Melissa Kaplan's
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Last updated January 1, 2014

Hormone Abnormalities as a Factor in Male Green Iguana Aggression

Some ideas to consider, based on what we know about hormone production, regulation and dysregulation in humans and other animals

©2006 Melissa Kaplan


After responding yesterday to an e-mail list post about the breeding season-related aggressive behaviors of a particular 4 year old male iguana, I woke up this morning thinking about those male iguanas who are extremely aggressive during breeding season, and those who are very aggressive all year round, and what it is that makes them different from the far greater number of males who are sociable and agreeable outside of breeding season, and whose behaviors during their breeding season range from comical to annoying, rather than outright dangerous to others.

Being in captivity is part of the problem with some of these iguanas--being forced to live in an enclosure that is significantly smaller than their territory in the wild. Even if the enclosure is a 2000 square foot house in which the iguana free-roams, it is still smaller than a wild iguana's territory, especially since the captive one includes at least one human, probably more, and a parade of visitors and possibly even some other resident animals.

Some of you may have read the account of Bon's iguana, iZ, who became fiercely aggressive as a result, it was later found out, of pain caused by a tumor (see Tough Love, or Attigtude Adjustment). So, assuming routine veterinary exams have ruled out impactions and tumors and other illness, might there be a physiological or biochemical reason for some of the cases of extreme breeding aggression seen in some male iguanas?

Some of you through the years may have wandered through my Chronic Neuroimmune Diseases website and come across my article on Estrogen Dominance. The quick overview (see my Glucuronidation article for more complete information): the body produces an acid that binds with the body's used estrogen, keeping control of it as it is escorted out of the body. Thus, the body keeps producing estrogen, but also gets rid of the estrogen previously produced.

However, an enzyme may be created by, it is believed, one of the 500 or so cousins of E. coli that live quietly (and benignly or commensally) in our gut, that breaks this bond or otherwise prevents it from being formed. When that happens, the bound and should-have-been-bound estrogen is free to keep circulating through the body, wrecking havoc as it goes.

Could this same thing happen with testosterone? Is there something that is supposed to be binding the 'used' testosterone? If so, could something cause this bond to be broken, or kept from being made in the first place?

Another possibility: Could some part of the conversion cascade that results in testosterone and its metabolites be faulty, thus increasing total testosterone (higher up the conversion cascade) or free testosterone (that which circulates in the blood at any given time)?

In doing some general searching on the Web, using "binds testosterone" and, later, "excess testosterone", I found that, indeed, there are similar processes going on in males that can go wrong, just as they can in females. In fact, women will recognize many of the hormones, since they play roles in our own monthly cycling and cause problems as we deal with fertility problems and progress through perimenopause.

The following is from a booklet on low testosterone, but it simply and clearly describes how things work properly and why they might become dysfunctional:

Testosterone Production (page 4)
The body carefully controls the production of testosterone. Chemical signals from two locations - the pituitary gland at the base of the brain, and a part of the brain called the hypothalamus - tell the testes how much testosterone to produce.

The hypothalamus controls hormone production in the pituitary gland by means of gonadotropin-releasing hormone (GnRH). This hormone tells the pituitary gland to make follicle-stimulating hormone (FSH) and luteinizing hormone (LH). LH signals the testes to produce testosterone. If the testes begin producing too much testosterone, this is sensed by the brain which sends signals to the pituitary to make less LH. This, in turn, slows the production of testosterone. If the testes begin producing too little testosterone, the brain senses this and sends signals to the pituitary gland telling it to make more LH, which stimulates the testes to make more testosterone.

The role of binding testosterone in males and females is played by a hormone cleverly called sex-binding hormone protein (SBHP). Needless to say, SBHP binds more testosterone in females than it does in males, once they reach puberty. Since both males and females produce testosterone and estrogens, it isn't surprising the SBHP production and function is a complicated one, resulting from the amounts of free estrogen and testosterone, which affect the body's production of SBHP--or lack of it.

A research paper, Sex Hormone Binding Globulin and the Assessment of Androgen Status, goes into this rather densely but might be useful in talking to your vet about getting your iguana's hormones tested.

Now, true, this paper is written more from the perspective of women's diagnosis and health issues arising from increased levels of testosterone, but the male is the flip side of the female (physiologically if not socioemotionally) and so, where there are dysfunctions and dysregulations that can cause a decrease in total and free testosterone in males and females, and increases in females, there are going to be things that cause testosterone increases in males.

(Here's another easier-to-read discussion of the free and bound testosterone, at

Excess testosterone can cause irritability, even rage. The steroids abused by body builders and athletes are synthetic testosterone; regular long-term use leads both to the irritability and rage (as well as severe acne and impotence, which probably does little to ameliorate the irritability and rage.).

The impotence, by the way, is caused by the fact that excess testosterone is converted into estrogen, with the result that males become feminized, leading to weight gain and reproductive problems (which leads us back to Environmental Estrogens, for those who want to look at the long term effects of male feminization of a species and the sources of environmental--xeno--estrogens).

What I am wondering is this: Could there be an excess testosterone situation occurring in breeding season male iguanas, perhaps due to a pituitary, hypothalamus, or adrenal problem, or a problem with the liver detoxification pathway associated with SHBP production/function, that is not leading to feminization because breeding season--and the cyclical increase in testosterone--occurs for only a relatively short period of time each year?

Remember: in the wild, males are in season for only 28-30 days, not months, as they are in captivity. A wild male iguana may have this type of problem, but he lives in a much bigger area and is only dealing with testosterone overdose for a month, if that, out of the entire year. In captivity, where breeding season for males lasts much longer (and may be triggered twice in one year), this could be a significant problem for an iguana with a genetic or acquired dysfunction in their hormone production or regulatory system.

So, perhaps a talk with the vet about testing to check out the androgen/testosterone cascade, including free and total testosterone and SHBP, might start yielding clues. The same for checking out the pituitary, hypothalamus, adrenals, and liver structure and function.

The problem with liver function tests (LFTs) is that they really don't tell you much - just whether enzymes are elevated or not, or whether the liver is cirrhotic or nearing failure. They don't tell you anything about what detox pathways are impaired. There is such a test for humans, but it involves ingesting caffeine and aspirin before the 24 hour saliva and urine collection, so I'm thinking it won't be particularly usable with iguanas. But perhaps there are analogs that can be used to start gathering data needed to start making assessments and comparisons.

Another way avenue to use to compare and contrast 'normal' male green iguanas to those who are extremely aggressive during breeding season only, and those who are extremely aggressive all year round, would be to do MRIs (fMRIs would be even better but the challenging would be to figure out 'tasks' for iguanas to engage in while being scanned), PET, and SPECT scans to look at brain structures, activity and perfusion.

Other hormones worth reviewing are the neurotransmitters that can affect behavior, especially serotonin. Less widely known is that thyroid hormone imbalance can result in psychosis and rage in some individuals, as can infection by various parasites not normally tested for or for which there are no definitive (100% accurate) laboratory tests. Microbial organisms are increasingly being associated with schizophrenia, autism and other neurological or mental/behavioral disorders that were once thought to be "all in the head".

Related Articles

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Testosterone, Aggression...and Green Iguanas

Testosterone Rules

Biosocial studies of antisocial and violent behavior in children and adults: a review

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