Enforcing the Generation Gap
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.
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.