Frogging by the Panama canal
Gamboa: What do Shakira and the red-eyed treefrog have in common? They’re both Latin American cuties with an arse-shaking dance that’ll knock your socks off. No offence to Shakira, but I’d much rather spend my evening with the bug-eyed little green fella, Agalychnis callidryas, and so we meet up with frog researchers Michael Caldwell and Justin Touchon at the Smithsonian Tropical Research Institute in Gamboa, halfway down the Panama canal.
Michael and Justin spend many of their nights sitting hour after hour in a mosquito-infested swamp watching the nocturnally active frogs do their thing. The red-eyed tree frog has a fairly simple life: sitting around looking cute, avoiding predators, eating, vigorously defending his 2-metre-wide territory from other males and trying to seduce females with his best/loudest chirp.
Getting lucky with a lady frog – which is less a fairy tale transformation into a prince, more a case of rodeo: jumping on her back and holding on for grim death – leads to a clutch of fertilised eggs, which are carefully deposited on the underside of a leaf above water. The eggs go from clear little pearls, through to maturity in just 6 days, upon which tadpoles emerge and fall into the water below. We find clutches at various stages of development – some nearly ready to pop, with big eyes inside.
The frog is not a clever creature. Most of his head space is taken up housing those huge eyes (which he sucks into his head for sleeping), leaving little room for cerebral thought or judgement. Nevertheless, bug-bitten nocturnal researchers here, noticed that embryos as young as 4 days old seem to be making complex life and death decisions – and in 80% of cases, getting it right.
Early life is a tricky time for the tiny creatures: the clutch protects them from waterborne predators and certain drowning (if they hit water before 4 days of gestation); but red-eyed treefrog eggs are a common snack for tree snakes and wasps. What researchers discovered was that if either of these should begin to munch on the clutch, and the clutch is at least 4 days old, it triggers emergency hatching by the rest of the eggs. Somehow, just 4 days after being precisely nothing, these tiny embryos make the decision to risk potential predation in the water to avoid certain predation by a snake or wasp in the dry.
Working out how they do it has opened up a whole new field of scientific investigation: vibrational vertebrates (- say that 3 times, very fast!). The egg-critters are able to discriminate between the particular vibrational patterns made in the clutch by a snake or wasp, and those made by other disturbances such as rainfall. Analysing the highly complex acoustics recorded from various types of leaf tremblers, the researchers couldn’t find any key differences between them, but it is only when they reproduce the exact same vibrations of a snake or wasp attack that the spontaneous hatching commences.
Now, Michael’s seen adult treefrogs using vibrations – the kind of booty-fluttering pulse that Shakira draws crowds with – to signal to each other through a plant. In the cacophony of competing frog calls, it’s a good way of communicating when your croak can’t be heard.
The discovery marked the first known example of vibration signalling in a vertebrate above the ground (mole-rats use vibrations below the ground). “It opens up huge possibilities,” Michael enthuses. “Potentially, everything that lives on plants could be using vibrations. We just haven’t been looking for it.”
The problem has been that treefrogs are shy and don’t want to perform their dance under the bright lights of researchers’ torches, so people who have spent their entire lives studying these frogs have missed the behaviour. As we trudge through the marshy froglands, we find several of the charismatic frogs hanging out, but none is getting jiggy. Michael had to use infrared light to capture the special 12 Hz vibrations, which the frogs use instead of – or in conjunction with – threat calls to rivals. Now, Michael’s been using infrared to look at other arboreal species and finding several frogs and potentially a chameleon that chat through the plant-telephone.
Vibration signalling could be a way for animals on the ground to tell which tree it’s worth climbing up for a mate or food, or to locate a tasty morsel in the same way spiders do on their web. Michael even thinks it’s possible that frogs twang stems, like plucking a guitar, in order to message others. For me, the green-eyed treefrog just got way more cool.