December 27th, 2007
Darwin’s lizards: like Galapagos’ finches, anoles of the Greater Antilles have proved to be eminently adaptable - includes related articles on Cuban anoles in Florida, and the crested and Gundlach’s anoles in Puerto Rico
The term “tropical biodiversity” is more likely to conjure up images of Amazonian rain forests than Jamaican beach resorts. Yet to study the causes of species richness, we travel to Negril, home to such vacation get-aways as Sandals and Hedonism II. Like most denizens of Negril, our subjects are sun lovers; their preferred basking spots, however, are not beach towels but tree trunks. Beautiful, scaly, and found just about anywhere in Negril but on the beach itself, Jamaican anoles (lizards of the genus Anolis) are a living test case for our investigations into the workings of evolution.
Even a leisurely stroll around Negril reveals an abundance and diversity of lizards. At the bases of trees and wooden posts, two species perch head downward. One, the jamaican lined anole, prefers shadier spots, whereas the brown anole basks on fence posts out in the open. Four other species are found farther up in the trees. The two most common run and jump from branch to branch, using trunks and leaves as necessary. They have slightly different temperature preferences but mainly differ in size and color, the beautiful blue Graham’s anole being twice the weight of the smaller and paler opalescent anole. The king of the treetops, and the giant among Negril’s tree-dwelling anoles, is the fifteen-inch-long “green guana,” as the locals call it. This fearless, lime-colored lizard supplements its diet of insects with small vertebrates, including other anoles. The anole that is possibly the most numerous is also the most rarely seen. A cautious demeanor and camouflage markings make the Valencienne’s anole difficult to spot as it creeps along narrow branches and twigs in search of hidden prey. This short-legged lizard is not a sprinter or leaper; it eludes predators by avoiding detection in the first place.
Each of these six species is adapted to its own ecological niche, in particular to the surface on which it lives and moves. The stubby legs of Valencienne’s anole, for example, may not give the animal speed but are well suited for maintaining balance on narrow twigs. In contrast, anoles that spend their lives closer to the ground have extremely long hind limbs that provide great sprinting and jumping capabilities (as we determined in the field, using a portable lizard racetrack and long-jump pit). These lizards sit motionless for long periods, their athletic prowess held in check as they scan the ground surrounding their perch. When an unwary insect wanders within range, the lizards dart out to capture a meal.
Anoles have also adapted to life in the trees by evolving adhesive toe pads, like those of their cousins the geckos. These pads, which are covered by millions of microscopic, hairlike structures, allow lizards to cling to the smooth and irregular surfaces of leaves and narrow branches. Species that dwell high in the trees have a greater need to maintain their grip; they generally have more well-developed toe pads than do species that live closer to the ground.
This array of anoles constitutes a classic case of adaptive radiation, a common phenomenon on islands in which the first species to arrive, finding a realm of untapped ecological niches, gives rise to a diversity of descendant species, each adapted to use a different part of the environment. The most famous case of adaptive radiation is that of Darwin’s finches, in the Galapagos Islands, but there are many others, including Hawaiian honeycreepers and East African Rift lake cichlids.
The adaptive radiation of Caribbean anoles, however, is exceptional in two regards. First, Caribbean anoles have experienced not one but four adaptive radiations, by diversifying independently on each island of the Greater Antilles — Cuba, Jamaica, Puerto Rico, and Hispaniola (which encompasses the countries of Haiti and the Dominican Republic). Second, and more surprisingly, these independent radiations have produced remarkably similar sets of species on each island (see table at left). For example, the slender, twig-hugging Valencienne’s anole lives on Jamaica, but each of the other islands hosts a species extremely similar in build and coloration that uses the same habitats and behaves in much the same way. Each of the four islands also has at least one long-legged species that perches motionless near the ground using a sit-and-wait foraging strategy to ambush prey; a shorter-limbed arboreal species with large toe pads, which moves throughout the trees and has the ability to change colors dramatically (from green or blue to brown); and a large species that lives high in the crown of trees. Although such convergent evolution is a widespread phenomenon in both the animal and plant kingdoms, convergence of entire sets of adaptive radiations has rarely been documented — and never before in quadruplicate.
The outcomes of the independent radiations, however, have not been identical. Cuba and Hispaniola, but not Jamaica and Puerto Rico, host a tree-trunk specialist with a flattened body, while Cuba alone has a large anole that lives near streams, catches fish, and runs across water to escape predators, much like the Central American basilisk lizard. Similarly, in Hispaniola, a small anole inhabits the leaf litter of mountainous rain forests; the unusual structure of its vertebrae may be an adaptation for hopping. All four Greater Antillean islands have stream and rain forest habitats, so the existence of only a single stream specialist and a single leaf-litter specialist is a mystery. Further investigation may eventually reveal why certain ecological niches are adapted to repeatedly while others are not.