Scientist at Work Blog: A Night of Saurian Spotlighting

Jonathan Losos, professor and curator of herpetology at Harvard?s Museum of Comparative Zoology, writes from Colombia, where he is studying the biodiversity of anole lizards, an evolutionarily successful group that has produced 400 species throughout Central and South America and the Caribbean islands. For more on anoles, see anoleannals.org.

March 5, 2013

Nightfall signals the end of a very successful afternoon of lizard monitoring. To celebrate, we head into Colonia Torva ? billed as ?Germany in the Caribbean? ? to grab some cervezas at the local kaffeehaus. We were elated at the unexpected abundance of tiger anoles we had found, as well as the equally unexpected show they put on.

Yet one question remained: Why did we find four times as many males as females? Is that truly a reflection of a population sex ratio out of whack? Or, as seems more likely, is it just a reflection of males? being more active and using more conspicuous parts of the habitat?

To find out, our plan is use one of the oldest tricks in the anole lizard finder?s book: night lizarding. We?ll go out after dark and look for them while they sleep. It?s then that males and females are more likely to be found in proportion to their true abundance.

Most anoles sleep on leaves and thin branches. This choice of boudoir has a built-in alarm system. It can protect the snoozing saurians from approaching predators, whose footfalls (or in the case of snakes, undulations) cause the sleep site to vibrate. The bed tremors wake the lizard in time for it to jump to safety. But their natural tripwire is completely ineffective against a more recently evolved predator: a bipedal species sporting a strong flashlight. Indeed, for hard-to-find anole species, the most effective strategy is to sleep all day and work all night.

So, off we stroll to town for dinner, planning to head straight from there to the forest we visited earlier in the day. Being herpetologists, we can?t help turning on our headlamps as we walk, scanning our surroundings for who knows what. A frog in the tree? A possum in the bush? You have to look.

And then it happens.

As we pass by a patch of overgrown weeds, Anthony Herrel spots a telltale pale blob attached to a grass blade. A quick panoramic scan indicates the field is chock-full of sleeping lizards, and close examination confirms that they are tiger anoles.

In a day full of surprises, this is possibly the biggest of them all. In all of our many collective years of lizard hunting, none of us has ever seen a twig anole in a weedy field. Twig anoles simply don?t live in such habitats; they are arboreal and occur in bushes and forests. In fact, an anatomically divergent type of habitat specialist called the grass-bush anole has evolved many times to use these habitats.

Finding a twig anole in grass-bush habitat is the anoline equivalent of finding a kangaroo in a tree. How did this happen? But then I remember: This is no ordinary twig anole. It may look like a twig anole, but we found this afternoon that it behaves in a distinctly un-twig-anolish way.

A Different Kind of Tiger

The rest of our trip goes by quickly. Our nighttime surveys revealed the tiger anole?s abundance to be even greater than we had imagined. In a typical night out we might find a couple of lizards per hour, but here we find 40 in 20 minutes. As expected, the sex ratio is approximately even. Further behavioral surveys over the following days are as action-packed as the first afternoon. In one marathon session, Rosario Casta?eda, the Colombian biologist currently working in my lab at Harvard, remains motionless for nearly two hours because every time she takes video of one lizard, another appears nearby. And visits to the weed patch confirm that the lizards are active there, as well as in the forest.

Heading back home, I reflect on the surprising turn of events. We had expected the two species with twig anole-like morphology to behave and use habitats like their Caribbean cousins. We thought the toepad-deprived beach anole should be utterly different from any of the pad-enhanced island habitat specialists.

We found that the variable-scaled anole does, indeed, fit the twig anole bill ? score one for convergent evolution. But the tiger anole is another story altogether. It?s got twig anole-like morphology and uses twig anole habitats. But it also uses habitats occupied by different habitat specialists in the Caribbean, and behaves in a manner distinctly unlike the twig anole. How can convergent evolution in anatomy be explained when species aren?t using the same ecological niche?

And, finally, the curious case of the padless anole. The explanation that pads were lost as an adaptation for walking on the sand isn?t satisfying when one finds that the lizards don?t seem to be using the sand. Why, then?

As always, more questions than answers. Although it would be fascinating to come back and study these species in greater detail, that?s not our plan for the moment. Rather, we need to more broadly survey the evolutionary diversity of mainland anoles, to find out which patterns are the norm, and which are the exceptions. Once we grasp the bigger picture, we can better choose which species to study in greater detail. With data for 30 mainland species in hand, that leaves only 220 more to go. Time to start planning our next trip, in July.

Source: http://scientistatwork.blogs.nytimes.com/2013/04/25/a-night-of-saurian-spotlighting/?partner=rss&emc=rss

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