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The lizard kings: small monitors roam to the east of an unseen frontier; mammals roam to the west
Natural History, Nov, 2003 by Samuel S. Sweet, Eric R. Pianka
A small lizard, caught in the open, flushes ahead of a pursuing monitor. The prey, desperately seeking escape, begins to run a winding course. The tactic could throw a predator off, but the monitor doesn't bite. Rather than engage in a tail chase, the monitor heads straight for a pile of rocks--the only nearby feature to which the hunted animal could possibly escape. The smaller lizard, outsmarted, arrives at the refuge too late.
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Such a display of intelligence in monitor lizards, the animals of the family Varanidae, is not unusual. As a rule, monitors do not have to chase their prey very far, and in many cases they seem to anticipate some gambit by their prey. When arboreal lizards are being hunted and run for a tree, they usually spiral around to the back side to ascend; one of us (Sweet) has watched pursuing monitors of two species (Varanus tristis and V. glauerti), on at least three occasions, spiral around the tree in the opposite direction to catch the prey unawares. (Experienced human lizard-catchers do the same thing.)
The black-palmed rock monitor (V. glebopalma), a three-foot-long lizard from northern Australia, hunts by taking up perches on three- to six-foot-high boulders along the margins of ledges, where it has a good view of some area of more-or-less open ground. If it spots prey--such as, in Sweet's observations, a skink or a frog--it literally projects itself off the boulder, dashes after the prey, and then returns with its quarry at top speed to some rock crevice before doing anything like chomping or whacking the prey and gulping it down. "Lizards" don't do this: if they have something in their mouth, they eat it then and there--no matter that something else may be zooming in at top speed in hopes of a double lunch. But monitors do.
Predators and their prey are locked into a co-evolutionary arms race, in which any advantage gained by one calls for a countermeasure by the other. Less sophisticated, or perhaps just unlucky, prey individuals perish. On average, those with better means of escape survive. More effective escape, in turn, favors predators better able to capture evasive prey, and the bar for both species rises in a reciprocating fashion. Similarly, competing lineages of predators--cats and foxes, for example--are also subject to the Red Queen's dictum that "it takes all the running you can do, to keep in the same place."
A common result of such pressures--less adept animals either don't catch a meal or can't avoid being eaten--is the evolution of larger brains and more sophisticated nervous systems, as well as a potential for increased intelligence. A successful carnivore might have better neuromuscular coordination than its peers or its prey; more refined senses (and brain to process the information); or enhanced problem-solving capabilities. Those aspects of neurophysiology co-evolve in turn with ecological and behavioral differences among various kinds of carnivores. The range of possibilities for a predator's behavior--whether it hunts alone or in a pack; whether it lies in wait to ambush or actively chases down its prey; and the degree to which it relies on visual, auditory, or olfactory input to find its meal--all affect the nature and sophistication of the animal's brain.
None of the logic of this arms race leads to the conclusion that effective brains and neural sophistication are restricted to mammals; monitor lizards make that much clear. Superb predators, these animals surpass all other lizards in intelligence. They are alert and agile. Their styles of hunting rely on acute vision and extremely sensitive chemoreception to cover what are typically huge areas relative to their size. In these and other ways, convergent evolution has led to many similarities between monitors and mammals. Herpetologists have relied on terms such as "mammal-like" and "near-mammalian" so often to describe the monitors that such phrases have nearly become cliches.
The descriptions, however, divert attention from a question that is far more intriguing than mere similarities in habits between the two groups of vertebrates: Are the two groups so similar that they are ecologically incompatible as top carnivores? In other words, does the presence of one group in an ecosystem restrict the presence of the other? An analysis of the capabilities of monitor lizards and small mammalian carnivores, combined with the study of their biogeography, may throw some light on whether, in some ecosystems, the monitor lizards became a fair match for the mammals.
The similar adaptations of monitor lizards and mammalian carnivores are certainly not the products of a shared family history. The most recent common ancestor of the two groups lived more than 300 million years ago. It was a far less sophisticated animal, lacking the metabolic scope, visual and chemoreceptive abilities, and complex information processing that characterize both groups today. Most contemporary features of monitors and mammals that function in similar ways are clearly not the results of similar anatomical endowments.
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