Feeding

With the exception of the fruit-eating Gray's monitor, all monitor lizards are totally carnivorous (Auffenberg 1988). With the exception of the Komodo dragon , monitor lizards only catch animals that are smaller than themselves (Auffenberg 1981). Although monitor lizards are capable of overpowering and swallowing prey of around a third of their own body weight, most feed largely on much smaller animals. Several big monitor lizards are known to consume large numbers of tiny invertebrates. particularly orthopterans and beetles, whilst other species of a similar size show a tendency to feed mainly on vertebrates. Similar differences in prey preference exist between dwarf monitors but in general the diets of all monitor lizards include a variety of animals of different sizes and they are often thought of as generalised feeders that will consume anything they are able to catch. This is an oversimplification however. In order to grow and reproduce monitor lizards must obtain more energy from their food than they use to catch it. Most food items will be lacking in some essential nutrients and so it may be necessary for the lizards to obtain a varied diet. In different habitats and at different times of the year the number and types of prey available to the lizards fluctuate and so feeding strategies must change accordingly. Lizards find food either by actively searching for it or by concealing themselves and ambushing prey as they pass. The evidence for this "sit and wait" behaviour is largely anecdotal because direct observations are very difficult to make (Pianka 1968, Horn & Schurer J978, Auffenberg 1981). Foraging behaviour is much better documented from observations of wild and captive animals and from interpretation of footprints and other marks left in soft ground, but data on the precise foraging movements of monitor lizards are difficult to obtain in the wild (Thompson 1992, Tsellarius & Cherlin 1991) and surprisingly few detailed observations from captivity have been published (Auffenberg 1983c, Traeholt 1993; Kaufman et al 1994). Many species systematically visit areas likely to contain prey (cow pats, water holes, trees. piles of leaf litter, tennite mounds, burrows. shorelines etc.), often travelling over 1km per day and moving directly from one foraging area to another. Whilst walking they continually taste the air and watch for any movement. If suitable animals are encountered in the open the lizards may rely on their superior strength and stannina to outrun and overpower them. When they detect an intere sting smell the rate of tongue flicking increases and the path taken becomes more convoluted and concentrated. When the source of the smell has been determined the claws and snout (and occasionally the tail) are used to uncover the prey, which are pounced upon as soon as they move. These observations srrongly suggest that monitor lizards find a lot of their food by sniffing it out, but rely on eyesight to locate the prey with enough precision to ensure it does not escape.

Whenever possible monitor lizards swallow their prey whole. Small animals are swallowed alive, larger prey are killed first to make the process easier. Prey is usually seized by the head or neck and either shaken violently or slammed against a hard surface before being swallowed head first. The shells of snails and crabs are crushed, pierced or removed, whilst animals with long legs may have the offending appendages ripped or levered off before being consumed. The tongue has lost the ability to manipulate food in the mouth (although it may be used by some species to lick up small insects (Mertens 1942a), and "inertial feeding" is used to get food into the stomach (Smith 1986). The monitor holds the food in its jaws and momentarily lets go, whilst at the same time jerking the head backwards and forwards and then grasping the food again, so that it ends up further inside the mouth. This is repeated until most of the prey is inside and then the head and neck are raised and the food falls further down the throat. It is forced into the stomach either by the lizard flexing its neck to the left and right or by muscular contractions from within. Animals that are too large to be manipulated with inertia are crammed into the mouth by employing a rock or the ground as a lever. With the aid of these methods food of a surprisingly large size or inconvenient shape (such as thorny lizards and large tortoises) can be swallowed intact.

Many monitor lizards will feed from the carcasses of dead animals, including human corpses. Some will eat flesh that is in a very advanced state of decay but most prefer fresh carrion. Monitors waste very little of their food compared to mammalian carnivores such as cats (Auffenberg 1981) and will even eat bones with no meat on them. Many are able to consume almost half of their own body weight in food and their very efficient digestive systems enable them to break down even the largest meals within a week if body temperatures are high. Gorged monitor lizards cannot run very fast and so it is important that the digestive pause be as short as possible. Only feathers, claws, fur, scales, fragments of bone and chitinous shells pass through the lizards undigested. This material is usually expelled in faeces but may sometimes be regurgitated as gastric pellets (Petzold 1967).

Much has been made of the Komodo dragons' habit of preying on buffalo weighing up to 15 times as much as themselves (Auffenberg 1981). Amongst other reptiles only the crocodiles kill larger animals, and they do it from the relative safety of water. The evidence for Komodo dragons catching massive prey is largely anecdotal, but some certainly attack animals five times heavier than themselves. However even if such attacks are successful they are unlikely to be able to consume much more than 10% of the prey themselves. Why, then, do they attack such dangerous animals? Mammalian predators that hunt bigger animals tend to do so either as a group or in order to feed a family. Monitor lizards have no such social spirit. The partly eaten prey attracts many other dragons to the site in search of a free meal and such congregations provide the opportunity to meet potential mates. It seems possible therefore that a few very large and experienced males risk catching huge prey in order to attract females, who, if they are wise, will consider the great strength and skill needed to kill such animals to be admirable characteristics. The successful hunter will have the opportunity to assert his dominance over the other males dragons, very few of who would be capable of such a feat themselves. If this were the case we would expect that only males would attack very large animals. Unfortunately there is no evidence for this.

>Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.