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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.


The Green Iguana (Iguana iguana) on the Venezuelan Llanos – Notes from the Field


green iguana on Venezuelan LlanosI grew up “knowing” green iguanas to be largely arboreal lizards of thickly-forested habitats.  In time, I was able to confirm that impression by observing them high in the canopy of a Costa Rican rainforest, and in the tall trees of Tortuguero Island, Costa Rica…where, in accordance with what I had read, they dove into the river when disturbed.

So I was quite surprised, upon arriving in Venezuela’s flat, largely treeless llanos, to find these flooded grasslands well-stocked with the huge lizards.  I was there to study green anacondas (also a surprising find, given my past impressions, but we eventually tagged over 500) but found it impossible to limit my attention to them, so overwhelming was the diversity of wildlife.

The green iguanas grazed on the flat, treeless terrain like so many cows…in fact, they were often among cows, as much of our work was on a cattle ranch.  When threatened (i.e. by me trying to catch one), they took off at incredible speeds and dove into the water.

Anaconda surfacing on Venezuelan LlanosOne stout brute of 5’11” in length (please see photo showing side-view) absolutely refused to enter the water when he reached its edge.  He held his ground, thrashing his tail (please see photo showing cut left on my arm by a smaller animal’s tail) and lunging at me.  Once subdued, I was able to see that he had numerous old wounds that had likely been inflicted by piranhas, and was missing several toes.  Perhaps his stretch of the river had a particularly aggressive piranha population, and he preferred a battle on land to another swim!

Venezuelan LlanosI’ve included a photo of typical iguana habitat in Venezuela’s central llanos country, to perhaps show you why I was so surprised to find the lizards there (the creatures in the foreground are capybaras, world’s largest rodent).  Also included is a photo of another reason that an iguana might choose to run or fight rather than swim – a huge green anaconda basking at the surface.

Today we can see ample evidence of the green iguanas’ adaptability right here in the USA – feral animals live in areas ranging from beaches to suburban gardens throughout south Florida!


Detailed information on green iguana behavior and typical habitats is posted by the Green Iguana Society at:


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