Hurricanes do not affect all lizards equally. Research has shown that survivors of hurricanes are generally those individuals with both larger toepads and shorter femurs than the average population before the hurricane. The larger toepad phenomenon can be explained by grip force; those with larger toepads were able to hang on to their perch in the strong winds, and those with smaller toepads likely got blown away. However, until now, the phenomenon of shorter-legged survivors was unexplained. A group of researchers lead by Shamil Debaere at the University of Antwerp, Belgium, investigated the reason behind the shorter femurs in surviving Anolis lizards and why longer legs may be disadvantageous in high winds during tropical hurricanes.
During windy storms in the wild, anoles generally hold on to vertical branches, with their head facing up, arms close to the body and legs either stretched out down the branch, or bent in a crouched position. In this way, the branch acts as a windshield that the lizards can effectively hide behind to withstand stronger winds. Debaere's team suspected that longer legs may stick out from that windshield, allowing them to be torn free of their hold by fierce winds.
To test whether longer legs might affect the amount of drag experienced by the reptiles in different positions, the researchers calculated the aerodynamic forces exerted on the lizard body, in much the same way that engineers calculate the aerodynamic forces on a sports car. Using this method, they were able to see which parts of the lizard were causing the most drag. They used previously recorded videos of Anolis lizards gripping on to vertical perches while being blown by a leaf blower mimicking a hurricane, to make accurate 3D models of the buffeted animals. The researchers then modeled the forces exerted on the 3D models across five different postures, two of which were given 10% longer legs – one with more flexed knees and one with the pelvis protruding further from the virtual perch.
They found that the postures with 10% longer limbs had the highest drag of all five postures. The longer femurs force the lizard's pelvis to extend further from the branch, outside of the wind-shielding effect of the branch. Imagine hurricane-force winds attempting to remove you from your perch; any object sticking out of the aerodynamic shadow from the branch will cause drag, much like sticking your hand out of a moving car.
Tucked in, hiding from the wind behind the branch, the lizards are better able to hang on during hurricanes, resulting in a surviving population of lizards with significantly shorter hindlegs. Because shorter legs are less likely to get caught in the wind, those lizards can hang on better than their longer-legged counterparts and are more likely to be able to hold on tight to their perch.
