Amazing social life of the Green iguana

Hooray: another of those articles that I've been promising to publish for weeks and weeks.

Thanks mostly to the importance of the species in the international pet trade, the Green iguana Iguana iguana is typically imagined as a rather uninspiring lizard that sits around on branches all day long, occasionally munching on 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 the social life of 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...

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, often selecting trees that are dead or sparsely vegetated. They proclaim their territory with lots of head-bobbing and displaying of the large dewlap. Any males that try to move in to the area are chased away, but between four and eight females move into the territory, where they compete among themselves for access to high-quality territory-holders (Burghardt 2002). Males go without eating during this territorial breeding phase, they have to put energy into changing their appearance (they change colour from greenish to orangish and increase the size of their jowls), and they also have to chase off rival males, and woo and mate with females. But as has just been revealed by a new study of another lekking iguana, the Galapagos iguana Amblyrhynchus cristatus, females may incur high energetic costs in discriminating among potential mates (Vitousek et al. 2007), so they don't necessarily have an easy time either.

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: we've previously encountered this sort of thing when covering 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.

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).

Iguana nests are not just scrapes in the ground: rather, 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 when creating their own nests.

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 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.

Juveniles form pods, 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. Those iguanas that hatched on Slothia have to, at some point, leave the island and swim to the mainland, and to do this, they 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 [unfortunately I couldn't find any available photos of wild groups of juveniles, hence the captive group shown in the adjacent image].

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, because we know from studies on diverse species 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 young Green iguanas 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 might be 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. If you want to know more about this study, you can read it yourself here (from Rivas' iguana page).

So there we have it. The significance of this behavioural complexity will not, I'm sure, have been lost on you. The stereotyped idea that lizards and other reptiles are far 'simpler' in behaviour and social life than birds and mammals is demonstrably false, and many forms of behavior long imagined as uniquely avian/mammalian are actually far more widespread. Other complex behaviours that I haven't discussed have also been documented in iguanas, including appeasement behaviour for example. Recall that we now think that play behaviour is practiced by reptiles including turtles, crocodilians and monitor lizards (for more on this subject see the ver 1 article Dinosaurs come out to play). So go forth and spread the word.

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. Extra points if you know what the hell I'm talking about.

PPS - I've just learnt that Discovery News did a piece yesterday about the Galve paper I recently published with Barbara Sánchez-Hernández and Mike Benton: the news article is here.

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.

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

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

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 [available here]

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.