The cane toad is an example of an amphibian that has adapted to survive extreme environments.
On this blog, we’ve discussed the challenges facing tropical frogs, especially those susceptible to chytrid fungus. Little discussed, however, are the amazing amphibian survivors in drier climates like deserts, grasslands and savannahs.
These frogs and toads have adapted to survive in an environment that’s very different from the tropical rainforest. Unlike the rainforest, frogs in a dry environment have to seek out rare pools of water to breed. They also have to find ways to prevent massive water loss through their thin skin during extended dry spells.
There are several species of frogs and toads that have developed amazing adaptations to survive in some pretty extreme environments. One of the most common adaptations is called aestivation (also “estivation”). During aestivation, an animal becomes dormant during a dry period to better conserve water or keep cool. Think of it like the dry weather counterpart to cold weather hibernation.
The water-holding frog (Litoria platycephala) of western Australia is a prime example of aestivation. Western Australia is prone to dry spells lasting months or even years. When rain does come, it’s usually in the form of tropical moisture, which means a lot of it and all at once. The water-holding frog takes advantage of this short breeding period to lay their eggs in the pools that form. Once all the water is gone, they bury themselves underground and shed several layers of skin that are thick enough to not only prevent dehydration but also store water. A convenient hole in the skin near the nostrils allows the frog to breathe slowly waiting out the next rainy period.
Aborigines discovered that these frogs could be used as an emergency source of water by squeezing the frog and emptying the almost fresh water for drinking. This doesn’t immediately kill or harm the frog, but it does make it harder for them to survive to the next rainfall.
Around the world, there are other species of frogs that aestivate in the same or similar manner to the water-holding frog. These include the African bullfrog (Pyxicephalus adspersus), cane toad (Bufo marinus) and plains spadefoot toad (Spea bombifrons). All three species are currently listed as “Least Concern” by the IUCN.
Despite their incredible ability to survive dry seasons, there is a limit to how much stress these frog species can handle. Climate change is likely to increase weather extremes, both floods and droughts. Increased drought length or severity could push some of these frogs to the edge where parasites or diseases could severely impact an already weakened population. In order to save all frog species, we’ll have to look high, low and even underground in some pretty dry places!
—Andrew Franks, Zoo New England
Photo by Brian Gratwicke, Smithsonian Conservation Biology Institute