There’s something peculiar going on off the coast of Brazil: An invasive species of toad that crept onto an island has become severely deformed, and scientists aren’t too sure why. While their abnormalities are well documented, the consequences of these changes on the feeding and reproduction of the species were unknown. Now, a new study has revealed that the animals display remarkable plasticity, adopting profoundly different behaviors to overcome their handicap. The impact this has had on their body conditions and reproduction have been published in the Journal of Zoology.
As the name suggests, invasive species generally do not spell good news for the environment they burst into. They can bring with them foreign disease-causing microbes, mess up food chains and compete with existing organisms that play a similar role in the community. But things don’t always go swimmingly for the intruder, either: If the species isn’t well-suited to its new abode, it can soon start to die-off or experience potentially severe handicaps.
One fascinating example of such an invasion going wrong is the Cururu toad that made its way to the archipelago of Fernando de Noronha a number of decades ago. Since arriving, the amphibians have progressively become more and more deformed, with almost half of the population now showing one or more deformities affecting the mouth, limbs or eyes. For example, some have missing or extra digits, and others have misshapen jaws. Around 20% of adults were also found to lack one or both functional eyes and are consequently completely or partially blind.
These handicaps have to spell bad news for the toads, right? Not necessarily. It seems as though the animals are still persisting in spite of them. However, researchers did find that the animals are adapting to their abnormalities, adopting new ways to find dinner. For example, unlike normal toads that rely on vision to locate and capture prey, blind ones have opted for a more laid-back approach.
Quite simply, they sit and wait for an unsuspecting insect to cross their path, then they gobble them down. Such behavioral plasticity, the authors write, is an “amazing example of amphibians’ ability to adapt to adverse conditions, which could be deleterious for many vertebrates in natural systems.” This flexibility is likely what is allowing the animals to linger in equal numbers to normal individuals.
But this new opportunistic diet is not without consequence: Since blind individuals can no longer discriminate prey traits and will eat anything they touch, they are unable to select an optimal diet like normal toads. This was supported by the finding that the stomachs of normal toads contained more fast-moving prey, but the contents were less diverse than those of blind individuals.
These differences in diet are ultimately leading to different body conditions between normal and malformed individuals. The latter, for example, weigh less and produce fewer eggs. That being said, deformed individuals still produce thousands, the researchers tell BBC Earth, so this reduction has not hindered population maintenance. Furthermore, as an invasive species, the toads do not have any natural predators on the island, so they can afford to produce a smaller number.
Although scientists are still scratching their heads over the reasons behind these striking deformities, future research may shed light on this. Current ideas include chemical pollutants in the environment, inbreeding or perhaps a pathogenic microbe.
[Header image "Cururú (Bufo paracnemis)" by Christian Ostrosky, via Flickr. CC BY-NC-ND 2.0]
[Via the Journal of Zoology and BBC Earth]
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