Home | Breeding | Aggression in Male Green Iguanas (Iguana iguana) and Nesting Behavior in Females: the Effects of Hormonal Changes and the Breeding Season

Aggression in Male Green Iguanas (Iguana iguana) and Nesting Behavior in Females: the Effects of Hormonal Changes and the Breeding Season


Green IguanaBreeding season aggression is a potentially serious concern for owners of male green iguanas.  Spurred by reproductive hormones, mature males, even those which have been placid for years, may suddenly become dangerously aggressive.  This most often, although not necessarily, occurs in the late winter to early spring in the USA, so I thought that a word of caution might be in order at this time.

The Results of an Iguana Bite

If you are caught off-guard, an iguana attack can be quite severe.  A former coworker of mine, well-seasoned in reptile care, was surprised by an aggressive male who had been living without incident in the collection for over 10 years. The animal latched onto his throat and then fell to the floor, leaving a wound that required 18 stitches to close.

Timing and Indications of Sexual Maturity

Depending upon dietary and other factors, male green iguanas may reach sexually maturity as early as 18 months of age or as late as 7 years or more.  Unfortunately, the onset of aggression may be quite sudden, may not occur every year, and can last from 2 weeks to several months.  As you can see from the preceding story, animals may be even-tempered for many years before suddenly changing their behavior.

Males in breeding condition may exhibit a deepening of coloration in the orange-tinted areas of the head and body, and may head-bob and erect their crests.  Waxy secretions are usually present along the femoral pores, and dried semen is often found in their cages.  However, these changes do not always precede aggressive behaviors, so caution is always a necessity around large males.

Dealing With an Aggressive Iguana

A number of strategies for dealing with aggressive male iguanas have been proposed, with results varying widely.  I suggest that a plastic garbage can lid be held as a ready shield when working around aggressive males…a broom works well if the animal is at large in a room.  Fighting back with these tools may convince the animal to seek an easier target.  Avoiding close contact with the animal until the “mania” has passed, and keeping a shield at hand, are the safest options.  An alcohol-soaked rag (please see below) and, if possible, a helper, are very useful as well.

Some pet owners deflect aggression by providing their iguana with a towel to attack (or mate with!), while others keep the animal in a darkened room for 3-4 days to lessen the production of testosterone and other hormones.  Surgically neutering the animal is also an option, but the results of such have been mixed – in a significant number of cases the aggressive behavior remains largely unchanged.

If You Are Bitten

If you are bitten, do not pull your hand away.  Stabilize the animal so that it cannot thrash about and apply an alcohol-soaked rag to the lizard’s nostrils (avoid the eyes).  Liquor is fine to use (for the rag, not for drinking while working with your pet!) and should always be within reach during the breeding season.  Once the iguana releases its grip, secure it in its enclosure and call your doctor for instructions as to wound care.  Please do not neglect this step, as a tetanus shot or other care may be required, even for relatively minor wounds.

The Tail as a Weapon

The iguana that I am holding in the accompanying photo caused the cuts visible on my arm with a mere flick of his tail…the jaws are capable of doing much worse.  I came across him and the larger male (who, at 5’11” long, was the largest I encountered in the field) while working on anaconda research in Venezuela’s central llanos country.

Gravid Female Iguanas

Mature female iguanas often develop eggs as spring arrives.  Such will occur with or without the presence of a male, so be sure to watch even those animals which are housed alone.  Ideally, unmated females will resorb the eggs into their bodies, and have no need for nesting sites.  However, sometimes the eggs develop fully and, consequently, must be deposited.

Gravid (egg-bearing) female iguanas will usually cease feeding, or feed less vigorously, and will become very restless.  They often injure themselves by rubbing along cage walls and screening at this time, and they may attempt to dig through the cage floor.

Providing a Nest Site and Avoiding Egg Retention

Females without access to an appropriate nest site may retain their eggs, which will lead to potentially fatal health problems.  Provide gravid females with a large plastic container filled to a depth of 18-24 inches with moist top soil and moss.  Covering the container and warming the earth’s surface with a lamp may induce her to accept it as a nest site.  There are a number of other tricks you can try…please write in it you need further information.

Veterinary intervention may be required if the female refuses to lay her eggs.  In many cases, an injection of oxytosin is effective in causing the eggs to be expelled.

Further Reading

I’ve written about my experiences with wild green iguanas in another article on this blog.  Please see The Green Iguana on the Venezuelan Llanos.

An interesting article on iguana farming and conservation efforts in Belize is posted at http://www.thewildones.org/Belize/iguana.html.



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    • avatar

      Hello, Frank Indiviglio here.

      Thanks for your interest in our blog and kind comment, much appreciated. Please stay in touch…I look forward to hearing from you in the future.

      Best regards, Frank Indiviglio.

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    • avatar

      Hello, Frank Indiviglio here. Thanks for your kind comment and interest in our blog.

      I look forward to your future questions and comments; please let me know if I can be of any assistance.

      Best regards, Frank Indiviglio.

  3. avatar

    Thanks Frank for taking the time to write such a great read! I am an owner of a 5yr old male Green Iguana named Francisco, and a 5yr old female beardie named Ebert. (thought she was a male at first, and since Francisco is close to sisco, I figured I would name Ebert and him after the famous movie critics sisco and ebert) They have been raised around each other since they were about 6 months old. As a matter of fact, I’ve managed to domesticate these animals to the point of being potty trained, and even uncaged. They sleep together usually on my chest,Francisco would use ebert as a pillow usually.. I had built a cage for Francisco which he was kept in for maybe a year or two (til he was about 1-2yrs old. Then, he started trying to dig himself out and would rub his nose raw to the point it would bleed on the screen mesh I had as the facing. I felt extremely horrible to see him like that so I ended up letting him out, which he then began bedding on top. So happens the cage happens to be about 4-5ft tall putting him at face level with me. I think this may be part of the reason for his “attitude”, that and his heat light being a factor.. I had a time where he went through his “puberty” so to speak, trying to hump things and what not.. And then his aggression started showing. This was probably around 3yrs old. Well, I must be one of the unfortunate ones who ends up having to deal with the aggression for months at a time.
    . I noticed in your blog however where you spoke of using broom and a trash can lid as a shield.. Quite humorous but I fully understand as to why.. He has actually had a stand off with me a few times where he puffs up, tilts to his side and tries to circle around me and get closer.. I have also noticed he never attempts to bite me at my feet or legs really, he seems to always want to try and climb up towards my face. I have been hittin maybe 3 times tops in 5 yrs, nothing major, he has never bit Down and locked his jaw, he seems to bite and back off almost instantly. Ebert on the other hand, she’s a complete sweat heart, has the largest personality of any lizard I’ve ever seen, and she would NEVER hurt a fly!.. Well, maybe a fly lol, but would never attempt to bite a person, the most she will do is lick you like a dog. But today actually(that is why my roommate found your blog) Francisco bit ebert pretty good… It looks as if he bit her down around her tail where it meets her back/belly.. I could actually see in one side of her cuts, all the muscle, could probably stick a kernel of corn in it.. I cleaned her up, put some anesthetic on it (new skin) works like superglue almost, and surprisingly she doesn’t seem to be bothered by the wound’s(its on each side so I believe he bit down each of her side got top/bottom jaw), nor the burn of the medication.I’m pretty sure her abnormally large size helped save her from severe damage.. I’ve been told by many she’s the largest beardie ever seen.. Her diet consist of fruits and veggies now, seems as if she lost interest in crickets awhile back when I tried coating them in calcium powder. I am pretty sure she will do fine and I hope it heals quickly for her sake, I almost wanted to strangle him.. He heard an ear full for a good hour or two along the lines of BAD BOY, NO BITE, YOU HURT SISSY, over and over.. And I actually believe he understands for the most part because with the stress from my hands, and my voice.. (I always grip both of his arms to his side with one hand, and his back legs to his tail with the other. He’s over 4ft too btw.. Very healthy and strong) His look completely changes, the look in his eye, he goes into submission mode.. Looks just like a child would when being scolded for picking on his sister.. I believe they can learn things,as a matter of fact he has because he attempts to go to the bathroom by himself even, rather than I taking him when he comes down from his cage to me. I am afraid however he may not learn quick enough and being he is still a growing boy, I don’t want him to end up killing ebert at some point. So as of tomorrow, I am going to separate them for the first time in three years so he can’t hurt her anymore..I just hope he doesn’t start hurting his face on his cage again, especially since he hasn’t been caged in over 3 years.
    Now to my question. Based on the info I’ve given, do you have any tips I could try to decrease the time of his agression? Also, I’ve heard mixed messages about how to punish them/deal with them. You say to use a broom and trashcan lid. However I was told that any fight doesn’t cause them to listen anymore, just causes them to become even more aggressive. I’ve basically just came to restricting him of movement by putting him in the “grip” or wrapping him in a blanket til he cools off and potentially becomes tired. I can see in his eyes tho, he is becoming quite intimidated of me when he gets me yelling at him like today and holding him. Basically showing my dominance and that I am not scared.. I am only afraid that this could receive a negative outcome causing him to be more afraid and in return more aggressive.. Any advice would be greatly appreciated! I Could also share pictures and videos if you’d like. These lizards have had human contact every day of their lives basically.. They are the cleanest, and probably some of the most domesticated/tamed lizards you would ever see. Hope to hear from you, and thanks again for the insightful information. – Danny McClain

    • avatar

      Hello Danny,

      Unfortunately, the behavior is hormonally controlled; while they do adjust to captivity , and certainly can learn to some extent (when to expect food, simple mazes, etc) they cannot be taught to control aggression. The broom etc I mentioned is to prevent injury to the keeper; if the animal’s attacks are always thwarted, it may avoid or limit attacks for a time. However, hormones will control what it does, and so it cannot in any sense be “trusted”. As captive conditions make it difficult to predict when aggression will surface, the animal should not be allowed near one’;s face, or of course near children etc., and should not be housed with other lizards. I hope all goes well, Frank

About Frank Indiviglio

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Being born with a deep interest in animals might seem unfortunate for a native Bronxite , but my family encouraged my interest and the menagerie that sprung from it. Jobs with pet stores and importers had me caring for a fantastic assortment of reptiles and amphibians. After a detour as a lawyer, I was hired as a Bronx Zoo animal keeper and was soon caring for gharials, goliath frogs, king cobras and everything in-between. Research has taken me in pursuit of anacondas, Orinoco crocodiles and other animals in locales ranging from Venezuela’s llanos to Tortuguero’s beaches. Now, after 20+ years with the Bronx Zoo, I am a consultant for several zoos and museums. I have spent time in Japan, and often exchange ideas with zoologists there. I have written books on salamanders, geckos and other “herps”, discussed reptile-keeping on television and presented papers at conferences. A Master’s Degree in biology has led to teaching opportunities. My work puts me in contact with thousands of hobbyists keeping an array of pets. Without fail, I have learned much from them and hope, dear readers, that you will be generous in sharing your thoughts on this blog and web site. For a complete biography of my experience click here.
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