In the rainforest in the Philippines, scientists laid out a tiny racetrack, just for skinks.
The racetrack and the high speed cameras surrounding it were part of research into a mysterious state of affairs. Skinks are lizards, but some species have lost their limbs over eons of evolution, giving them a snakelike look. However, other skinks whose ancestors jettisoned limbs have, for reasons still unknown, brought them back. They break an old rule of thumb in evolutionary biology, which holds that if you lose a complicated structure, you are unlikely to evolve it back.
Now, the researchers behind the racetrack write in Proceedings of the Royal Society B in a paper published Wednesday that skinks with limbs move faster and burrow better than their limbless compatriots. And the timing of limb gains and loss across the family tree of skinks in the Philippines appears to sync up with shifts in the local climate, which could have changed the texture of the soil where they lived. As it grew drier, limbs disappeared, and as it grew damper, they sprouted back in some species, giving a rare glimpse into how, under the right evolutionary pressure, organisms can force limbs back into being.
On land, many legless creatures live in dry, sandy areas, said Philip Bergmann, a professor of biology at Clark University in Worcester, Mass., and an author of the new paper.
“A lot of people for decades, maybe even a century, have been suggesting that a snakelike form would be an adaptation for a burrowing lifestyle,” Dr. Bergmann said.
In Asian jungles, however, skinks with and without limbs coexist in the same damp, tropical environment.
To see how skinks in these habitats actually move, the team captured 147 individuals of 13 different species. Some had no limbs, others had tiny ones, others had fully formed legs and feet. They encouraged the skinks to run along the racetrack and burrow in a tube of dirt, carefully recording their movements to analyze later in the lab.
The team found that skinks with limbs outperformed those without, moving and digging much faster. The limbless animals had their own manner of surviving in the forest, creeping slowly and keeping well out of sight rather than relying on speed.
Overlaying a reconstruction of the climate, based on published work by paleoclimate researchers, on the branching tree that contained all these species revealed interesting patterns. Sixty million years ago, when the skinks first lost their limbs, the area was much drier. Twenty million years ago, when some of them brought their limbs back, the climate had shifted to be wet and monsoonal.
“The climate in the past seems to correlate pretty nicely with our hypothesis,” Dr. Bergmann said. Perhaps in wetter climes, limbs had advantages they had not had before.
The idea that complex structures, once they’re gone, stay lost makes a certain amount of sense, Dr. Bergmann said. In theory, once the genes involved in making a limb stopped being used, they could run the risk of being damaged by random mutations and not being repaired, because they were no longer useful.
However, research has shown that many of these genes are actually involved in the development of many different parts of the body and need only be switched back on when needed, he went on.
“There is strong selection to maintain those genes,” he said. “So you could activate them again in the right place.”