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.

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

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.]

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.]

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).

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.

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

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 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.

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 - -

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

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