Eleutherodactylus iberia. Image: Miguel Vences.
DOI: 10.1098/rsbl.2010.0844
While you are sitting at your desk sipping your coffee, there is a biologist somewhere in a remote jungle in Cuba worming his way through damp leaf litter, trying to ignore the swarms of mosquitoes sipping his blood.
He pauses, motionless, concentrating, trying to not become distracted by the joyful dawn chorus of thousands of birds, trying not to hear the persistent whine of hungry mosquitoes. The biologist wonders, not for the first time, if the source of the sound that he is seeking was the result of his runaway imagination. Maybe he shouldn't have had that third cup of coffee before setting out this morning?
The biologist peers over the ruffled surface of the leaf litter, enjoying the rich scent of wet earth whilst trying to breathe as quietly as possible, aware of the dampness soaking through his clothes, mindful of the armies of insects and spiders crawling over his body. Sweat beads up and then trickles down his forehead, almost timidly at first, but it gains speed and confidence as it runs past his eyebrow and down his unshaven face.
An ant on the edge of a leaf is so close that he can barely focus on her. She waves her antennae intently, almost like a dog sniffing the air currents. What does she detect? After her momentary pause, she silently hurries on to her own mysterious appointments.
A mosquito settles on his eyelid (dammit!) but the biologist can't shoo her away for fear that his movement will betray his giant invading presence to the residents of this tiny world. But after meticulously turning over leaves for almost 20 minutes without any sign or peep from his quarry, the biologist is almost ready to give up searching this patch and move on.
No, wait! He freezes, holding his breath.
There it is -- that sound -- and much louder this time. Oh, he's so close now. The biologist finally hears those magical high-pitched chirps soaring above even the incessant whine of mosquitoes buzzing around his ears, chirps that originally brought him to his knees, then to his belly. He resumes turning over dead leaves with renewed purpose, carefully and slowly and one-by-one, in the area where the chirps seemed to emanate from.
Suddenly, out of the corner of his eye -- the one whose lid that the mosquito is still perched upon -- a tiny blackish something jumps out from under the leaf he is holding in his fingers, landing in the litter far far away.
It's a tiny frog! (or was it a cricket?) The biologist's heart pounds loudly in his ears, temporarily drowning out the mosquitoes.
The tiny animal's astonishing leap carried it farther than any human long-jumper's efforts ever could. The biologist marvels, not for the first time, at this animal's remarkable athleticism even as he moves carefully towards the place where it disappeared.
This time, the biologist is sure he saw a frog. After a few more minutes of searching, he finally captures the elusive animal. Yes, it's a miniature frog; a beautiful Eleutherodactylus iberia.
The biologist gently places his prize in a small cup and examines it closely. This tiny frog, smaller than a cricket, is a lovely chocolate brown color (almost black, really) with a brilliant stripe that changes from orange-to-yellow-to-white that races from its nose to the toes on its powerful hind legs. There is a second bright stripe, not readily visible except under close inspection, that runs from the frog's lower jaw and continues along its undersides.
Eleutherodactylus iberia perched on a researcher's finger. Image: Miguel Vences.
DOI: 10.1098/rsbl.2010.0844
The biologist is Miguel Vences, professor of evolutionary biology at the Technische Universität Braunschweig in Braunschweig, Germany. He is heading up an international team of researchers who just discovered that Eleutherodactylus iberia are members of yet another group of rainforest frogs that are highly toxic. But what sorts of toxins are these and where do they come from?
Dr Vences and his team wanted to learn more about these poisonous miniature frogs so they collected specimens of E. iberia in October 2009 and in January 2010 at Bahía del Taco in Cuba. They also collected specimens of close relatives at Yunque de Baracoa, Guantánamo province in eastern Cuba.
Since most poisonous frogs obtain their toxins from their diet, the team examined these frogs' stomach contents to identify what they were eating, being especially careful to identify oribatid mites to the family level because of these species' importance as a source of toxic alkaloids for poison frogs. In fact, two alkaloids discovered in E. iberia have previously been detected in arthropods: 307A in mites and ants, and 323A in ants, suggesting a dietary source for these compounds [DOI: 10.2307/1447647 & DOI: 10.1073/pnas.0702851104]
The researchers found that numerically, 71 per cent of the 62 prey items found in E. iberia's stomach were mites. They found other arthropods in much smaller proportions: the third and fourth most common prey items were springtails (Collembola; numerical proportion 10%) and ants (6%).
The researchers also identified alkaloids in the frogs' skin using gas chromatography–mass spectrometry and found that E. orientalis and E. atkinsi, two sympatric species that are close relatives to E. iberia, had no detectable levels of poisonous compounds, whilst E. iberia had significant amounts of various alkaloid toxins sequestered in their skin. The major toxic alkaloid that the researchers found is known as PTX 323A. They also found several other related compounds (Figure 1):
DOI: 10.1098/rspl.2010.0844In addition, the team found significant amounts of two isomeric alkaloids with unknown structures. Isomers are molecules that contain the same number and identity of atoms but they are arranged into different structures.
These data provide a few more clues into this frog version of the "chicken-and-egg" scenario where it is difficult to determine which came first; small body size, bright colours and contrasting patterns, diurnality or sequestration of toxic alkaloids. Field studies add interesting details: Cuban dwarf Eleutherodactylus frogs are mostly diurnal, but E. iberia's close relatives, E. atkinsi, E. cubanus and E. etheridgei, are not brightly coloured.
Are these other species toxic? Currently, scientists don't know if alkaloid sequestering regularly occurs in these close relatives, nor in fact do they know whether all populations of E. iberia are poisonous.
But as more evidence accumulates, scientists are finding the data support the hypothesis that specialization to small-sized prey, including ants but especially mites, was a first evolutionary step, followed by evolution of alkaloid-sequestering, aposematism and diurnality.
With any luck, Dr Vences and his colleagues will once again be lying on their bellies under towering Cuban trees, meticulously flipping yet more dead leaves in their quest to understand how a very small resident of the world came to be.
Sources:
Ariel Rodriguez, Dennis Poth, Stefan Schulz & Miguel Vences (2010). Discovery of skin alkaloids in a miniaturized eleutherodactylid frog from Cuba. Biology Letters, 6 DOI: 10.1098/rsbl.2010.0844
Miguel Vences [emails, 29 & 30 October 2010]