How Skinks Break The Evolutionary Rule To Lose Limbs And Grow Them Again

Hims little limbs are a bit useless when it's dry. Brachymeles boulengeri photo by Philip Bergmann

As a rule of thumb, in evolution, when you lose a limb it doesn’t come back. There are several examples in the animal kingdom where an arm or leg has become superfluous and as such the species adapt to no longer grow them, some of which we can still see the residual evidence of today such as the rudimentary hind limbs in modern whale skeletons. One species however is proving that the loss of a limb needn’t be forever, as new research published in the journal Proceedings of the Royal Society B has investigated the mysterious returning limbs of skinks.

Backtracking on an adaptation is known as evolutionary reversal and is the process through which lost structures can return to a species over time. The exact mechanism through which this is possible is unclear, but it’s something seen in a group of reptiles known as skinks, which have on several occasions evolved from having four limbs to a limbless, snake-like body before returning to being quadrupedal again.

If you look very closely you can see this Brachymeles bicolandia is still holding onto the teeniest of stumps. Photo by Philip Bergmann

To find out how this happened, a team of researchers gathered body form and locomotion data for a range of skinks from the snake-like to the four-legged. How did they do that, I hear you ask. By setting up a tiny race track complete with high-speed cameras. They then put it to the test with 147 skinks from 13 different species captured from the wild. Of their contenders, some were limbless, some had teensy stubs and others were packing four loud and proud legs and feet. The researchers set the skinks loose to charge along the track and burrow into a hole, the footage from which they later reviewed back in the lab.

After studying the footage, the researchers concluded that skinks with limbs were speedier and better able to burrow than their legless counterparts. The limbs particularly came into play when digging in the wet, packed dirt used in the race, suggesting that the importance of limbs may be affected by the climate.

Analysis of palaeoclimatological data supported this idea, as they observed that the loss and reemergence of limbs linked up with changes in the weather. When their habitats became drier, limbs were less important and were lost but then if it grew damp some species grew back their limbs. The discovery shows that under the right circumstances with the right environmental pressures evolution can be reversed, even in a case as extreme as regrowing an arm and a leg.

The diddiest of digging devices. Photo by Philip Bergmann

It’s important to note that these changes were not sudden. As delightful as it might be to imagine the first drops of rain splashing off the head of a snake-like skink as it spontaneously sprouts legs with a satisfying *pop*, such evolutionary reversals takes many generations before hints of a returning limb can be seen. One can dream, though.


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