The lizard kings: small monitors roam to the east of an unseen frontier; mammals roam to the west

One substantial difference is that monitors are ectotherms--loosely referred to as being cold-blooded. The more familiar term is something of a misnomer, because the "cold-blooded" monitors, at least, typically operate at tightly regulated body temperatures equal to or higher than those of mammals. Monitors, however, do without the costly molecular and physiological control mechanisms required by endotherms, the so-called warm-blooded animals. Both monitors and mammals can sustain their activities for long periods.

Monitors do not sense chemicals with the nasal olfactory chamber that is so well developed in mammals. Instead, they transfer compounds from their tongues into two elaborate sensory receptors known as the vomeronasal organs. Vestigial in mammals, these organs occupy paired cavities that open onto the roof of the monitor's mouth.

Many accounts of monitors in captivity cite behaviors unusual among reptiles that attest to sophisticated information-processing capabilities. White-throated monitors (V. albigularis) can count up to six. Komodo dragons (V. komodoensis) recognize their keepers. When chasing rats, crocodile monitors (V. salvadorii) anticipate evasive tactics. Few field studies, however, have explored the monitor intellect, and the wariness of monitors in the wild is legendary. But the work that has been done demonstrates that the animals can locate terrain features, mates, and food both by memory and with their remarkably sensitive chemical detectors.

Monitors are renowned trackers. Alexey Y. Tsellarius of the Severtsov Institute of Ecology and Evolution in Moscow and his colleagues found that Caspian monitors (V. griseus caspius) can distinguish male from female and resident from non-resident monitors merely by sampling their tracks with the vomeronasal organ. If the monitor then gives chase, it unhesitatingly follows the track of the other animal in the correct direction. Our observations in Australia corroborate Tsellarius's finding for both desert and woodland species. One of us (Pianka) once came upon the track of a large monitor known as a perentie (V. giganteus) that had intercepted his own. The track showed that the lizard "ricocheted" off the human footprints and fled in the direction it came from, illustrating its chemosensory talents.

Monitors that feed on the eggs of other reptiles can locate a clutch buried in sloping, backfilled tunnels. They do not gain access via the tunnel entrance, which is often three feet or more away from the eggs; instead, they dig straight down from above. Walter Auffenberg, a herpetologist formerly at the Florida Museum of Natural History in Gainesville, demonstrated that Komodo monitors can detect carrion from nearly seven miles away. Auffenberg also concluded that some monitors climb to ridgelines expressly to sniff the wind for carrion odors over a large area, a foraging strategy that requires substantial planning.

Monitors can apparently recall the positions of refuges within their home ranges. Pianka has observed that such Australian desert species as the perentie and the rusty desert monitor (V. eremius) remember exactly where good burrows are located: the lizards head directly toward them cross-country, which for perenties may be a mile or more. Lace monitors (V. varius) display a similar talent, though put to different use: They lay their eggs in active termite mounds, then return about nine months later to reopen the nests for the hatchlings to exit. Such a feat calls for map knowledge as well as an accurate sense of timing.