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

Neutering Green Iguanas

Douglas Mader DVM, Vet Q&A, Reptiles Magazine


I recently took Hank, my male green iguana, to the vet for a checkup. He recommended neutering Hank because it curbs aggressiveness in male green iguanas. Hank is 3 years old and very good-natured. He hasn't bitten or tail-whipped anyone. He does bob his head and turn orange when he's in an aggressive mood, but that's it.

I would never consider neutering Hank unless it would ultimately make him happier. What are your views regarding neutering iguanas? You have a wonderful and very informative column.

Kirsten S., California

Thanks for the compliment. I appreciate the feedback. You raise a very good question, and it's one that is subject to much debate. I have neutered more than 50 green iguanas for various reasons--some because the owners didn't want them to breed with their females, and some because they were aggressive. In addition, I have tried to keep track of how this procedure affects the animals' subsequent behavior.

Before I discuss my findings, let me make a few points about aggressiveness in the green iguana. I'm sure others may not share all my views, but this is what I've noticed in Southern California, where I practice. I point out the location of my practice for a very good reason: I believe that geography plays an extremely important part in an animal's behavior. Specifically, the exposure to day-length (very long days in Alaska during the summer versus moderate daylength in Southern California, for instance) and temperature (85 to 90 degrees Fahrenheit in Southern California during the fall, versus 50 to 60 degrees in Chicago).

Day-length and temperature will affect an animal's breeding cycle. In Southern California, we see two distinct breeding seasons each year (spring and fall). I know, through my veterinary and herpetology friends, that in high-latitude areas you would only see breeding activity during the spring. During this breeding time, males may become very aggressive. As you noted, their color changes, head-bobbing starts and they may attack both conspecifics and owners. In many cases, the iguanas may not be able to differentiate between the two. What this means is that, to the iguana, a person (such as the owner) may just be another lizard within its breeding territory.

There has been a report in a veterinary journal correlating aggressive attacks by male iguanas on their female owners during their monthly menstrual periods. The speculation is that women produce some sort of pheromone (airborne signal) during this time that triggers the iguana to attack. This is not to say that all women will be attacked by their iguanas during their periods, however, as there have also been reports of men being attacked.

It is important that you are able to distinguish the difference between the types of aggression that you may see in an iguana. First, there is defensive aggression, which is present in most every iguana, even the most docile animals. This occurs when the animal is startled, such as when you walk up to it while it's sleeping, or when you stick your hand into its cage. The iguana will stand erect on all four legs, turn sideways and puff up (they do this in an attempt to visually increase their body size, apparently to intimidate their opponents). They will often accompany this behavior with tail-whipping, but will rarely bite unless you grab them.

In contrast, offensive aggression occurs when you are sitting on your couch, minding your own business, and your iguana runs across the room, jumps up onto the couch and takes a chunk out of your arm. In other words, the difference between the two is that defensive aggression is provoked (such as by entering the animal's private space) and offensive aggression is not. That distinction is important, because when evaluating the effects of castration, it is important to be able to distinguish between the two.

I have tried to follow up on all my post-castration iguanas to evaluate their behavior modification. Because of various circumstances, I have only been able to contact about a third of the owners after one year post-surgery.

From my observations, it has been apparent that there has been no effect on either defensive or offensive aggression immediately following the surgery. The owners of the animals that I checked on a year later had various reports, from their iguanas being somewhat less aggressive to no change at all. They said current aggression was mostly defensive, but the offensive component was still present.

For the record, I have talked with other veterinarians who have castrated iguanas. I have heard completely opposite reports in a few cases, where the vet states unequivocally that "the once-mean iguana is now docile as a kitten." The problem with these reports is that they are anecdotal. There have been no controlled studies to rule out other possibilities for the aggression (such as other male animals in the same house, exposure to sunlight [always good for stimulating aggression], presence of female animals, ambient temperature and day-length).

As any experienced iguana owner can tell you, there's no way you can predict whether or not an iguana will be nice or mean. I've seen baby iguanas, raised as "only children," being doted over with attention 24 hours a day that grew up to be "docile as kittens." I have also seen animals, raised in an identical fashion, that grew up to be little T-rexes. Likewise, I have seen adult wild-caught males that were harness-trained. So, as I have alluded, understanding iguana aggression is a fit topic for somebody's Ph.D. thesis.

Aside from castration to calm aggressive male iguanas, a number of other methods have been tried. For instance, female progestational hormones can be given during the breeding season. These come as either pills or injections. But, just as with castration, the effects are variable and unpredictable. You can also attempt to temper aggression by decreasing day-length (by turning the cage's lights off early) and cooling the animal. Remember, you will eventually have to warm it back up or turn on the lights; you can't deliberately keep an animal locked up to prevent it from being aggressive--that would be cruel. Don't forget that most of these behaviors are normal for these animals. If your iguana exhibits behavior you find disagreeable, perhaps it is not the right animal for you to keep as a pet.

One last note regarding castration: I have castrated a limited number of prepuberty male iguanas. So far, and keep in mind that this is not a scientific finding, these animals have yet to become aggressive. However, since these animals were castrated before they reached puberty, they never developed the large crests, massive jowls and the beautiful scales for which male iguanas are known. So, if you were to have this done to your iguana, you might gain a friendly pet, but you would be losing the majestic look that only a mature male green iguana can have.

So, Kirsten, in answer to your question, I think the jury is still out regarding a definite answer. The choice is yours. The procedure is safe, if performed by a qualified reptile veterinarian, and you might end up with a great, gentle pet. But once again, there are no guarantees.

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