Michael Balsai on the Savannah Monitor Diet 

In the aftermath of an article published in a herp hobbyist magazine about savannah diets, there has been much discussion and radical changing of captive savannah diets, from primarily rodent based diets to primarily, or exclusively, insect based. Several individuals have written me telling me I should change my savannah care article to reflect this "new" information. I have some serious questions about this "new" information and as a result will not be changing my care article at this time. If you are thinking about, or have already changed, your savannah's diet as a result of the article and panicked posts on email lists and newsgroups, you may wish to read the following and perhaps rethink your decision.

The article in question claims that feeding captive monitors rodents causes intestinal impactions by fur, kidney damage, liver disease due to fatty rodents, and obesity. While I have not kept as many monitors as I have iguanas, I have had several savannah monitors through the years and have corresponded and talked with other savannah keepers. Those of us who feed strictly or primarily rodent diets to our savannahs (as soon as they are old enough to start on pinky mice, and working up from there as the monitors grow) have not had any problem with impactions, kidney or liver problems related to diet, nor obesity.

I did take in an obese savannah once. His prior owner got rid of him because "he cost too much to feed." No wonder! The savannah was being fed a couple of adult mice every other day! It was obese. It was sluggish. It was not happy when I put it on a diet by drastically reducing food intake and making him move around. Within six months, he was back to looking like a healthy savannah should look, was alert and active and remained so on a strictly rodent diet.

If you own any type of monitor and have done any reading on them, then you probably already know of Michael Balsai. A long-time monitor keeper himself, Mike has published many articles on monitor care, currently writes a column for The Vivarium, and has authored books on monitors. Mike and I touched base on this diet uproar and found that we were pretty much in agreement. The following comes from an recent exchange of correspondence in which Mike and I discussed savannah diet, both captive and in the wild, and issues relating to research and interpretation of research findings.


Kaplan: In light of the discussions as a result of the Reptiles article that stated savannahs should be taken off rodent diets and fed mostly insects, it has been suggested by several people that I change my care article to say that adults should be fed insects rather than rodents and that I should stress that only smaller insects be fed as large ones may pose a danger to the monitor.


    Balsai: I'd be more concerned that many readers will see insect prey and use only crickets and/or mealworms. And for adult savannahs, I doubt any insect that is readily available will be too large...rather I'd fear the opposite. I have seen many adult savannahs totally ignore most readily available insect prey that is normally offered to captives.

That has been my experience, too. Even moving insect prey (various worms, crickets, snails, etc.) have been ignored, usually from the time they started feeding on mice when very young. Speaking of worms, any thoughts on crushing kingworm heads before feeding them to savannahs?

    Probably a good idea for safety's sake, though the stomach of savannahs is presumed to be very highly acidic and hazardous to most, if not all, animal life forms.

In my experience, captive adult savannahs diets composed primarily of domestically bred mice do not cause obesity and fur impactions.

    This has never been a problem for me or many of my colleagues either. All of us encourage varied diets, but I see no problem at present with rodent biased diets when used responsibly.

One of the people who wrote me said that studies at the San Diego Zoo's CRES demonstrated that obstipation and constipation occurred when the monitors were fed mice, and that in the wild they tend to be scavengers and eat only the meat and entrails and rarely consume skin and hair.

    I disagree with this. They have not really demonstrated this at all. They have failed completely to isolate problems stemming from obesity, from those that arise from the diet itself. See my other comments for what we "really" know about wild savannah monitor diets (as opposed to white-throats, and even with this white-throated species, field studies are restricted to only a few isolated populations rather than from studies more evenly distributed in their range). For example, CRES has apparently only studied the white-throat population in Etosha National Park, which I believe is in Namibia. This is known to be a very arid region and it is known that white-throats range through several other habitat types, in addition to this region. What do white-throats feed on in areas that are not arid? I doubt any long-term studies have been conducted on these populations to be able to say anything meaningful about their diets in such areas. Naturally, such studies will be expensive and time consuming, but they are needed, before we can really have any true picture of wild monitor diets. In addition, we must have a thorough knowledge of any human changes inflicted upon the habitat that might actually indicate the lizards now have artificially induced food preferences, as Gaulke suggested for water monitors in the Philippines.

From what I have read and seen of wild savannah and white-throated monitors, there seems to be a tendency towards opportunistic feeding, that is, feeding on whatever is around: reptiles, mammals, etc.

    We actually know very little about what savannahs do in the wild, though they probably are opportunistic feeders! Most information we have traces back to a single study carried out on a single population in Senegal by Mamadou Cisse in 1972. Even the much cited study by Losos and Greene from 1988 also relies on information tracing back to this study and stomach content analysis done on museum specimens. In fact, the Losos and Green study reflects taxonomic ambiguity from that time, because since then, lizards called savannah monitors have now been recognized as at least two species; Varanus exanthematicus and V. albigularis. Some of the lizards that Losos and Greene called V. exanthematicus were V. albigularis. The animals from their study that were unambiguous seem to be savannahs also taken from Senegal and most likely the same population as Cisse's study.

    Some of my recent columns in The Vivarium have taken pains to point out that strident statements concerning savannah monitor diets made by certain other authors at various times are not necessarily supported with adequate data and are, at best, premature. My latest column, which I believe should appear in The Vivarium Volume 9, No. 5, addresses the problem of inadequate field data in more detail and points out some problems with David Good's recent and in my opinion controversial article in Reptiles [volume 6(2):48-65]. In addition, this author is a bit sloppy about some of his source material, because after taking great pains to try and distinguish between V. exanthematicus and V. albigularis, he cites field data for V. albigularis to support claims about what V. exanthematicus supposedly eats in the wild.

    I also point out the important point made by Gaulke (1991, Mettensiella 2: 143-153) that the apparent diets of wild water monitors in the Philippines may actually be a result of habitat destruction rather than actual preference. In a nutshell, much, much more field data is needed before any kind of claims can be made about diets for wild savannah monitors (or most other species for that matter) and anyone who claims otherwise, in my opinion, neither understands how scanty such data presently is or possibly even how the scientific method works.

    Feed savannah monitors responsibly with rodents and you should not normally encounter any difficulties. As I pointed out in one of my other columns (The Vivarium 8(5):18-19), I believe most if not all of these supposed "problems" with using rodents to feed savannah monitors, is that many keepers, especially neophytes, that do not apparently understand that reptiles do not require feeding at mammalian levels, are overfeeding their lizards and that the problems that result are due to obesity rather than use of rodents. I have not read a single thing, yet, to convince me that the problems are somehow due to the use of rodents because they, as food, are somehow physiologically harmful to these lizards. Rather, all these problems seem the result of seriously overfeeding the animal and that the detrimental symptoms are actually the result of obesity instead! Admittedly, rodents are a "richer" diet I suppose, but I advise people to use only lean adult mice to feed adult savannah monitors. I rarely use adult mice over about 1 to 1.5 years old. I also avoid feeding pregnant mice to these lizards.

    I also believe that advising people towards insect and/or invertebrate diets is to encourage, particularly among newbies, situations that could eventually lead to metabolic bone disorders in these lizards. In fact, after Good's article appeared, I saw numerous postings on the rec.pets.herp newsgroup, encouraging people to feed crickets to their savannahs. I am also aware of many individuals being advised toward such a diet by many pet store owners. Varied diets which utilize the judicious use of mice are a much better idea at the present time in my opinion. In fact, in that same issue that included Good's article, another article appeared which noted that a diet for V. albigularis, which included even as much as 50% rodents, should be OK if done judiciously. I will add that in the past I have used diets for savannah monitors that were very heavily biased towards the use of lean adult mice, but when done properly to ensure against obesity, the lizards were fine. No impactions from fur (I have never seen this and I have kept many, many savannahs over the years), no obesity, no kidney or fatty liver problems; none of these things. I also never had a savannah monitor suffer from metabolic bone disorders either.

As with many animals, food availability varies seasonally, especially in areas subject to regular flooding or droughts. It has been my impression, knowing what I do about the areas in which savannahs live as well as learning about them in captivity, that they are gorge feeders. That is, gorge when food is available to build up energy stores to get them through periods of decreased food availability.

    Many species of monitors do this, and this is why I mentioned that examining how diet varies seasonally for various monitor species could be important. In addition, some species have periods of inactivity (essentially hibernation of sorts), so they may gorge in anticipation of such activity (or inactivity, if you will). Savannah monitors and white-throats (V. albigularis) are two such species where at least some populations may show such behavior.

Obesity in savannahs seems to be caused by the same behavior that causes much of the obesity in other captive animals: overfeeding by the keeper. This seems to be a particular problem when the animal in question acts like it is still hungry even though it has taken in sufficient amounts for its age, health and physical status. This appears to be what happens with savannahs in captivity - their gorge behavior is interpreted as actual hunger, and so the animal is fed based on this behavior, rather than overall physical appearance.

    I think it is important to make more keepers aware of this "phenomenon" in many monitor species. Another species that shows this gorge behavior is V. gouldii horni (the argus monitors appearing in many pet shops of late). I am considering how to work a discussion of this gorge behavior into another column, at present.

Savannahs, from what I have observed and read, seem supremely lazy when not actively hunting or engaging in thermoregulatory behavior. They are similarly lazy (inactive) in captivity (unless given opportunity for exploration or movement, outside of an enclosure, for example). Combine overfeeding with lack of exercise, and you get obesity.

    It is true that they appear lazy as captives. I am uncertain at present how much of this reflects an artificial "behavior" induced by the abnormality of captivity. However, I have learned that many will move about more than one would credit them as doing, if they feel they are "safe" from observation and disturbance. Given adequate space, they could actually exercise more than one might suppose, so long as they feel safe from observation and other disturbances (loud noises, sudden motions towards them, etc.). I have also found that with tame specimens, if you place them in a safe area where they might walk about under subtle supervision. In fact, they will actually walk around and "explore" quite a bit.

    But, in general, you are correct, Melissa, when you note that overfeeding with the inability to exercise properly or at all, will quickly lead to obesity.

Because of the above, I have not raced to alter the information in my Savannah article. I would be interested in some in-depth articles on the studies, rather than those written for the herp magazines, especially their methodology, the diet fed to the rodents, etc.

    We probably have a long wait, Melissa. I am not one of those people who rushes to alter what I do every time somebody publishes something "new" or "radical" about something I happen to know something about. In fact, my dissertation area is vertebrate paleontology (I am studying the phylogeny and related subjects concerning varanid lizards, as a matter of fact) and I am notoriously conservative in my views. Usually I have found that most of these "radical" new ideas die off quickly and turn out to be wrong.

    I have shown above where some of my objections stem from. I agree that more scientific approaches are merited before any of us, who have been working with a particular species for many years, rush to change methodologies that work for us. What we mostly read in these magazines are little more than opinions, not much else. Why should I change what has worked quite well for me all these years just because somebody else says so? None have presented me with serious and well thought out experiments to support their opinions and methods as better than mine.

    In fact, I recently cited a definite study to demonstrate that the use of rodents does not necessarily lead to kidney disease in savannah monitors. This was a study carried out some years ago to examine uric acid clearance rates in savannah monitors (J. M. Maixner, E. C. Ramsey, and L. H. Arp 1987. Journal of Zoo Animal Medicine 18: 62-65). No one has yet tried to show the irrelevance of that study to my arguments. More real scientific studies are merited to support the arguments of this "new" camp, but they never really cite any true scientific studies...thus what they say is, in my opinion, nothing more than their respective opinions.

I am well aware that, in the area of herp medicine and our understanding of their environmental and nutritional requirements, there has been and will continue to be changes as more research is done and they are studied more under a variety of conditions. Because studies can conflict with one another for a number of reasons, I feel it is sometimes better to sit back and wait a bit before rushing in to change a regimen that has been working.

    I, one hundred percent endorse this view, and it is how I view all these articles that appear in these various herp magazines. They lack any real scientific rigor.

Related articles:

Savannah Monitor Care

Hepatic Lipidosis (Fatty Liver Disease): Prevention is the best cure

Look for Mike's new book, The Care and Maintenance of Common Species of Monitors and Tegus, published by Advanced Vivarium Systems, available through various herp booksellers and amazon.com.

Balsai, Michael, and Melissa Kaplan. 1998. Michael Balsai on the Savannah Monitor Diet.

Created: June 1998
Last Updated: 07/27/00