Through an extensive analysis of 228 toad species, constituting nearly 43 per cent of the known toad species of the world, scientists have now come up with a list of special traits that could have enabled these amphibians to conquer the world.
Ines Van Bocxlaer, a graduate student of the Free University of Brussels, and her associates list these special traits in the February 5 issue of the journal Science.
In a period of less than 10 million years -- which is a relatively short period in the history of the evolution of life -- toads have been able to spread across almost all continents of the world. Toads (Bufonidae) originated in the tropics of South America. The roughly 500 known species populating a wide range of habitat types are incredibly diverse in their characteristics, and some species have a larger distribution range than others.
The researchers looked at these characteristics and correlated them with their distribution range. “When we linked adult and developmental traits with the past distribution of toads, we could see that ancestral toads having a combination of certain traits were the ones that expanded their range and spread over the world,” Franky Bossuyt of the Free University of Brussels, under whose joint supervision the study was done, told The Hindu.
S.D. Biju, Associate Professor at Delhi University who collaborated with the study as research supervisor, says these traits could help determine which species of toads could become an invasive presence in a particular environment. He cited the case of the cane toad, which was introduced in Australia in recent times and had by now become infamous for its “invasion” of the whole of the island continent.
One common trait of the wide-populating toads is the ability to live away from water instead of being in high humidity, damp and even wet places to survive.
Other traits that facilitate spreading include the possession of parotoids or poison glands, which deter predators, and inguinal fat bodies, which provide extra energy to hop around expanding range, Dr. Bossuyt said.
The researchers also say that the toads that have come to colonise large parts of the world have relatively larger body sizes. They balloon in size and have greater body volume than other toads to store water, relative to the skin area from where to lose water.
The ability to lay eggs in all kinds of water bodies, large clutch sizes of eggs laid at a time, and the ability of their larvae to feed off nutrients in the environment (which means reduced dependence on the mother toads) are the other common traits found in toads that have been successful with range expansion.
“Much of the research on molecular evolution so far explains the past, but links between geographic expansions and speciation have rarely been demonstrated. This study will probably lead to further discussion about this adaptation,” Dr. Biju said.
In India, the common toad Bufo melanostictus is able to adapt to a diverse range of habitats and is hence able to have a large distribution throughout the country.