Mystery of the lizards that know the way home

Call it the case of the homing lizards. It’s a small mystery. The central question is one that has prompted plenty of scientific research: How do animals find their way home? ­­­­­­The lizards in this case are anoles — mostly small reptiles that thrive in the Caribbean.

The species is Anolis gundlachi. The lead detective is Manuel Leal, a biologist at the University of Missouri. He has been studying the behaviour of anoles for more than 20 years. For about three years, Manuel has been trying to understand how the anole finds its way back to its own territory after being carried into the rain forest. As he told an audience in June at the annual meeting of the Animal Behaviour Society in Anchorage, the case is far from closed. First, a bit of background. Anoles are particularly abundant in the dense vegetation of the rain forests in Puerto Rico, where Manuel studies them. Each species is tied to a very specific environment. For instance, many live on tree trunks, but only a particular part of the trunk. Trunk-ground anoles live only in the space from the ground up to 6 feet or so and the trunk-crown anoles live above them.

Several years ago, Manuel was studying competition between two species. If he removed all of the trunk-ground anoles, he wondered, would the trunk-crown lizards extend their territory farther down the tree? He ran into a problem, however. He would take the trunk-ground lizards far from their home territory to make room for their upstairs neighbours, and then release them. But in a reptilian version of the children’s song, The Cat Came Back, the lizards wouldn’t stay away. “Lizards kept showing up in the territory that had just been scoured for lizards,” he said. Manuel wondered whether new anoles were appearing in empty territory or the old ones were returning. But how could a lizard that had never left home find its way back through 25 yards or so of dense rain forest? First, he established that they do find their way back. A graduate student would take one, put it in a box, shake it to disorient the anole, walk about 80 yards or more away in a randomly chosen direction, shake the box again and let the lizard out. “The graduate students got lost,” Manuel said, but not the lizards. It took them a few hours or a few days, but about 80 percent of them found their way back. How did they do it? The box and the shaking ensured that an anole couldn’t keep track of the path it took. The dense rain-forest vegetation made it almost impossible for the lizards to be guided by visual cues, Manuel said. Some birds and insects can detect polarised light from the sun, which gives them a way to sense direction. And some animals can use Earth’s magnetic field. So the team glued tiny magnets to the heads of anoles. The lizards still came back. Same percentage. Same time period.

Then the researchers created Ping-Pong ball caps to the heads of the anoles to interfere with polarised light that might reach an organ in the brain called the parietal eye. Some research has shown that lizards can detect polarised light with that organ, not with their regular eyes. The team didn’t just block all light to the parietal eye, because it controls the daily rhythms of the lizard. Block it completely, Manuel said, and “after a day they just stop moving.” If it is always night for them, they stay quiet. And how did the lizards do with the hats? They came back.

Finally, Manuel’s team created computer simulations of random walks, on the idea that just by wandering at random, the lizards would arrive at home. The lizards in the simulation did not find their way home. Manuel turned to the audience. His next step is to attach a GPS tracking device to the lizards to see what kind of path they follow on their way home.