Monitors & Mice, The Myth Dispelled

The sad history of not feeding rodents to Savannah Monitors

Posted by Daniel Bennett

Savannah monitors aren't opportunistic feeders in the wild. They are after very specific types of prey and none of them are rodents. I think that's because they couldn't compete with all the other animals that are feeding on rodents; if they come across small enough rodents of course they will eat them but the search patterns they use turn up a rather narrow range of invertebrates, snails, and frogs with the odd reptile egg and that's what is in their guts almost all of the time. Of course the range of the species is very wide and our sample sizes are very small, but so far its feeding ecology looks rather specialised compared to opportunistic monitor species.

I think that you can recognise, or define, opportunistic monitors in the wild, by seeing if they are attracted to rotten carrion. If you try to bait monitor lizards with carrion in Africa you will get niloticus but you will not get exanthematicus. If you did it in Malaysia you would get biawaks and bengalensis but you would not get rudicollis and you probably wouldn't get dumerilii either. If you did in the Philippines you would only get biawaks. It's a useful definition because it's a very easy test. I'd guess that if you did it in Indonesia you wouldn't get prasinus type animals and you have some surprise non shows amongst the others.

What do we know about the natural diets of these monitors that can't be attracted with bait? Of course their range is wide and our sample sizes are very small but they seem to have very tight dietary niches compared to their opportunistic counterparts. There's a conspicuous lack of what I would call snake food.

Incidentally; these non carrion eating monitor lizards tend to perform poorly in captivity, especially the ones that live in wet forests. In contrast the carrion eaters are generally considered quite easy to breed.

How such idle speculation can be applied to the feeding of monitor lizards in boxes is uncertain, because all monitor lizards that can be bred in boxes will apparently thrive for indefinite generations on a diet of rodents and invertebrates. There's (almost?) no documented evidence of generations of captive monitors raised without rodents, but for many years now people have been banging on about how vital it is to omit rodents from the diets of savannah monitor lizards in particular. Why could that be?

David H. Good 1998, REPTILE magazine. Misunderstanding the Savannah Monitor: An Argument for Changed Husbandry.

This junk article was based on a talk the author had heard, but apparently entirely failed to comprehend, about wild savannah monitors. The talk was at the only monitor lizard conference that has ever been held in the USA, in San Diego. David Good spectacularly got every single detail in his article wrong, and the only thing he remembered correctly was that no rodents had been found in the diet of savannah monitors. He used this to suggest a rethink of savannah monitor husbandry that should have landed him in prison. Ever since it's been regularly rehashed by generations of monitor keepers who have spectacularly failed to breed their lizards. "They need a natural diet in captivity", what utter nonsense.

What David Good should have done was listen to another talk on the same day by a man nobody in the monitor lizard world had ever heard of. I can honestly say that nobody I knew in that room believed a word of it at the time, but it sparked a complete revolution in the way monitor lizards were kept. More importantly, it introduced a completely different philosophy to the husbandry of captives; that captive conditions should improve on natural conditions rather than replicate them. In nature monitor lizards are constrained by limited heat, water, food and shelters. By removing these constraints he claimed monitor lizards would grow at phenomenal rates and become fecund beyond all expectations. When members of the audience objected to basking temperatures so high they could easily kill the lizards, he claimed that the monitors sought such heat out of their own free will.

"so why would you not just try your best to give it as close to the natural living conditions as posible with out argument? sounds like kinda a crazy argument ?" Not really, because natural conditions are harsh and it's just possible that savannah monitors actually prefer rodents to milipedes.

Additional notes by Professor Sam Sweet

Varanid lizards (the whole family) differ from all other lizards in having structural and physiological features that permit them to use aerobic metabolic pathways while active -- in other words, monitors are more similar to mammals than to other lizards in that they do not tire out quickly even during long periods of high locomotor activity. This is certainly a specialized trait. Monitors are also specialized compared to other lizards in having acute long-distance (color) vision, sensitive hearing, and very well-developed senses of smell and tongue-based vomeronasal organ "taste". Monitors additionally seem to be able to integrate all these senses well, and are pretty smart, for want of a better term.

These are all features that make monitors effective as widely-foraging predators, and they are very good at this. However, it is a fairly good general rule that predators cannot afford to be particularly choosy about what they eat, or overly specialized in the ways they go about searching for prey. (As an aside, many snakes violate this generality by being highly specialized for finding and consuming only a few kinds of prey.) While monitors may capitalize on seasonally abundant prey (for example, turtle eggs), all of the published studies of monitor diets show that individuals eat a wide range of prey types (basically as items are encountered). In other words, monitors that encounter a lot of freshwater crabs eat a lot of freshwater crabs, and likewise for other lizards, large insects, and so on. The Philippine monitors V. olivaceus and V. mabitang are unusual in feeding in part on fruits (they also eat snails), but there is actually very little in their structure that signals this dietary difference.

This last point is the key. While you cannot make a strong argument that any monitor species is a dietary specialist, you can certainly show that many species are habitat specialists (and since this restricts the sorts of prey they encounter, you do therefore tend to see crabs inside of mangrove monitors, bearded dragons inside of Gould's monitors, and canopy insects inside of green tree monitors). It's the cart that follows the horse.

Ecological differences between species of monitors lie mostly in body size and habitat use, with differences in diet being the consequence of those primary distinctions. Even so, there is not that much difference in body form evident among tree- or rock-dwelling species, semiaquatic species, or those that make their living on dry ground. Limb and tail proportions and general stoutness vary, but this is not very dramatic across the group. This is one reason that systematists continue to regard all monitors as members of a single genus, Varanus.

Sam Sweet
Professor, UC Santa Barbara

Other Thoughts

Gregg M - Squamata Concepts LLC

When they do come into contact with rodents, I am certain a wild savannah would never pass up the chance to eat it.