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Amazing social life of the Green iguana

The views expressed are those of the author and are not necessarily those of Scientific American.


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An adult Green iguana in full regalia is a spectacular and beautiful beast. This very red individual (originally from Cuba) was photographed in Florida by Cary Bass, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

It’s still not as well known as it should be that ‘complex’ or ‘sophisticated’ bits of social behaviour are far from limited to mammals and birds among the tetrapods. Lizards, snakes, crocodiles, alligators and even humble frogs, salamanders and caecilians engage in such things as pair-bonding, parental care and kin recognition. Play behaviour (Burghardt et al. 2000), co-operation (Lenz 2004), social nesting, counting, sophisticated learning and problem-solving abilities (Leal & Powell 2011), social learning (Wilkinson et al. 2010), and the care and protection of siblings have all now been reported for various lizard, turtle and crocodilian species. Long gone are the days when interesting or ‘complex’ bits of behaviour must be assumed absent in non-mammalian, non-avian tetrapods.

Anole performing discrimination trial, from Leal & Powell (2011). These authors found individuals of Anolis evermanni to be quick learners (the lizards had to move a disk to get a reward).

In the interests of promoting some of the amazing things we now know about the social behaviour and behavioural complexity of one species in particular – the Green iguana Iguana iguana – I thought it was time to repost this Tet Zoo classic (originally from ver 2). [Image above by Cary Bass.]

Thanks mostly to the importance of the species in the international pet trade, the Green iguana is typically imagined as a rather uninspiring lizard that sits around on branches all day long, occasionally munching salad or sitting in its water bowl. It’s true that some captive individuals become remarkably charismatic and idiosyncratic, but for the most part the Green iguana is generally thought of as a rather dull animal that doesn’t really do much of interest. Today we’re going to change all that (I hope). Field studies stretching back over three decades have demonstrated beyond question that social behaviour in the Green iguana is remarkable and complex, and if you’re unaware of the sorts of behaviours that have been reported for these lizards, you might be surprised…

An active social life for wild lizards

Head-bob (or head-nod) signal practised by Green iguana, as illustrated by Distel & Veazey (1982).

Firstly, Green iguanas can be described as leading fairly active social lives, at least during the breeding season. They are territorial lizards with a lek-style breeding system: males choose exposed arboreal display sites, deliberately selecting trees that are dead or sparsely vegetated (Dugan & Wiewandt 1982). They advertise their ownership of this territory with lots of head-bobbing and displaying of the large dewlap, and patrol the territory – moving from perch to perch, head-bobbing with each perch change (Dugan & Wiewandt 1982). Males that try to move into the area are chased away, but females – in cases as many as eight – move into the territory, and here they compete among themselves for access to the territory-holding male (Burghardt 2002).

Males go without eating during this territorial breeding phase, divert energy into changing their appearance (they change colour from greenish to orangish and increase the size of their jowls), and also have to chase off rival males, and woo and mate with females. Females don’t necessarily have it easy during this time either, since – as we know from studies of another lekking iguana, the Galapagos marine iguana Amblyrhynchus cristatus – females may incur high energetic costs in discriminating among potential mates (Vitousek et al. 2007). After mating, males have been seen to stay close to females: this appears to be post-copulatory guarding (Dugan & Wiewandt 1982), a form of behaviour that prevents the female from mating with another male and hence prevents (or slows) sperm competition inside her body. [Image below by Paul Kehrer.]

Costa Rican Green iguana in a tree - this individual may or may not be territory-holding, I don't know. Photo by Paul Kehrer, licensed under Creative Commons Attribution 2.0 Generic license.

Incidentally, not all male Green iguanas are big, showy, territorial animals. Some are small, superficially female-like, and more cryptic. Rather than attracting females, they try to mate with them forcibly. Cryptic males that morphologically mimic females have now been documented in quite a few tetrapod species: I’ve previously written about this sort of thing in newts and sheep.

During January and February, mated females migrate to favoured nesting areas. And they really do migrate: in the case of the well-studied Panamanian iguanas that breed on the island of Slothia, in Gatun Lake, the females travel up to 3 km in order to reach their nesting area (as demonstrated by radio tracking: Montgomery et al. (1973)). They walk across land and then swim to the island. They clearly come to Slothia to nest and not much else, and don’t live on the island during the rest of the year. While Green iguanas in parts of their range nest individually, those that come to Slothia nest colonially and therefore synchronize their nesting. As many as 150-200 female Green iguanas gather together at favoured clearings, and here they compete with one another for access to the best nesting areas. [Image below by Franz Xaver.]

This photo - taken in Costa Rica by Franz Xaver - shows three iguanas in close proximity in the same tree. I don't know anything more than that, but it might be a large, territory-holding male and two females. Image licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

Nesting and the behaviour of babies

On Slothia, iguanas shared a nesting area with an American crocodile Crocodylus acutus over three successive nesting seasons. It’s possible that the iguanas did this so that their nests and/or juveniles might receive protection from the crocodile, but it seems most likely that the two species were brought together because of similar nest requirements, and neither seemed to benefit from the proximity: the crocodile both interrupted iguana nesting activities and killed and ate some of the iguanas, and the iguanas sometimes dug up the crocodile’s eggs (Dugan et al. 1981, Bock & Rand 1989).

Baby Green iguanas - emerging synchronously, but cautiously, from their nest - on the cover of Science, 18th February 1977. This is the issue that included Burghardt et al. (1977). Image (c) AAAS.

Iguana nests are not just scrapes in the ground. The iguanas build complex burrow systems that become deeper and increasingly complex over the lifespan of a nesting colony (Bock & Rand 1989). While female iguanas do not nest-guard as crocodiles do, they may stay with the burrow for a day or two after laying, defending it from other females. Presumably this is an attempt to stop later-nesting females from digging up the clutch while creating their own nest.

However, reported cases in which iguanas returned to their nests four days (in Panama) and even as much as 15 days (in Mexico) after laying their eggs suggest that Green iguanas in some populations return to check on the safety of their clutches (Wiewandt 1982).

During the first week of May, the baby iguanas start to hatch. Emerging from the central chamber of the buried nest, they dig their way to the surface. This takes up to seven days. However, they don’t simply emerge and then dash off into the forest, alone. On emerging, they sit with just their heads poking out, sometimes for as long as 15 minutes, and sometimes repeatedly disappearing and reappearing from the nest entrance. It seems likely that the iguanas are looking out for predators, but what’s particularly interesting is that the iguanas don’t just look out for predators, they also spend a lot of time observing other baby iguanas emerging from other nest holes (Burghardt 1977, Burghardt et al. 1977). By observing the behaviour of other hatchling groups, iguanas from one clutch seem to decide whether or not it is safe to leave the nest. Burghardt (1977) reported cases where baby iguanas belonging to four different clutches all emerged synchronously, an observation which led him to conclude that “nest emergence seems socially facilitated by visual cues” (p. 183). This is a far cry from the stereotypical image of the baby reptile crawling from its nest and immediately dashing off headlong into cover.

'Night-hopping' behaviour observed in hatchling Green iguana, reported by Burkhardt (2004) and illustrated by Tim Winkler.

Incidentally, what happens when baby iguanas emerge from their nests at night? Burkhardt (2004) reported some bizarre behaviour (observed through a night-vision camera) where babies would emerge, and then jump upwards, towards the brightly moon- or star-lit sky (Burkhardt 2004). I don’t think anybody has any idea what might be going on here and what, if any, significance this behaviour might have – it’s an interesting little mystery.

Pods of babies and looking after siblings

Here's a larger version of that photo from the 1977 Science cover. There are far better photos of this behaviour, but I don't have them to hand. Image (c) AAAS.

Juvenile iguanas form groups termed pods or chuletas (Burghardt et al. 1977), usually consisting of about four individuals. They indulge in a tremendous amount of social behaviour of the sort typically regarded as unique to mammals and birds, rubbing their bodies and heads against one another, displaying their dewlaps, nodding their heads and wagging their tails at each other. They engage in allogrooming (grooming other members of the social group). The young iguanas stay associated at night, when they sleep in close physical contact with other pod members, sometimes even lying on top of them. Baby iguanas definitely recognise their own kin, apparently using olfactory cues (Werner et al. 1987), and continue to stay with them for many months after hatching (Burghardt 2002).

While these pods obviously consist of siblings, they seem to exhibit some sort of structure, with one iguana acting as leader. Burghardt (1977) described and photographed cases where juveniles followed one another in a line through vegetation and across the ground, with the iguana or iguanas in the lead often looking behind to, apparently, check on the progress of the followers. At some point juveniles have to leave Slothia and swim to the mainland, and to do this, the iguanas have to make their way through a reed-bed before setting out across the water. Prior to departing, the juveniles were seen to engage in lots of head-rubbing and other physical contact, and the individual that appeared to lead the group was the one that engaged in the greatest amount of these activities. Invariably this was the first animal to enter the water and start swimming. If its companions failed to follow, it would return to shore. The iguana identified as ‘leader’ was also reported to disappear into the reeds and reappear with additional recruits.

It's difficult to get good photos of juvenile Green iguanas. I found this one on the web a while back; sorry, I cannot find the source anymore.

It is perhaps tempting to think from this that iguanas ‘look out’ for each other, or at least for their siblings. Such an interpretation might seem anthropomorphic, but it isn’t necessarily: we know from studies on diverse animals that kin selection can lead members of some species to exhibit behaviours that might favour the survival of their brothers and sisters. In Green iguanas, the idea that individuals really do ‘look out’ for siblings has received robust experimental support from studies of anti-predator behaviour. Noting that male and female Green iguana babies exhibited quite different types of anti-predator behaviour, Rivas & Levín (2004) flew model hawks at both lab-based and wild groups of Green iguana siblings. They showed that, while females tended to hide, stay motionless, or run away from the potential predator, males exhibited far more interesting and unusual behaviours: they ran in front of the model hawk, appeared from beneath cover (rather than hiding within it), and – most interestingly – covered their smaller female siblings with their own bodies, thereby concealing them from view.

It’s possible that these behaviours are selfish: the unusual responses of the males “might surprise a searching predator and give the escapee more time to escape at the expense of the remaining animals” (Rivas & Levín 2004). But it appears more plausible that this ‘covering behaviour’ is a hitherto undocumented form of fraternal care, where males are actually protecting their female siblings.

So there we have it. The significance of this behavioural complexity will not, I’m sure, have been lost on you. The stereotypical idea that lizards and other reptiles are far ‘simpler’ in behaviour and social life than are birds and mammals is demonstrably false (for some species at least), as is the idea that non-avian reptiles are more limited in what they can do relative to birds and mammals; many forms of behavior long imagined or often characterised as uniquely avian or mammalian are actually more widespread. And there are yet other complex behaviours that I haven’t discussed here which have also been documented in iguanas, including appeasement behaviour. So go forth and spread the word.

Let me make again the point that some male Green iguanas (this one is from Colombia) are spectacularly well adorned. That's not for camouflage - it's sex and show. Photo by BobisBob, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

PS – this article was originally going to be titled ‘Sorry Sarah Connor: that iguana really doesn’t like you’, but I couldn’t get a screen-capture of the appropriate scene in time. Some of you know what I’m talking about.

For previous Tet Zoo articles on interesting aspects of social behaviour in reptiles (living and extinct), see…

Refs – -

Bock, B. C. & Rand, A. S. 1989. Factors influencing nesting synchrony and hatching success at a green iguana nesting aggregation in Panama. Copeia 1989, 978-986.

Burghardt, G. 1977. Of iguanas and dinosaurs: social behavior and communication in neonate reptiles. American Zoologist 17, 177-190.

- . 2002. Walking with iguanas. BBC Wildlife 20 (5), 60-65.

- . 2004. Iguana research: looking back and looking ahead. In Alberts, A. C., Carter, R. L., Hayes, W. K., Martins, E. P. (eds) Iguanas: Biology and Conservation. University of California Press (Berkeley), pp. 1-12.

- ., Greene, H. W. & Rand, A. S. 1977. Social behavior in hatchling green iguanas: life at a reptile rookery. Science 195, 689-691.

Burghardt, G. M., Chiszar, D., Murphy, J. B., Romano, J., Walsh, T. & Manrod, J. 2002. Behavioral complexity, behavioral development, and play. In Murphy, J. B., Ciofi, C., de La Panouse, C. & Walsh, T. (eds) Komodo Dragons: Biology and Conservation. Smithosonian Institution Press (Washington, DC), pp. 78-117.

Distel, H. & Veazey, J. 1982. The behavioral inventory of the Green iguana Iguana iguana. In Burghardt, G. M. & Rand, A. S. (eds) Iguanas of the World: Their Behavior, Ecology, and Conservation. Noyes Publications (Park Ridge, New Jersey), pp. 252-270.

Dugan, B. A., Rand, A. S., Burghardt, G. M. & Bock, B. C. Interactions between nesting crocodiles and iguanas. Journal of Herpetology 15, 409-414.

- . & Wiewandt, T. V. 1982. Socio-ecological determinants of mating strategies in iguanine lizards. In Burghardt, G. M. & Rand, A. S. (eds) Iguanas of the World: Their Behavior, Ecology, and Conservation. Noyes Publications (Park Ridge, New Jersey), pp. 303-319.

Leal, M. & Powell, B. J. 2011. Behavioural flexibility and problem-solving in a tropical lizard. Biology Letters 8, 28-30

Lenz, S. 2004. Varanus niloticus. In Pianka, E. R. & King, D. R. (eds) Varanoid Lizards of the World. Indiana University Press (Bloomington & Indianapolis), pp. 133-138.

Montgomery, G. G., Rand, A. S. & Sunquist, M. E. 1973. Post-nesting movements of iguanas from a nesting aggregation. Copeia 1973, 620-622.

Rivas, J. A. & Levín, L. E. 2004. Sexually dimorphic anti-predator behavior in juvenile green iguanas Iguana iguana: evidence for kin selection in the form of fraternal care. In Alberts, A. C., Carter, R. L., Hayes, W. K. & Martins, E. P. (eds) Iguanas: Biology and Conservation, pp. 119-126.

Vitousek, M. N., Mitchell, M. A., Woakes, A. J., Niemack, M. D. & Wikelski, M. 2007. High costs of female choice in a lekking lizard. PLoS ONE 2 (6): e567. doi:10.1371/journal.pone.0000567

Werner, D. I., Baker, E. M., Gonzalez, E. del C. & Sosa, I. R. 1987. Kinship recognition and grouping in hatchling green iguanas. Behavioral Ecology and Sociobiology 21, 83-89.

Wiewandt, T. A. 1982. Evolution of nesting patterns in iguanine lizards. In Burghardt, G. M. & Rand, A. S. (eds) Iguanas of the World: Their Behavior, Ecology, and Conservation. Noyes Publications (Park Ridge, New Jersey), pp. 119-141.

Wilkinson, A., Kuenstner, K., Mueller, J. & Huber, L. 2010. Social learning in a non-social reptile (Geochelone carbonaria). Biology Letters 6, 614-616.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. vdinets 1:05 pm 09/17/2012

    I’ve just spent a year working with Gordon Burghardt, studying (among other things) play behavior in taxa not usually thought of as playful. There is a lot of those… It was a very useful experience to me, because now I am always on a lookout for play behavior, and I’ve already seen some where I didn’t expect.

    Link to this
  2. 2. naishd 1:09 pm 09/17/2012

    That’s great, lucky you. Last time I wrote about play behaviour in reptiles, I noted that some of the peculiar things pet turtles do could and maybe should be interpreted as play. The terrapins I used to have in my office, for example, used to detach the filter box from the side of the tank and then push it around in the water. They did this every day.

    Darren

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  3. 3. vdinets 1:14 pm 09/17/2012

    Yes, Gordon is doing some research on turtles as well :-)

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  4. 4. David Marjanović 2:05 pm 09/17/2012

    I don’t think anybody has any idea what might be going on here and what, if any, significance this behaviour might have – it’s an interesting little mystery.

    So they see each other better?

    Link to this
  5. 5. Heteromeles 4:39 pm 09/17/2012

    Speaking of play behaviors, I’ve seen stingrays playing “king of the hill” on the side of the tank at the Birch Aquarium in San Diego. They were swimming up the vertical wall, one atop the other, under the water inflow, until the topmost one couldn’t hold on and got swept off. Then they’d do it again.

    Still, they’re not as cute as ignuanas. I expect ICanHazIguanas to be up soon.

    Thanks for reposting this, Darren.

    Link to this
  6. 6. Bastique 5:10 pm 09/17/2012

    Thanks for you using my photo and attributing it. One correction. It is a Cuban iguana, but it was not photographed on Cuba. The beast in question was photographed in my back yard in Fort Lauderdale, Florida.

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  7. 7. Dartian 3:38 am 09/18/2012

    Darren:
    Some of you know what I’m talking about.

    Oh yes. We’re never told or shown what happened to Pugsley, but I’d like to think that the T-800 spared him.

    Link to this
  8. 8. naishd 4:01 am 09/18/2012

    I assume that T-800s don’t waste time on non-targets (except where their termination is mission critical, as with dogs that raise alarms), so I’m sure Pugsley was fine, so long as he kept his head down in Sarah’s apartment.

    Darren

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  9. 9. Dartian 5:03 am 09/18/2012

    I assume that T-800s don’t waste time on non-targets

    A T-800 once spent almost the entire length of a music video looking for the members of Guns N’ Roses – only to conclude in the end that shooting them would be a “waste of ammo”. So there!

    (Hmm. Were we supposed to talk about iguanas?)

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  10. 10. Rajita 1:24 am 09/19/2012

    “Incidentally, not all male Green iguanas are big, showy, territorial animals. Some are small, superficially female-like, and more cryptic.”

    That is interesting. Is this smallness genetically determined in Green igu-s? There were some theoretical studies modeling such multi-morphic males using the rock-scissors-paper game. In most cases I have read about there are three morphs that allow stability via the rock-scissors-paper model. Is there a middle morph known in these green igu-s which allow such a situation?

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  11. 11. David Marjanović 5:26 am 09/19/2012

    Won’t squamates prefer rock-paper-scissors-lizard-Spock?

    Link to this
  12. 12. Finback 8:22 pm 09/19/2012

    “I don’t think anybody has any idea what might be going on here and what, if any, significance this behaviour might have – it’s an interesting little mystery.” – maybe a little bit of worship of BOKRUG, THE WATER LIZARD?!

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  13. 13. monty_zoom 4:34 pm 09/20/2012

    Long time lurker, first time poster (here). I have lived with the same iguana for 18 years, and I just wanted to thank you for this. I just wanted to note that I have heard that some mothers stick around until after the babies hatch. The story goes that babies do not have the needed gut bacteria to digest local food. Thus, they eat mothers fecal matter and acquire said gut fauna. Do not know if it is true, but I thought I would add it.

    During this time of year (mating season), my iguana eats less, but still eats. He is also quite active. (Climbing, looking, exploring…) He always goes for his own reflection (in mirrors or windows or what have you.) Sometimes he charges, sometimes he just hangs out. They are very interesting creatures.

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