"The night is dark and full of terrors."

That was what Melisandre said, and it is true, especially if you are a tiny little lizard living in a dark, dense, rain forest. The terrors of the night come in many forms - giant centipedes with strong jaws and potent venom, giant geckos weighing ten times your body weight and capable of crunching your skull with a single bite, heat-seeking pit vipers that climb slowly up a branch, cat snakes with sharp night vision and the ability to track you down from smell alone even in pitch dark, owlets waiting for the slightest movement to pounce on you from above.

A large (~ 25-30 cm long) centipede in Long Island making a meal of an unfortunate bent-toed gecko. These centipedes are voracious predators.  This species is known to cause severe bites and reaches a maximum length of 33 cm (that's 7 cm longer than the Guinness Record holding species from South America)

When giant centipedes such as the one shown above can make short work of you, you have to be alert even while sleeping. Bay Island forest lizards almost always sleep facing the main stem of the plant - may be they can spot approaching danger better in this position.

 When you are a small (7-8 g) agamid lizard in a rain forest, pretty much anything larger than you is danger. Bay Island forest lizards have excellent day light vision. Their typical hunting strategy is to sit on a tree trunk or twig all day long, watching the forest floor for any sign of movement of an insect. The moment they spot one, the lizard sprints at the insect and grabs it. I once saw a Bay Island forest lizard jump from a tree trunk to catch a little wasp that was sitting on a leaf of a neighbouring plant, which he did in mid jump. Their alertness and visual capabilities allow them to escape predators during the day. Once, in Long Island (a small Island close to Middle Andaman, not New York), I came across one of these little guys running from a green bronzeback (Dendrelaphis andamanensis). Both parties ran across the road, probably without noticing me, but as soon as the snake saw me, it abandoned the chase. We have no idea how good their vision is at night, but many terrestrial animals with excellent day light vision do not have good night vision, and this seems to be the case with Bay Island forest lizards too. As soon as night falls, these lizards climb up to their favourite nighttime perch, a delicate branch or leaf, and settle in.

A long, thin Andaman cat snake, capable of climbing up the thinnest branches. Cat snakes have large eyes giving them good night vision, but it is the forked tongue that smells down stationary prey. Slight vibrations from its movement may be the only clue about approaching danger for the lizards.

Andaman scops-owl, a potential nocturnal predator

 In our recently published study, we explored this behaviour in two sympatric species, the Bay Island forest lizard and the short-tailed forest lizard, in the Andaman Islands. These two species take sympatry to extremes: one can often see both species sitting side-by-side on the same tree trunk. When it was time to sleep, both species seemed to prefer similar looking perches: a horizontal leaf or delicate branch, making us wonder why! We also thought that since the two species are very similar, there might be some sort of niche partitioning, perhaps sleeping microhabitat. We found that the larger the lizard, the higher its sleeping perch tended to be. The larger of the two species, Bay Island forest lizard (Coryphophylax subcristatus), tended to sleep on higher and outward branches of plants. Other than that, surprisingly, the two species did not seem to differ in their choice of sleeping sites and there seemed to be little evidence for these two species competing for sleeping perches. Even more surprising was that most of the lizards (74% C. subcristatus and 90.98% C. brevicaudus) slept with their head pointed towards the main body of the plant, and almost always on a delicate, unstable, horizontal perches. This was in sharp contrast to their diurnal behaviour: most of the time, one would find these lizards perched vertically on a tree trunk. Could this selection of sleeping perch and position be because they are better able to detect a snake or a centipede approaching from the same plant? 

However, this opens up another question: do these exposed perches make them easy targets for nocturnal raptors? We do not have any clear answers for this question. One possibility is that owlets and owls, which use primarily visual and auditory clues, may have a harder time detecting sleeping lizards as long as the lizards do not move. This would mean that most predation at night comes from predators such as snakes, centipedes, shrews, etc that can detect a sleeping lizard based on smell alone. If this is indeed the case, sleeping on exposed, unstable, branches or leaves, away from other vegetation is a good choice.

This behaviour of sleeping on thin, isolated branches and leaves is not restricted to Bay Island forest lizards. Elsewhere, I have seen other diurnal agamids such as long-tailed agamas (Bronchocela sp.), Oriental garden lizards (Calotes versicolor), blue-throated lizards (Ptyctolaemus gularis) doing the same. In most cases, they sleep with their head oriented towards the plant and tail hanging out from the distal end of the branch or leaf. On the other side of the world, there are studies on Anolis lizards exhibiting near identical behaviour, perhaps a case of convergent evolution of behavioural strategies as a response to similar selection pressures.

Girl on top: a female Coryphophylax in the Nicobar Islands decided to sleep with her hind-quarters on a male's head.

So, have we answered all questions? Do we really know why they do the things they do? Nope, we haven't even scratched the surface yet.