begun growing in the wild. What all of these so-called invasives have in common is that their populations are becoming established in a new environment, often in the absence of natural factors that would control their spread. Invasive species can be plants, animals or disease-causing pathogens. Many have the potential to cause harm to wildlife, people or to a region’s economy.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
mechanism The steps or process by which something happens or “works.” It may be the spring that pops something from one hole into another. It could be the squeezing of the heart muscle that pumps blood throughout the body. It could be the friction (with the road and air) that slows down the speed of a coasting car. Researchers often look for the mechanism behind actions and reactions to understand how something functions.
membrane A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.
muscle A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.
mutation (v. mutate) Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.
native Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.
potassium A chemical element that occurs as a soft, silver-colored metal. Highly reactive, it burns on contact with air or water with a violet flame. It is found not only in ocean water (including as part of sea salt) but also in many minerals.
potent An adjective for something (like a germ, poison, drug or acid) that is very strong or powerful.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
quoll A small, meat-eating marsupial that has a spotted coat and looks similar to a cat. These animals are native to Australia and New Guinea.
reptile Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.
rodent A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs, hamsters and porcupines.
secrete (noun: secretion) The natural release of some liquid substance — such as hormones, an oil or saliva — often by an organ of the body.
sodium A soft, silvery metallic element that will interact explosively when added to water. It is also a basic building block of table salt (a molecule of which consists of one atom of sodium and one atom of chlorine: NaCl). It is also found in sea salt.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
toxin A poison produced by living organisms, such as germs, bees, spiders, poison ivy and snakes.
Wales One of the three components of Great Britain (the other two being England and Scotland. It’s also part of the United Kingdom (whose other members include England, Scotland and Northern Ireland).
Journal: B.M. Marshall et al. Widespread vulnerability of Malagasy predators to the toxins of an introduced toad. Current Biology. Vol. 28, June 4, 2018, p. R654. doi: 10.1016/j.cub.2018.04.024.
Journal: M. Vences et al. Tracing a toad invasion: lack of mitochondrial DNA variation, haplotype origins, and potential distribution of introduced Duttaphrynus melanostictus in Madagascar. Amphibia-Reptilia. Vol. 38, 2017, p. 197. doi: 10.1163/15685381-00003104.
Journal: B. Ujvari et al. Widespread convergence in toxin resistance by predictable molecular evolution. Proceedings of the National Academy of Sciences. Vol. 112, September 22, 2015, p. 11911. doi: 10.1073/pnas.1511706112.