Bit of a mix of views below on what monitors do with their tongues and how this can help them make their livings. Three parts here, then: first, what's going on when monitors use odors; second, a quick comparison of vision, olfaction and hearing as information media; third, what monitors seem to be learning about their environment by using their tongues.
Part i. Most lizards use their tongues both as sensory organs and to manipulate prey while chewing, but advanced Scleroglossan lizards (teiids, lacertids, anguids, and varanoids) increasingly specialize the tongue for a sensory role. In varanids and snakes the tongues are wholly sensory, and what used to be the rear, manipulative part of the tongue of other lizards has become the sheath into which the foretongue retracts. Properly speaking, the tongues of monitors and snakes should be called foretongues, but we'll ignore that. The tongues of monitors have a stiff, smooth surface enclosing an elongate muscle. There are no 'taste' or 'smell' receptors, and only a small number of 'touch' receptors on the tongue itself – it's just a chemical-collecting wand.
When a monitor retracts its tongue, materials adhering to its surface are wiped against the roof of the mouth at the openings to a pair of blind sacs, the vomeronasal or Jacobsen's organs. These are lined with ciliated cells (that move molecules around on the wet inner surface of the sac), and with various kinds of chemical receptors. The vomeronasal organs (vmo for short) are not connected to the nasal cavities, and have their own branches of the olfactory nerve. Monitors are also able to 'smell' (as we do) using a range of specific olfactory receptors in their nasal linings. Monitors smell while they breathe. There isn't a term for what they do with their tongues, which is neither smelling nor tasting, but something somewhere in between. For the heck of it, let's just call tongue/vmo use "sensing", to distinguish it from "smelling".
Monitors smell airborne odors, but "sense" heavier odorant molecules that are usually resting on some surface. Smells can travel a long way, while "odorants" usually are too large to be airborne; likewise, smells come and go quickly, while odorants can lie around for a long time before they are washed away, oxidized, or broken down by UV light, bacteria and so forth. Many odorants undergo chemical changes as they age, and this carries information too.
Part ii. There are lots of ways to learn about the outside world, but only vision, hearing and olfaction are "distance" senses. Vision is the most informative: most things reflect light (in characteristic ways), and light travels in straight lines – there is a huge amount of information to process, but it is all laid out in 3-D as a geometrically accurate and complete "picture" of the surroundings. Hearing sucks by comparison (as long as you are using only existing sounds, rather than creating sounds and processing their echoes), because most things don't make any sound at all, and you need to compare arrival times and intensities between both ears to localize the source, and to already know what it is to determine how far away it is. To tell if a sound source is moving, you need to remember both where it was and when you last heard it. At least sound tends to travel in straight lines, but it bounces around some too.
Compared to these two, olfaction REALLY sucks as a way of getting realtime information about 99% of the world around you. Odors are only vaguely concentrated toward their source, get blown all over, etc., etc. About all they tell you is that something stinks (or maybe used to stink) sort-of over that way. It's a little better with the heavier odorants, because they tend to stay put. You can follow what left them.
Part iii. So what's a monitor doing with its tongue? It's not smelling (that's in its nose). Monitors do not have huge numbers of receptor cells in their noses (nothing like a dog, for example), and the betting is that these are tuned to a fairly small number of especially relevant odors (such as those of cycling females, carrion, maybe some slow-moving predators such as snakes, things like that). They could be super-sensitive on those channels, but there's no way they'd beat a decent dog. Contrary to some of the suggestions in threads below, there is no evidence at all that monitors are primarily 'smelling' animals – they simply haven't got enough of the receptors to make that possible.
Things get closer to the mark with the vmo "sensing" capabilities, although the numbers and kinds of things you can find that way depend on what leaves such scent trails. It is certainly not a way to get around in life in the absence of other sensory information, especially vision, but vmo sensing tells you about things that used to be there, and maybe where those things went. Many of these odorants are waxy or oily and can last a really long time. It's not at all out of the question that a monitor walking across an area learns a great deal about the recent history of that place – that some female (likely known individually) went that-a-way yesterday but isn't cycling, that a python went back and forth around here several times in the last week or so (watch out, and don't hole up here), that there are more skinks running around here than over there, or that this is my turf because it stinks like me everywhere. Vmo sensing is an important adjunct to vision, but not a substitute for it.
One last thing, to tie this into monitors in captivity and give my pal Frank and his ignoranti something to rave about. In the wild, odors and vmo compounds initiate behaviors – in other words, there are new ones all the time, and different information crops up in different places. Some of these vmo compounds surely act as pheromones – chemicals that are physiologically active, or kick off hormonal responses, like increases in circulating testosterone or corticosterones. In captivity, all this stuff builds up, to levels that are hundreds or thousands of times greater than would ever be encountered in nature. Either of two things happens then – the animals get 'saturated' and no longer respond to these cueing stimuli at all, or they respond-respond-respond and get their hormonal balances all out of whack. This aspect of monitor-keeping is ignored by just about everyone, but ask yourself, what are the alternatives? If monitors are so cued-in to chemical signals, what happens when they lose the ability to use them?




