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

Iguana Breeding Season Basics

©2000 Melissa Kaplan


While the other articles on my Behavior, Breeding, and Reproduction page go into things like the meaning of head-bobs, male breeding season aggression, and the female's cycle of egg development and laying, this article will fill in some of the blanks and tie together information scattered throughout several articles.

Onset of Sexual Maturity
Iguanas in the wild - and healthy iguanas in captivity - will attain sexual maturity at about 18 months of age. If you got your iguana as a hatchling (about 2.5-3"/6.3-7.6 cm SVL), he or she was about 1-2 months of age at that time.[1] By the end of the iguana's first year, the SVL should be 8-9"/20-22 cm, and you will be able to sex the iguana.[2] This means that, about six months before onset of sexual maturity, you will be able to tell if you have a male or female. This gives you six months or so to prepare for the possibilities: male aggression [3, 4] or a gravid female.[5]

If you have a male, expect to start seeing some color changes. Not only will his adult coloring and markings begin to gel as they have gradually changed from the time he was a hatchling[8], but you will most likely start to see some orange or rusty red appearing. It may start on his head and neck, back, and the tops of the legs. As time marches on towards the 18 month mark, the color will grow more vivid and spread over a wider area. This is also the time period in which iguanas who previously lived together compatibly may start showing signs of incompatibility.[6]

There may or may not be behavioral changes. With some males, the only way you know they are in season is because of the color change, the occasional deposits of seminal plugs[7], and startlingly milky urates (due to ejaculates being deposited along with the urates and feces). You may also, for the first time, see the hemipenes "flashed", either as they are everted during defecation or, well, just because males of some species seem to have a need to strut their stuff.[10]

Other males will begin to behave differently. You will see posturing, including increased head bobbing, restlessness, lateral compression of the torso ("hatchet mode" or "hatcheting", dewlap flaring, crab-walking, and ritualized tail movements. Some iguana owners, male and female, get stalked by their iguana, who manages to move forward while crab-walking sideways in hatchet mode. Human males are often seen as competitors, while human females are seen as potential mates. How you respond will go a long way towards dictating what kind of season you will have this year, and in subsequent years.[3, 4]

Some people think that neutering an iguana will stop the aggression. This is not the case, however. Because of where they are located deep inside the body, it is a lot harder to neuter (remove the testicles) an iguana. Since the testes are themselves not the seat of aggressive behavior, nor the only organs that product testosterone and other hormones related to aggression or augmenting existing aggressive tendencies, removing the testes does not eliminate the hormones associated with aggression.[11, 12]

The biggest question in the minds of owners of male iguanas, especially male iguanas who get very aggressive and/or obnoxious during the season, is: how long does breeding season last? The answer, unfortunately, is: it varies. In the wild, males are in season for about 30 days. During that time, they exhibit the color and other changes we see in our captive males. Since females roam throughout the territories of several males, and are themselves only receptive to mating for only 7-10 days, males mate with several females during the month they are in breeding mode. A female who isn't receptive is left alone after a generally simple rebuff (head bobbing, foot swatting, or tail lashing). When a male ignores the "back off, buster" signs, other females in the area may come over and gang up on the male until he finally gets the message that no means no, and goes off to wait for a more receptive female to come along.

So, why aren't captive male seasons only 30 days? We don't know for sure. We do know that the eyes and the pineal gland play a role in circadian rhythms, thermoregulation, and hormone production.[15] We know that the farther away you get from the tropics, where the iguanas are originally from, the quantity of ultraviolet wavelengths and intensity of light changes. The farther north or south you get from the iguana's native range, the longer breeding season lasts, despite the use of timer-driven artificial lighting in a windowless room. So, you can expect the season to last anywhere from one month to four months or so.

The changes a female iguana undergoes are often the same, in terms of their skin turning orange or red. Most often, it is only dominant females who do so. This makes it a bit more difficult for iguana owners to figure out what is going on, so they must look for the other signs discussed in the article on egg development[5], and make sure the female's diet is well supplemented with extra calcium to avoid the health problems associated with gravidity-induced metabolic bone disease.[15]

A Caution For Both Males And Females
One difference between males and females is that you won't see any seminal plugs or milky urates. But you will see thickened, orangy urates as the eggs get larger and their bodies divert more fluids towards supporting the developing eggs. This makes it critical to keep your females as hydrated as possible. Males may show signs of dehydration, too, as their season progresses and the period of time without food and reduced water intake takes their toll. Because the male's season in captivity is so much longer than in the wild, and because captivity overall is far less humid than the iguana's native range, it is important to make sure both males and females stay well hydrated during the season as well as the rest of the year.[19]

Breeding Season Cycles
Once breeding season hits, it will occur about the same time every year. If your iguana reaches the 18 month mark and his or her first season in September, that will be about the time every subsequent year that the season will start. Unless you move. If you move farther north or south, the timing will change, as discussed above, with it starting later the farther away from their native range you get, and sooner the closer to their range you get.

If an iguana does not show any sign of going into its first season until it is 3+ years, the reason can generally be traced to captive diet. Diets lacking in what the iguana needs to develop and mature at the same rate as their wild counterparts will affect onset of sexual maturity. So will prolonged periods of acute stress and illness. Social structure, the design/disposition of the captive environment, and number of cagemates can also affect onset, with juvenile males being delayed while they are in forced proximity to mature males.[16, 17]

Multiple (Double) Seasons ("Double Clutching")
Some iguanas will go into season twice in a year. This appears to happen only in years in which there has been abnormal weather patterns, with two distinct wet seasons following by distinct dry seasons. In the wild, iguanas mate towards the end of "summer" (wet season). The females carry the eggs and lay them so that they incubate through through the "winter" (dry season, when less food is available). By the time the hatchlings emerge, the rains have started again and plants are growing more rapidly, offering adults and hatchlings alike with plenty to eat.

With wild iguanas going into season only once a year, and males only for a finite period of time each season, we can posit that captive iguanas who do so twice a year are experiencing greater stress. The same is especially true of females who develop two clutches in a year. Until we know more about what exactly factors into triggering a breeding season, there isn't any apparent way to stop it from happening. It does mean that we need to be extra alert to the signs of the impending season and to make sure our iguanas, male and female, are well supported nutritionally and environmentally to reduce as much as possible the level of stress they are under, and to make sure they are as healthy as possible to help them withstand the strain of double seasons.

Harem or Surrogate?
Some people think that if they get their male a female or two, everything will be just fine - the male will stop being so obnoxious, and, well, adopting females is a good thing, right? Maybe get some babies to sell or give away to good home? Unfortunately, if you've learned nothing else by now, you've learned that, when it comes to iguanas, things are rarely simple.

Since males mate with potentially 15-18 females in the wild (they generally mate on one day, rest the next, mate the third day, rest the fourth, etc.), you would need a large harem just to start taking the edge off. Most people don't have the room to properly house a single iguana, let along 15+ iguanas[18] Keep in mind, too, that females don't have to be mated, or even within a mile, of a male to become gravid, so not only would you still have a male on the make, you could conceivably (no pun intended) with 15+ gravid females, each needing her own nest box, each doing her best to claw her way out of the enclosure, through the carpeted floor, or through your plate glass sliding backdoor. And did I forget to mention that iguanas in season go potty more than their usual once or twice a day? Males do it as a sort of territorial marker, females because as the eggs start taking up more room in their abdomen, their internal organs get compressed and they are unable to store wastes as long as they used to (something any previously pregnant human female can tell you about).

As for the "wouldn't it be great to have lots of little iguana babies!", think again. People who have bred iguanas with the thought that they could easily sell the babies found out otherwise, and it's already getting impossible in many areas for reptile rescuers who have taken in iguanas who need homes to find good homes for them. So, unless you are able and committed to caring for the male, 15+ female adults, and their potentially 30-60 offspring (let's a minimum, that would be 30 eggs x 15 females = only 450 hatchlings, all of whom need to be housed, fed and cleaned up after...and tamed, socialized and potty trained if you can't find good homes for them within the first month or two), this is not really an option. Well, not unless "home" for you happens to be a predator and largely-free-of-humans Caribbean island within the green iguana's native range...

So, getting a mate for your male iguana seems to be a non-option. That leaves you with sexual surrogates. Now, now, stop that snickering! I hear you out there! "Surrogate sex partners for iguanas!?" Look at it this way: would you rather your iguana took his territorial aggressions and, er, horniness out on your body? Iguanas have been known to seriously attack both their male owners and female owners. Since aggressive attacks and mating both involve lots of teeth and claws, it really doesn't matter what the human's gender is, only that they are old enough for the iguana to be able to detect the pheromones (did I forget to mention their very keen sense of smell, through the use of their forked tongue and vomeronasal [Jacobson's] organ?) that tell them what we are.[3, 4]

So, do feel free to donate a towel, old sweatshirt, baseball glove, sturdy plush toy, inflatable pool toy (I hear that snickering again...), or make a a Luv Sock. But just in case, make sure you keep your medical insurance premiums paid up.

To learn more about the points discussed above, please see the referenced and related articles. 

1.  Age and Expected Sizes

2.  Sexing Iguanas

3.  Dealing with Iguana Breeding Aggression

4.  Male Iguanas in Season and Human Females

5.  Egging and Incubation

6.  Introducing and Housing Multiple Iguanas

7.  Seminal Plugs/Secretions

8.  Iguana Skin Color

9.   Head Bobbing

10. Hemipenes

11. Testosterone, Aggression and Green Iguanas

12. Neutering Male Iguana

13. To Spay or Not To Spay

14. Parietal "Eye"

15. Dystocia

16. Lizard Tough Guys

17. Differential resource use, growth, and the ontogeny of social relationships in the green iguana

18. Basic Cage Design Problems

19. Fluid and Fluid Therapy in Reptiles


All of which leads up to
Wally and Sylvia, 1999

Iguana Mating: Wally & Sylvia, 1999.  Photo by Melissa Kaplan.
Click to see larger image

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