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Enforcing the Generation Gap

Amos Esty

With its prehensile tail and strong, opposing toes, the common chameleon is a natural climber. But you're not likely to see younger member of the species Chamaeleo chamaeleon among the trees and bushes of their native Mediterranean habitat, and the reason has nothing to do with that famous talent for camouflage.

The tail of a juvenile chameleon...Click to Enlarge Image

Why do young chameleons prefer to stay close to the ground, where they face more predators, can have difficulty finding shade and risk looking terribly awkward scurrying through the grass, when they seem so clearly adapted for arboreal life? Apparently, the powerful force behind this behavior is generational conflict. It seems that adult chameleons are quite territorial—and even willing to resort to killing and eating younger members of their own species to keep the trees to themselves.

In a recent study published in Behavioral Ecology and Sociobiology, biologists Tammy Keren-Rotem and Eli Geffen of Tel Aviv University and Amos Bouskila of Ben-Gurion University of the Negev argue that cannibalism in the common chameleon has resulted in an ontogenetic habitat shift. That is, as individuals develop, their choice of habitat changes. In this case, juvenile chameleons tend to stay in low grasses, whereas adults make better use of their anatomical gifts by living primarily in trees. About four months after emerging from their eggs, when they have grown too large to be eaten by adults, chameleons will finally begin to venture farther off the ground.

The common chameleon is not the only species to undergo this type of change. Frogs stick to the water as tadpoles, of course, but can take to land once they develop limbs and lungs. Some fish change territory as well, often due to corresponding changes in diet. Other causes of ontogenetic habitat shift can include the selection of perch sites (some juvenile lizards, for example, forage on small branches that can't support the weight of adults) and changes in the amount of heat or sunlight required.

Keren-Rotem, Bouskila and Geffen narrowed the list of possible explanations for habitat shift in the common chameleon by conducting experiments with individuals found in a nature preserve near Mt. Carmel, along the Israeli Mediterranean. First, they observed the habitat preference of 15 adults and 15 juveniles in the absence of other chameleons, confirming that adults spend most of their days and nights on the middle and upper branches of trees, while juveniles usually remain in grasses or on lower tree branches. They then examined what happened when adults and juveniles were placed near each other. Young chameleons showed little change in behavior when around another juvenile, but most either froze or moved quickly away on spotting an adult.

The third experiment tested whether an attack was likely when there was close contact between the generations. Chameleons capture prey with a projectile-tongue technique unique among lizards, rapidly extending their tongues as much as two body lengths to snare victims, then pulling the prey into their mouths and swallowing it. The biologists placed a one-way mirror between an adult and a juvenile, so that the adult could see the juvenile but not vice versa. The projection of an adult's tongue toward a juvenile counted as attempted cannibalism. About a third of the adults tried to attack the unsuspecting juveniles within 30 minutes of observation. The authors also witnessed one incident of cannibalism in the field when a young chameleon made the mistake of getting a bit too close to an adult.

Although the authors do not draw any definite conclusions about why cannibalism would have developed in the common chameleon, it may just make evolutionary sense. "Treating any juvenile as a food item," says Geffen, "may increase the proportion of the cannibal's gene copies in the population"—assuming, of course, that the unlucky chameleon is not the offspring of the perpetrator. There have not yet been any tests of whether relatedness affects the odds of cannibalism, but according to Geffen it's unlikely that chameleons can recognize their offspring. Another possible explanation is resource competition: Killing a young chameleon leaves that much more food for the adult. As an added bonus, notes Geffen, young chameleons make an excellent source of protein.

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