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

Iguana Age and Expected Size

©1997 Melissa Kaplan

There is no way for someone who has not had a great deal of experience with iguanas to tell how old they are (and, even then, experienced folks can only make an approximate guess). Iguana growth is determined by diet, heat and activity, as well as by the population characteristics in the population from which the iguana was taken or bred. That being said...

Age/Size Norms
Sexual Dimorphism
Effects of Diet and Care on Growth

Age/Size Norms
The following table represents the sizes and weights typical of properly fed and housed iguanas. Most people are amazed at how big iguanas get, and how big their less-than-adequately cared for iguanas should be. This is because the captive care information that has been available through pet stores and pet trade books has served to retard their growth and development, through poor diet, inadequate temperatures, and lack of sufficient space to exercise.

svl = snout-vent length
stl = snout-tail length
Iguana tails range from 2.5-3 x the svl.

Females are generally smaller than males. There is some regional difference in size/mass so actual size will be dependent upon where the iguana was originally from, but overall will be well within the above ranges.

Sexual Dimorphism
When animals are sexually dimorphic, it means that the males and females have different external physical characteristics that enables one to tell them apart. Sexually mature humans are obviously sexually dimorphic. Many animals are dimorphic but you need to know what you are looking at. Some are so wildly different that they may appear to be difference species. With reptiles, as with many animals, the physical differences may be so subtle as to almost not be there at all, and there is enough natural variation in individual members of the species that it may be almost impossible to be completely accurate without resorting in invasive methods.

Many animals, such as iguanas, start life looking very much alike, with the dimorphic characteristics developing only when the animal has attained sexual maturity. The document on sexing iguanas discusses how you can visually determine the sex of an iguana who is at least one year old, the age at which the femoral pores begin to expand in males. Another male iguana characteristic is the soft bulge in the base of the tail on the ventral (bottom) surface. The bulge that begins to develop around age 16-18 months are the inverted hemipenes, the bi-lobed reproductive organs sported by lizards and snakes. They are located in the tail, just south of the vent, and are sometimes everted sometimes during defecation or dominance displays. And, of course, during breeding. Only one lobe is used at a time during copulation.

Effects of Diet and Care on Iguanas
Here are some age and size examples from some of my iguanas. The measurements and weights in the list below reflects the iguanas as they were in 1994. Some of these iguanas have gone on to new families; other have sadly just gone, victims of their early poor care and diet. Their ages are based on what their prior owners told me. (SVL = snout-vent length.)

So, as you can see, there is some great disparity between ages. Sylvia I have had the second longest. Freddy I got a year ago, and the others all came to be since February, most since June of THIS year (1997). All were fed diets consisting mainly of lettuce, monkey chow, banana, spinach, broccoli or commercial iguana foods. (August 1997 update: Zeik, Freddy, Merlin, Lito, Sylvia and Elvys have since died of kidney failure. Ziggy died of complications arising from spinal fracture and compression due to severe metabolic bone disease. Wally is now 18" svl and 10+ lbs. The others have been adopted out, with new iguanas having come to fill the basking areas. (If I tend to get a bit testy about people feeding animal protein and monkey chow, it is because I've seen kidney failure in action, held the iguanas through their seizures, then held them while the lethal injection, administered to cut short the pain and fear, did its job.)

My older small males all have well developed hemipenal structures, with enlarged femoral pores and well developed spikes and dewlaps - quite obviously male. Captive males become sexually mature at about 1.5 years but their male characteristics other then femoral pores don't really begin to get bigger than females (and the hemipenal bulge become visually apparent) until 2+ years. Despite this fact, however, 4 igs (Merlin, Ziggy, Lito and Gecko) came to me with their owners either not knowing their gender or having decided (or been told by pet stores and vets) that it was the opposite of what it was.

In February 1995, I took in another little iguana. Rugwort, at 20-22 months of age, was 7" svl, and his lower jaw was blown out by fibrous osteodystrophy until it was twice the width and dept of the rest of his head. Weighing in at well under half a pound, his hind legs were also grossly swollen from the metabolic bone disease, and his spine had already started to go crooked. I had been urging his owners (primary caretaker: a thirteen year old boy) to get him to a reptile vet for over six months. They never did, just watched while he got sicker, weaker, more and more swollen as his bones lost ever more density. By the time they finally decided that the cat food they were shoving down his throat wasn't going to make him any better, they asked me to take him. Rugworts MBD is now fully resolved, though his lower jaw and head are misshapen, and he has a bit of a Quasimodo-like hump in his back. He is quite active, climbing and running. Despite the fact that Wally always chases him off, he always tries to remain in close proximity to him, and always manages to sleep next to him (or under him, as Wally uses him as a pillow) at night. Rugwort hit sexual maturity by 2.5 years of age, with distinctive femoral pores and hemipenal bulges. He is now (August 1997) 11" svl.

Final note...
I raised Wally since he was a baby (about 2.5-3" svl; his snout-vent length equaled the length of my thumb). I raised him just like The Green Iguana Manual (1992. P. De Vosjoli; Advanced Vivarium Systems) said. By about 10 months of, he kept developing the thickened thighs and tail lumps typical of metabolic bone disease. So I kept boosting the amount of calcium added to his food and poured on the spinach and rhubarb compote (after all, they are both high in calcium, right?). The rest of his diet included broccoli, redleaf, romaine and the expensive European (also called Spring, or baby) lettuce mixes, cantaloupe and other melons, hibiscus, roses, figs, and carrots, with softened dog kibble and tofu for protein (and do I shudder thinking about it now!).

No matter what I did, however, the MBD lumps would return or never fully resolve. So I started researching iguanas, and diet and nutrition, in other sources. When I changed his diet, deleting the spinach, lettuce, melons other than cantaloupes, tofu and broccoli, and began using the green beans and squash in their place, I found a dramatic increase in both his growth rate and activity level. His color also improved, and he no longer developed signs of MBD, with all the existing lumps disappearing as his normal bone density was resolved..

Wally lost his tail (well, okay, he didn't lose it: it is still wrapped in foil, sitting in my freezer, since I slammed it in the car door) two years ago. While it never regenerated, it didn't stop the rest of him from growing. He just turned 5 years of age, and is 18" svl and weighs just over 10 lb. Where once his entire body was the same size as the length of my thumb, his head is now longer than my thumb, and the base of his tail taller than my far-from-delicate wrists...and he's still growing. (1997 update: Wally got very sick last summer. During the resultant struggle to regain his usual health and activity levels, his growth plateaued, and he has grown very little in the past year.)

Sources for information on sexual maturity and breeding in green iguanas
The following chapters are in Gordon Burghardt and A. Stanley Rand, editors. 1982. Iguanas of the World: Their Behavior, Ecology and Conservation, Noyes Publishing, Park Ridge NJ.

Dugan, Beverly. The mating behavior of the green iguana, Iguana iguana. pp. 320-341.

Dugan, Beverly and Thomas Wiewandt. Socio-ecological determinants of mating strategies in iguanine lizards. pp. 303-340

Harris, Dennis M. The phenology, growth and survival of the green iguana, Iguana iguana, in northern Colombia. pp. 150-161.

Rand, A. Stanley and Harry W. Green. Latitude and Climate in the phenology and reproduction in the green iguana, Iguana iguana. 142-149.

Van Devender, Robert W. Growth and ecology of the spiny-tailed and green iguanas in Costa Rica, with comments on the evolution of herbivory and large body size. pp. 162-183.

Wiewandt, Thomas A. Evolution of nesting patterns in iguanine lizards. pp. 119-142.

Rodda, Gordon H. 1993. The Mating Behavior of Iguana iguana (Part 1). Iguana Times 2(2):2.

Rodda, Gordon H. 1991. Sexing Iguana iguana. Bull. Chicago Herp. Soc. 26(8):173.

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