Coin-sized frog becomes mite-y thanks to poisonous diet

Miguel Vences was dissecting a frog no bigger than his fingernail when he smelled an unusual acrid smell. “Maybe it can be compared with vinegar,” he says. “It is a totally different smell, but somehow the same kind of bitter-burning feeling when you get it into your nose.” He remembered the distinctive scent from his experiences with other species of frogs, all of which have powerful poisons in their skins. He reasoned that the species he was cutting open – a beautiful Monte Iberia eleuth – was similarly armed with toxins. A chemical analysis of its skin confirmed Rodriguez’s suspicion. The frog’s skin was laced with toxins, including a group of muscle-paralysing poisons called pumiliotoxins that are common among poison dart frogs.

Frogs don’t make pumiliotoxins for themselves; they steal them. In 2004, another group of scientists found that poison dart frogs eat ants that are loaded with pumiliotoxins, absorbing the poisons from their meals for their own use. Rodriguez showed that the Monte Iberia eleuth does the same, but it relies on a different source. When he cut open the frog’s stomach, he found mites and lots of them. These tiny meals made up around two-thirds of the eleuth’s diet (while ants made up just six per cent).

Along with its toxic skin and thieving antics, the eleuth shares many characteristics with other frogs that use a similar class of poisons. It’s brightly coloured with chocolate-brown skin and yellow-white lines; it’s active during the day; and it’s incredibly small. Indeed, it ties with the Brazilian golden frog for the title of world’s smallest frog. At less than a centimetre in length, it could happily sit on a fingernail or a small coin.

The fact that so many frogs share these characteristics is no coincidence. Rodriguez thinks that their small size allowed them to specialise on tiny prey such as ants and mites. This special diet allowed them to evolve the ability to store defensive poisons from their meals. That, in turn, led to the evolution of bright colours to advertise their potent defences, and the luxury of operating in broad daylight. It’s an intriguing idea, but scientists will need to compare the diets, defences and sizes of other closely related species to see if it holds true.

Reference: Biology Letters http://dx.doi.org/10.1098/rsbl.2010.0844

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