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.
Neutering Aggressive Male Green Iguanas Doesn't Usually Work
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:
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.
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.
on Herps and Dominance and/or Aggression
and Lyme Disease
Research: Of Mice and (Maybe) Men
Articles about testosterone
Hormones in Context
Relating to Brain and Behavior
Books on Testosterone and Behavior
Heroes, Rogues and
Lovers: Testosterone and Behavior.
The Trouble with Testosterone
and Other Essays on the Biology of the Human Predicament
Need to update a veterinary or herp society/rescue listing?
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