Animals do the most amazing things. Read about them in this series by Janaki Lenin.
Peacocks flaunt their ornamental trains to win over mates. Monitor lizards use their long flat-sided tails as a whip against assailants. Kangaroos use theirs as a fifth leg. Cats’ tails act like counterweights when the animals jump, while snow leopards keep their noses warm on cold nights by burying their faces in them. Many snakes, chameleons and New World primates use their prehensile tails as belays while climbing trees. And how could fish, crocodiles, lizards and marine mammals swim without their propelling tails? Geckos may rely on their tails a great deal more than other animals: like the proverbial cat, they gain many lives.
The thick tails of leopard geckos (Eublepharis macularius) store fat. When a predator is in hot pursuit, they snap off their tails on the run. This voluntary severing is called autotomy. Even as the twitching severed extremities keeps the pursuer occupied, the lizards escape. The sacrifice of a body part comes at a cost. For one thing, the geckos lose their energy store. The tail can weigh as much as a quarter of the body mass. Losing such a large appendage that sticks out behind the body shifts the balance, the centre of mass, forward. To compensate for this change, geckos alter their profile by hugging the ground.
When leopard geckos walk on the desert sands of Gujarat and Rajasthan, their fat tails swing from side to side without touching the ground. Why don’t they hold their tails stiff? Why do they sway? If they lose their tails, do they change the way they walk?
Kevin Jagnandan, of Chapman University, and Timothy Higham, University of California, Riverside, both in California, studied ten leopard geckos with intact tails. Using two synchronised cameras, they filmed the geckos walk on a horizontal track. One camera viewed the lizards from above and the other from the side.
“As with most studies involving live animals, the biggest challenge is getting the animals to cooperate in the experiments,” Jagnandan told The Wire. “It’s not uncommon to have geckos attempt to climb out of their enclosures rather than walk down the trackway into the camera’s field of view.”
Then the researchers filmed them again after gluing hollow graphic rods to the tails. The rods made the tails stiff, preventing them from moving sideways. Since they still possessed tails, they had the same centre of mass as whole geckos but the immobilisation deprived them of their function. Lastly, the researchers pinched the base of the tails to make them drop off, and filmed the now tailless geckos tread the track.
“Immobilisation of the tail by nerve ablation [surgical removal] or chemical treatment might have worked, but then the lizard would probably have dragged the tail, introducing new problems,” Aaron Bauer, a professor of biology at the Villanova University, Pennsylvania, told The Wire. Bauer wasn’t involved with the study.
Jagnandan and Higham calculated the speed, length of strides, stance time (the duration of time when a foot was in contact with the floor), duty factor (percentage of the total stride during which a foot was in contact with the track) and the angles of the limb joints. They also examined the side-to-side movement of the tail.
Restraining their tails forced the geckos to walk as if they had lost them entirely. They compensated for the loss of stability and balance by stiffening their pelvic girdles, flexing their legs and lowering their bodies close to the track. If the geckos changed their posture even though they didn’t lose their tails, perhaps the shift in the centre of mass wasn’t to blame. It was their inability to swing their tails from side to side that made them hunker down.
“We were initially surprised to find that restricting the tail movements could have the same effects on leopard gecko locomotion as removing the tail altogether,” says Jagnandan. “However, the data ended up being extremely useful for forming new hypotheses on the role of the tail in gecko locomotion.”
Walking low to the ground forced the geckos to take shorter steps. Although this didn’t affect their walking speed, they may run slower.
Gecko tails come in many sizes, shapes and lengths. The leaf-tailed geckos of Australia have broad, flat tails, knob-tailed geckos have runty tails while flying or gliding geckos of southeast Asia have growths on their tails. Would the loss of their tails change how other species of lizards walk and run?
“Absolutely!” replied Jagnandan. “Many leaf-tailed geckos are capable of autotomy like the leopard gecko, although their tails evade predators via camouflage. However, an experiment with a similar design (comparing treatment groups before and after autotomy) would be useful for determining the utility of the tail in camouflage and how important it is for gecko survival.”
As their tails regrow, geckos would regain their ability to run faster. Would having a little nub of a tail be better than no tail at all? At what stage in their growth do tails help the lizards regain their top speed?
“A series of serial autotomies might be a good idea in the future as this would reveal if the entire tail is needed to maintain normal locomotion, or if loss of a certain percentage of the tail has no effect,” says Bauer. “Understanding the way in which the tail influences locomotion may help us get a handle on the interplay of selective forces that help to shape tail morphology. Fat storage, balance, use as a rudder in gliding, defensive tail squirting in Pacific geckos, symbol of social status, etc. – all of these play some role in determining tail form and the ease (or not) with which tails are shed.”
The leopard geckos walked on a horizontal track for the experiment. Other species walk on walls, leap from tree to tree. “In future experiments, we hope to determine how the tail of geckos (and other animals) may be used for navigating turns and obstacles,” says Jagnandan. “Additionally, we hope to explore the evolution of tail function by examining a broad diversity of species with tails that vary in size, shape and utility in locomotion.”
Tailless geckos are at a double disadvantage: they are slower to evade capture, and they have nothing to throw as decoy when predators close in.
The study was published in the journal Scientific Reports on September 7, 2017.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.