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

Testosterone, Aggression...and Green Iguanas

©2000 Melissa Kaplan


The issue of the role of testosterone and aggression comes up fairly regularly in groups of green iguana keepers due to the potentially serious outcomes when an aggressive male iguana attacks other iguanas, humans, or other animals. Some male iguanas are aggressive all the time; others only during breeding season. Unfortunately, non-breeding related aggression may not be aggression but the actions of a dominant lizard, one who is dominant ("alpha") over his owners as he would be over males in his area in the wild. Based on my own experience in taming aggressive adults, and those of others I have worked with, this type of aggression is not physiological but behavioral - the dominance behavior was inadvertently reinforced by the responses an subsequent actions of the humans; in effect, they were trained by their iguana rather than the iguana tamed by them.


Why Neutering Aggressive Male Green Iguanas Doesn't Usually Work
Neutering male iguanas has been done to reduce aggression in males. Unfortunately, because the production of hormones in the body was not taken adequately into consideration, nor the interplay between naturally aggressive tendencies and testosterone, neutering has not been nearly as successful in green iguanas as it has been in other species. This failure rate, I believe, has to do with one or more of the following:

1.      Iguanas are not pack animals like dogs. While they do live in social proximity to each other in the wild and at times in captivity, interactions between alpha males, or a dominant male and subordinate males who are dominant to others, is more one of tolerance during most of the year, rather than cooperative living as seen in family groups of canids, felids and other species where there is a dominant male.

2.      Iguanas have not been selectively bred for traits of docility, loyalty, gentleness and the many other traits which have been bred for - intentionally or not - to make dogs "man's best friend." Farmed iguanas are bred just to produce as many iguanas as possible for the food and pet trade, not for any particular trait.

3. The fact that aggressive tendencies or aggressive behavior occurs in the absence of high levels of testosterone indicates that far more than just the testosterone levels are responsible for aggressive behavior. In human behavioral research, it has been found that abnormalities in the frontal cortex of the brain are significant indicators of aggression. Serotonin uptake, magnesium deficiency, and other chemical imbalances can also lead to aggression.

4.      Testosterone is not just a product of the testes, just as estrogen (and testosterone) is not just a product of the ovaries; the adrenal cortex in both males and females produces testosterone. Thus, removing the testes or ovaries will not completely eliminate these hormones from the body.

5.      Other organs and glands affect the body's response to situations, including the amygdala, hippocampus, hypothalmus, and pituitary gland. These operate independently of testosterone and are not affected by surgically removing the testes.

There are three basic components to emotional response:

  • Behavioral (situationally appropriate muscle movements)
  • Autonomic (facilitates the behaviors and energy stores required to carry them out)
  • Hormonal (reinforces the autonomic response)

In other words, there is far more happening in an aggressive act than a single hormone. Aggression during breeding season is also not just hormones - there are overriding imperatives for species to procreate. In a sense, the body is just the gamete's way of making more gametes. Basically, life boils down to reproducing by whatever the most effective means are to join the best of the available gametes together to ultimately create a machine to create more gametes. It ain't romantic, but when you get down to it, romance is a more effective gamete machine assembly line than rape.

Confounding these variables are congenital anomalies of the brain itself or the supporting organs, such as those of the endocrine system, and acquired defects due to physical or chemical injuries. Such defects can be caused by the early interactions of a neonate or juvenile with others, as documented in human and other primate research. People who work with animals, especially those involved in the pet trade, are rarely gentle and nurturing to the product they sell; another variable, then, to add to the above is early interactions between the aggressive wild animal and humans. In humans, injuries to the prefrontal cortex may occur from which the patient apparently uneventfully recovers. However, upon later examination of the brain, it is found that there was indeed lesions in the prefrontal region associated with aggression and quasi-psychopathic behavior. The roughness with which pet trade iguanas are handled, and the injuries they may sustain as they adapt to captivity (most iguana owners are familiar with the high-speed dash head first into walls or plate-glass windows that all iguanas seem to do at least once), can be severe enough to cause brain injury. Since brain examinations are rarely done as part of a veterinary exam, such lesions or areas of hyperactivity or hypoactivity are unlikely to be found.


In conclusion...for now
There is not a lot of research in the biochemistry of aggression in reptiles. There is a growing body of literature that looks at behavior in relation to things which can cause aggressive responses: territory, mate competition, food competition, fear, and feeling trapped with no escape. The following groups of links related to reptiles as well as to aggression in general. The articles themselves range from research paper abstracts to articles written for the general public, to research papers and university lecture/course notes. There are no answers here, if you are looking for a concrete "X causes my iguana to be aggressive." There are, instead, a range of possibilities, and a picture of the continuing evolution of our knowledge about brain function and neurochemistry.

If you have an aggressive iguana, one who is aggressive outside of breeding season, you need to look at your own actions as being precipitators, as well as his set up, where he is housed within your home, other factors that can influence behavioral responses (other people, sounds, pets), and his overall health. If it is happening during breeding season, and the season seems to be longer than the usual 2-3 months, you again will need to look to the environment, both his physical environment (day length, UVB duration) and the external environment (multiple dry seasons and/or wet seasons, unseasonal overcast days, early or unseasonal heat waves, etc.) as they can affect your iguana's brain chemistry.




The amygdala is part of the limbic system that controls some of the more basic drives such as aggression and sexuality.
autonomic responses Autonomic responses include heart rate, blood pressure, respiration, sweating, pupillary dilation, tearing, and hormone secretion.
gamete Gametes are cells connected with sexual reproduction. A gamete is either a male sperm or a female egg. Each gamete contains one set of chromosomes. When joined with another gamete of the opposite sex, they form a fertilized cell (zygote).
frontal cortex The frontal lobe (cortex), which includes the motor cortex, is the center of personality and emotion, and performs tasks that involve logic and reasoning.
hippocampus The hippocampus is at the core of the temporal lobes and controls the more primitive pleasure stimuli and aversion stimuli pathways and associations. Some long term memory is stored here as well.
hypothalamus The hypothalamus functions as the primary center for emotion and drives and controls sex, anger, temperature control, hormone release, eating, drinking, sleep, and pleasure pathways.
pituitary The pituitary gland secretes somatotrophic hormone (growth hormone) and controls the action of other glands like the thyroid gland, adrenal glands, ovaries, and testes.


Articles on Herps and Dominance and/or Aggression
Arginine vasotocin increases calling-site acquisition by nonresident male grey treefrogs
Female territorial aggression and steroid hormones in mountain spiny lizards
Functional versus physiological puberty: an analysis of sexual bimaturism in the green iguana, Iguana iguana
Lizard Tough Guys
Male morphs in tree lizards, Urosaurus ornatus, have different delayed hormonal responses to aggressive encounters
Neutering Green Iguanas
Social inhibition of territorial behaviour in yearling male collared lizards, Crotaphytus collaris
Territory acquisition in lizards: I. First encounters; II. Establishing social and spatial relationships; IV. Obtaining high status and exclusive home ranges


Non-Herp Articles on Aggression
The following websites have articles or university lecture notes that address the issue of aggression and the brain.

Aggression and Lyme Disease
From Abuse to Aggression: The psychophysiological consequences of maltreatment
Impairment of social and moral behavior related to early damage in human prefrontal cortex
The Neurobiology of Violence and Aggression
New Clues to the Causes of Violence
Nondrug Management of Aggression in Nursing Facilities
Pacific Neuropsychiatric Institute: Aggression
Penetrating Insights into the Brain (For more on the evolving theory of brain function, see The Music of the Brain)


Additional Information
Articles about various brain chemicals and other substances illustrate that it is not just testosterone which is implicated in aggressive behavior:

Aggression Research: Of Mice and (Maybe) Men
Aggression, Suicide: Zeroing in on a chemical culprit
Iron: Help for Conduct Disorder?
Low Saliva Cortisol Correlates with Early, Persistent Aggression
Magnesium Deficiency Apparently Increases Violent Crime
New Research Supports MAOA/Violence Link
P.E.T. Study: Looking inside the minds of murderers
School Study: Supplementation decreases delinquent behaviors and increases IQ
Serotonin and Impulse Aggression: Not So Fast
Side Effect of Seizure Drug: Reduced Aggression
Suspect list shortens for maternal aggression's brain origins
Uric Acid Studied


Articles about testosterone and aggression.
I suggest first reading Mazur's Testosterone and Dominance in Men article, a meta review of the research literature on testosterone and aggression, to get a handle on the generally inconclusive findings relating to testosterone causing aggression and familiarize yourself with the theory of reciprocol causation.

Testosterone Rules
Effects on Behavior of Modulation of Gonadal Function in Men with Gonadotropin-Releasing Hormone Antagonists


Hormones in Context

An Introduction; Systems and Control; Oestrogens and Androgens; Testosterone and Aggression


Online Resources Relating to Brain and Behavior
Crime Times
Glossary of Psychiatry and Neurology
Biological Psychology
Understanding and Preventing Violence, Volume 2: Biobehavioral Influences (online book)


Books on Testosterone and Behavior

Heroes, Rogues and Lovers: Testosterone and Behavior.
James McBride Dabbs and Mary Godwin Dabbs. 2000. McGraw-Hill, New York.
a1 Books,,,, B&, and

The Trouble with Testosterone and Other Essays on the Biology of the Human Predicament
Robert M. Sapolsky. 1997. Scribner, New York.
a1 Books,,,, B&, and

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