Melissa Kaplan's
Herp Care Collection
Last updated January 1, 2014

Animal protein and green iguanas

©2001 Melissa Kaplan

From my Iguana Care, Feeding and Socialization article:

For years, it was thought that iguanas were omnivores, consuming both animal and plant matter. While there have been some observations of some individuals eating insects and carrion, most have occurred in places where iguanas have colonized nonnative habitats, or in highly disturbed native habitats. Long term research into many different populations has shown that this is not a species-wide behavior in iguanas in their native habitats.

A single misread sentence leads to decades of misinformation and confusion...
Authors, be they biologists, veterinarians, or pet care book writers, all refer to earlier sources for their statements regarding iguanas-as-omnivores. Trace these sources back far enough, and you come to "Swanson, P.L., 1950. The iguana Iguana iguana iguana (L.). Herpetologica 6(7): 187-193."

The only sentence in Swanson that refers to juveniles is: "Very young iguanas are probably almost entirely insectivorous, but I have made no personal observations on their feeding habits." [emphasis mine]

It took only one person to misread or misunderstand this sentence to condemn 50 years of pet iguanas to deadly diets in captivity.

See also Dr. Adam Britton's excellent article, Animal Protein Issue, for further information and analysis.

According to Thomas Boyer (JSEAM 1(1)), Gordon Rodda (Herpetological Review, 25(2):85), and John Iverson (Adaptations to Herbivory in Iguanine Lizards, in Iguanas of the World: Their Behavior, Ecology and Conservation, GW Burghardt and AS Rand, eds., 1992, Noyes Publishing, Park Ridge NJ), iguanas are folivores (leaf eaters) in the wild. Iverson found, in his study of the intestinal contents of iguanas of all ages that, despite previous reports of their insectivorous and carnivorous natures as juveniles, they are in fact herbivorous from their very first meal. Though they may occasionally ingest bits of carrion or an insect perched on a leaf, animal protein consumption is accidental--it is not a conscious dietary choice. The iguana digestive system, like those of the other strictly herbivorous lizards, is structured to process a high-fiber plant diet, and to extract much (but not all) of their water needs from the foods they eat.

During the 1980s, as research into more efficient farming of iguanas for human food consumption progressed, it was found that feeding them animal protein forced a more rapid growth. This practice grows the hatchlings into a size big enough to dissuade most predators of young iguanas, and so enables farmers to let them loose in surrounding trees where they live off of the usual assortment of foliage. To entice the iguanas to stay in the vicinity, the farmers provide some attractive foods in elevated feeding stations. When the iguanas are big enough, they are harvested as needed for food. (For more information on the use of farming to conserve both animal and plant resources, please visit the Costa Rica: Iguana Park website and read Sorrel Downer's Survival of the Tastiest). The fact that this animal protein-enriched diet kills iguanas by the time they are 5-7 years of age matters little as they are slaughtered and consumed long before then.

Iguanas have been kept as pets for decades. The recommended diets during all that time were largely comprised of marginally nutritious greens and other plants, and animal protein: insects, mice, cat food, dog food, chicken, beef, pet and zoo carnivore foods, eggs, and more. In the early 1990's, iguanas exploded in popularity in the pet trade in the United States, followed shortly thereafter in Canada and northern Europe. The iguana pet care books written twenty years or more before were still being sold, sometimes just republished with newer cover photos, and new books were published that essentially rehashed the same misinformation in the earlier books. Up to this time, the mainstream veterinary literature for the most part picked up on whatever the available popular pet iguana books listed for diets. Vets hadn't really seen many iguanas up till then and so really were not able to make much of an assessment as to what worked and what didn't.

As time went on, however, and more iguanas were being brought to veterinarians, some of the vets began to put together cause and effect. Other vets and herp researchers began researching the health problems and investigating the processes leading up to them. As a result, we saw a radical change in veterinary recommendations. Many vets who used to recommend the feeding of animal protein stopped doing so. Some examples of former recommendations include: Jeffrey Jenkins DVM, client information sheet, 50% for hatchlings; Nancy Anderson DVM, The Compendium 13(8), 33%; Stephen Barten DVM, Veterinary Clinics of North America Small Animal Practice/Exotic Pet Medicine 23(6), 15%; Fredric Frye, 1993, who recommended plant and animal-based proteins. Others, such as Thomas Boyer DVM, recommended a higher volume (twenty percent) of certain animal-based foods such as trout and cichlid chows, vertebrates and bird chows, but a lower volume (five percent) if fed monkey biscuits or dog chow (Journal of Small Exotic Animal Medicine 1(1). By 1993-1994, these and other vets had started recommending that no animal protein at all be fed to green iguanas.

The reason why no animal protein came to be recommended is that even very small amounts of animal protein seemed to cause serious problems. Another concluding factor was that iguanas fed a properly constructed plant diet did just fine, thank you, without any animal protein in their diet. As iguana nutritional research continues in the pet and biological sectors (as opposed to in the food production sector), it continues to support a diet free of animal protein. So, while a bite of animal protein certainly won't kill your reptile, since we do not know of any amount that is safe, it makes sense to just not feed any at all.

Why is animal-based protein such a problem?
While all foods are made up of the same 22 amino acids, how those aminos are deconstructed and utilized by the body differs depending upon whether the source of the aminos was a plant or an animal. Plants form pyramidines, animal protein forms purines. The more purines in the diet, the higher the strain on the kidneys, and the faster they will fail. So, instead of supporting the iguana for 15-20 years or so, the kidneys fail much sooner, leading to a rapid, distressful and distressing, death.

A couple of other factors may be at play in this problem. Research in the lab and in the field has already documented how efficient the green iguana is in digesting its food and extracting nutrients. Their gut evolved to break down tough plant material. Plants have both a cell membrane and a tough cell wall; animal cells have only cell membranes. Iguanas, under optimum conditions, can extract 40% of the nutrients from the plant food they eat, making them one of the most efficient herbivores, topping even rabbits in this regard. Given this strong, specially adapted gut, they may be breaking down more animal matter into bioavailable components than they can do with plant matter, thus making more animal protein (and fats) available than they would eating the same quantity of plant material, as well as netting a higher level of purines. (For interesting discussions on colon and digestion, I refer you to the Iguanas of the World book referenced above, both to the Iverson chapter on herbivory and to the McBee and McBee chapter on digestion.)

Another factor of growing concern is that, in captivity, it is virtually impossible for the person keeping iguanas at home, in their office, or at school, to provide the high humidity in their iguanas' environments. Most green iguanas come from areas where the air is saturated with moisture.

Despite all the research and articles written by knowledgeable vets that have been published in the reptile periodicals and veterinary journals, there are still books being published today which recommend foods such as monkey biscuits, primate, trout, carnivore and omnivore chows, chicken, beef, other animal flesh, dairy products, eggs, worms, mice, crickets, cat food, and dog food. Some of these books also recommend feeding bird gravel ("for digestion") which is not only completely unnecessary, but dangerous as the gravel will cause internal injuries and serious, if not fatal, impactions. When looking for iguana care books, you will need to not only look at the publication dates, but also at the diet information it contains. Often times, if it is a newer book but it recommends animal protein, it may be inaccurate in other recommendations or facts as well.

The Bottom Line
Not only is there no amount of animal known to be safe, but the research of wild iguanas in their native habitat - both observation of feeding as well as laboratory analysis of gut contents - shows that they do not eat animal protein (insects, vertebrates, live or carrion) at any stage in their development.

Will a bite kill them? Of course not. Half a bottle of baby aspirin probably won't kill a child either. Does that mean its okay to feed it to them? Nope. Why give an animal who is already subject to serious long term health issues just by virtue of being kept in captivity something that is is going to ultimately cause damage to its internal organs and so hasten its death?