The lizard will approach you, flicking its tongue in your general direction. If your nerve breaks and you make a run for it, you are very likely done for. If you remain motionless, the chances are the lizard will lose interest and you will live to chirrup another day.
How do we know this is a worthwhile survival wheeze? Because a couple of zoologists from the United States co-opted some grasshoppers to try it - and it worked.
The experiment was part of a wider series of investigations carried out by John Phillips and Allison Alberts from the Zoological Society of San Diego to find out if newly-hatched monitor lizards can tell the difference between snakes that they can eat and those that can eat them.
The matter is of interest to zoologists because although it is known that prey can often detect predators by chemical signals - a phenomenon called chemoreception - and that predators use chemoreception to detect prey, in the case of the monitor lizard snakes can be either prey or predators.
'Because snakes are generally formidable predators, any misclassification of a predator species as prey is potentially very costly,' say Phillips and Alberts.
Working in the Etosha National Park in Namibia, Phillips and Alberts subjected groups of newly-hatched monitors to a variety of snake snacks (all the snakes used were victims of road accidents). The menu comprised sand snake, spitting cobra or horned adder. In the wild, monitors eat sand snakes, but get eaten by the other two. The snakes were served up either whole or as small portions, with or without skin. Also presented were grasshoppers - common prey for monitors - which had coats of snakeskin. The grasshoppers were either restrained by forceps or allowed to run free.
What happened was this. The lizards flicked their tongues at the whole carcasses of the sand snakes four or five times before attacking and eating them. But with the spitting cobra and horned adder this tongue-flicking was followed by aggressive behaviour, and out of 54 encounters with these carcasses, the lizards attacked only nine times and not once attempted to eat the dead snake.
Similar patterns were seen when intact sections of snake were used. But when the skin was removed, the lizards ate all three types of snake indiscriminately.
What this goes to show, say Phillips and Alberts, is that young lizards can identify snakes by chemicals that come from their skin. Furthermore, this ability is innate rather than learnt.
The chemicals are probably collected from the air by the tongue - hence the tongue-flicking - and passed to a specialised sensor, the vomeronasal organ, where they are processed.
Finally, grasshoppers clad with skin from the venomous snakes were not eaten provided they were restrained. When they roamed free, they were invariably attacked and eaten.
This seems to confirm the central role of the snakes' skin, but suggests that movement of prey is a more important factor in the monitor lizard's feeding behaviour 'Our results for monitor lizards indicate that visual cues associated with prey movement can override chemical cues,' the scientists conclude.
For the lizards, their ability to discriminate between predators and prey at a distance has two advantages. First, it allows early avoiding action to be taken before physical contact with predators. But it also makes the monitor a more efficient hunter: it doesn't have to expend scarce energy resources by attacking something that turns out to be inedible - or, worse still, potentially lethal.
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