A very brief overview of digestion in green iguanas and other herbivorous lizard species
©1995 Melissa Kaplan
Herbivorous lizards, such as green iguanas, utilize a combination of high heat, fermentation and gut microbes to effect a breakdown and utilization of their food.
Green iguana dentition is consistent with the tooth design of other completely herbivorous species, with their teeth used for cropping large pieces of plant matter for ingestion.
The structure of their gut is similar to that of herbivorous mammals including a greatly enlarged, elaborate colon. Food passes through the small intestine into the large, pouch-like anterior colon. The colon of herbivorous lizards contains many folds and partial partitions which may act to slow the passage of food through the colon, giving increased time to fermentation and time to work on the ingesta by the microbial inhabitants of the colon (protozoans, bacteria and nematodes) which are instrumental in breaking ingesta down into utilizable materials.
Volatile fatty acids in very high levels are found in the colon of these lizards, consistent with the high levels found in herbivorous mammal rumens. The comparatively low levels found in the small intestine and stomach indicate that very little breakdown of structural carbohydrates occur in these areas. The low levels of VFA on the other side of the anterior colon, in rapidly decreasing amounts as ingesta passes through the large intestine, indicate that VFA are being absorbed through the anterior large intestine.
Herbivorous lizards tend to be larger than their omnivorous and carnivorous counterparts as mass-energy requirements decrease in proportion to increasing body size. With plant diets being less energy-rich than omnivorous or carnivorous diets, larger body size is more efficient, especially when there are significant seasonal variations in the amount and quality of food available.
Large body size may also be a factor of the gut symbiont process as these organisms need high heat to function properly. Thus, while it takes longer for a large ectotherm to heat up to optimum range, they are able to maintain the optimum temperatures longer after the external source of heat is no longer present. It is likely no coincidence that herbivorous lizards may be found across a rather narrow band around the world. All but a very few are tropical. Chuckawallas and desert iguanas are two of the exceptions, but, living as they do in the warmest parts of the North American southwest deserts, they are able to meet and maintain the necessary core body temperatures.
Rate of digestion, as it relates to both the time of passage through the gut and to the efficiency of digestion, is a factor of body temperature, quantity and quality of food. Efficiency relates to the breakdown and extraction of nutrients necessary not only to maintain basic body functions but also to enable growth and reproduction.
Zimmerman and Tracy found, in their look at herbivory and ectothermy:
Higher temperatures + larger quantities of food high in fiber = faster passage but with less efficiency (typical of feeding during lush seasons)
Higher temperatures + smaller quantities of high fiber = slower passage but greater efficiency (typical of feeding during dry seasons)
High temperatures + large or small quantities of low fiber = rapid inefficient passage
Low temperatures + smaller quantities of high fiber = slower passage with reduced efficiency (during cooler periods, ectotherms will conserve energy by seeking cooler places, thus reducing the drain on energy reserves)
Low temperatures + small quantities of low fiber = slower inefficient passage
"Food is an important resource in the environment for organisms, and factors related to transforming food into nutrients are likewise important. Given the potentially accentuated influence of body temperature on rates of digestion of plant material in herbivorous ectotherms, we propose that daily, seasonal, and geographic patterns of distribution and abundance of herbivorous ectotherms may coincide with daily, seasonal and geographic patterns of availability of temperatures in the environment needed to digest vegetation at rates adequate for maintenance, growth and reproduction." (Zimmerman and Tracy, p. 405)
More recent research by Baer et al., looks at dietary fiber, intake quantity, metabolization, and growth in green iguanas. They found that while the daily dry matter intake was about the same, the digestibility and metabolization of energy decreased as the level of fiber increased. Greater growth was seen in the iguanas fed the diets containing low and medium fiber (19% and 24% neutral detergent fiber, respectively) versus high fiber (27% NDF).
On the face of it, the two studies appear to be contradictory. However, it points to the fact that fiber helps slow the overall rate of passage of food through the gut, giving the gut flora more time to work on breaking down the ingesta. The end result that when more ingesta is broken down, more is available for uptake. There comes a point, however, when enough becomes too much. This is why the benefits of the lower fiber diet outweigh (literally, in this case) the medium fiber diet, and both are better, in terms of net metabolizable energy, than the highest fiber diet. Finally, during the lush times of year, not only is more food available, but the food is of high quality and more of it is eaten, so there is an overall net gain for herbivores.
There are circumstances in which faster early growth is more desirable than slow growth. We see in wild iguanas that hatchlings grow quiet rapidly, growing from 2.5-3 in. SVL at hatching, to 8 in. by the end of their first year. Oftedal, Werner, and others who were working on captive breeding of green iguanas to promote them both as a food source and to conserve local resources, found that by adding animal protein to the herbivorous iguana diet, hatchlings grew much faster, resulting in a reduced risk of being preyed upon. The more recent Baer, et al. study also uses a manufactured diet, but there is no indication of which diet this was or what it contained, so its usefulness, in terms of comparing it to a leafy greens only vs. greens/vegetables/alfalfa/fruit diet is limited.
As discussed above, larger ectotherms take longer to warm up, but stay warmer longer, than do small ectotherms. This may impart advantages, such as digestion going on longer every day even after the cooler temperatures of night set in. Larger animals also have larger stores of energy to draw upon in case of stress or illness, and so should be able to withstand such periods longer before showing signs of illness than smaller animals. For species in the pet trade, more rapid growth in youngsters may help them withstand the generally deleterious conditions of the importer and pet stores (and expo vendors). More rapid growth over the short term may also help the animal cope with the stresses caused by new pet keepers who often know nothing or obtained incorrect species care information when they first bring their new pet home. Pet species are often subjected to substandard care for several months or longer before their keepers finally find out what their pet's requirements really are and make the necessary adjustments to their pet's diet and captive environment.
Diets vs. Wild Diets
When we compare the availability and nutrient cycles of plants in the wild to the produce available to us in our markets, we find a very different situation. While our markets may show some seasonable variation in what is available, the increasing globalization of trade in fresh produce results in most produce being found year round, at least in major urban centers. The produce trade, geared towards human diets and preferences rather than those of herbivorous reptiles, rarely includes plants typically found in wild herbivore diets. The variety of produce in any one store is further dependent upon the amount of physical space the store chooses to devote to their produce department.
There are certain plants stocked in most produce departments that are generally considered to be staples. Examples of these plants include head and romaine lettuces, cucumbers, tomatoes, etc. As people continue to move around and consumers try to meet dietary guidelines that place an emphasis on eating a variety of vegetables and greens, the types of food plants found in many markets in most urban areas have expanded accordingly. Nonetheless, the plants in our markets do not reflect the variety or nutrients-per-plant that our herbivorous and omnivorous reptiles would eat in the wild.
Some people insist that by including anything other than dark leafy greens in the diet is dangerous to herbivorous lizards. They claim that the regular feeding of vegetables, even when they are a part of a total diet that includes leafy greens, is a problem as mixed greens/vegetable diets are higher certain nutrients, including plant protein, than a diet that is 100% (or nearly so) leafy greens. Feeding a "rich" greens/vegetable diet will lead, they say, to an early death, just as does feeding animal proteins.
When reading about different types of fresh food diets, it must be kept in mind that there have been no longitudinal studies comparing the various 100% leafy green diets promoted on some sites and the combination leafy greens/vegetable/alfalfa/fruit ("mixed") diet. A diet based solely on leafy greens will generally contain more calcium oxalate [binds calcium and causes mineralization of soft tissues] and goitrogens [binds iodine, leading to thyroid gland impairment]), factors mitigated by a healthy mixed diet.
Until someone sits down with large numbers of herbivorous hatchlings (or neonates) that can be raised for 15+ years (or, ideally, for more than one generation) in a strictly controlled setting to study the long term effects of the different diets, it is all going to be just so much conjecture.
Baer DJ, Oftedal OT, Rumpler WV, Ullrey DE. 2001. Dietary Fiber Influences Nutrient Utilization, Growth and Dry Matter Intake of Green Iguanas (Iguana iguana). Journal of Nutrition Vol. 127 No. 8 August 1997, pp. 1501-1507
Hayes WK. 1995. What do wild green iguanas eat? Iguana Times 4(3):48-49.
Iverson, JB. 1988. Adaptations To Herbivory In Iguanine Lizards. In, Iguanas of the World: Their Biology, Ecology and Conservation, GM Burghardt and AS Rand, eds. Noyes Publications, NJ.
McBee RH and McBee VH 1982. The hindgut fermentation in the green iguana, Iguana iguana. In, Iguanas of the World: Their behavior, ecology and conservation. GM Burghardt and AS Rand (eds.) Noyes Publications, Park Ridge, NJ, pp. 77-83.
Nagy KA 1982. Energy requirements in free-living iguanid lizards. In, Iguanas of the World: Their behavior, ecology and conservation. GM Burghardt and AS Rand (eds.) Noyes Publications, Park Ridge, NJ, pp. 49-59.
Zimmerman, LC and Tracy CR. 1989. Interactions between the environment and ectothermy and herbivory in reptiles. Physiological Zoology, 62(2):374-409.
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