The cane toad (Rhinella marina) is a large, prolific amphibian native to Central and South America. Its success as an invasive species stems from its rapid reproduction and powerful chemical defense system. This species presents a challenge to local fauna unfamiliar with its toxicity. Despite this defense, some animals have developed strategies to consume this prey.
The Cane Toad’s Defense Mechanism
The toad’s primary defense is a cocktail of toxins stored in large glands behind its ears, known as the parotoid glands. These glands contain a milky secretion of compounds called bufotoxins, which function as cardiotonic steroids and neurotoxins. The toad uses this defense passively; the toxin is exuded when the gland is squeezed or bitten by a predator.
Upon ingestion, bufotoxins rapidly affect the predator’s cardiovascular and nervous systems. Effects often include rapid heart failure and neurological damage, frequently resulting in death. Because the toxin is densely sequestered, a single bite in the wrong location can deliver a lethal dose. The production of this toxin is also energetically costly, which is why it is used as a final line of defense.
Predators in Native Habitats
In its native range of Central and South America, the cane toad co-evolved with predators that developed a high tolerance to its toxins. These interactions resulted in physiological adaptations over millennia. For instance, some snakes, such as the banded cat-eyed snake, exhibit resistance to bufotoxins.
This long-term exposure means native predators can consume adult cane toads with minimal adverse effects. Large predators like caimans also prey on the toads. Evolutionary pressure has resulted in genetic changes that decrease their physiological vulnerability, allowing them to include the toad in their diet.
Behavioral Adaptation in Invasive Ranges
When the cane toad invades new territories, such as Australia, it encounters predators that lack co-evolved physiological resistance. However, some native animals have developed behavioral strategies to overcome the toad’s toxicity by avoiding the parotoid glands.
Australian birds like the Torresian crow and the Australian magpie flip the cane toad onto its back. By feeding on the less toxic underside, they bypass the lethal parotoid glands. Native freshwater crocodiles have learned to consume only the toad’s hind legs, which contain less toxin than the rest of the body.
Even in reptiles, where learning is generally slower, rapid evolutionary shifts are occurring. The red-bellied black snake has developed an innate avoidance of toads in populations exposed for multiple generations. Some snake populations are exhibiting a genetically based reduction in head size, which prevents them from eating the largest adult toads. Native rodents, including the dusky rat and grassland melomys, also show tolerance for the toxins and readily prey on the toads.
Targeting Early Life Stages
Predation pressure often focuses on the cane toad’s early life stages, bypassing the adult’s high toxin load. Cane toad eggs are highly toxic, even more so than the adult in terms of toxin concentration, and can be lethal to predators. However, toxin levels drop significantly in developing tadpoles, reaching their lowest point around metamorphosis.
This shift makes the tadpole and newly-metamorphosed toadlet stages vulnerable to specialized predators. Aquatic insects, such as dragonfly nymphs, and certain specialized spiders prey on the tadpoles. Terrestrial invertebrates, most notably the meat ant, attack and kill the small, newly-metamorphosed toadlets. They are unaffected by the bufotoxins, and the toadlets lack effective behavioral defenses against them.