Storytelling Science

Dangerous Predators in your Home Zoo

Amitabha Mukerjee


 

Chikku, and her brother Tikku, are neighbourhood kids who are good friends of mine. Once, Chikku had a school trip to the zoo. It was a great picnic, and they learned about many animals.

That evening Tikku came to my house. He was very sad, because Chikku had gone to the zoo and not he. So I arranged a private zoo visit for Tikku, right there in my house. I picked him up, and asked him to lift a framed photo which hangs on the wall. Behind it, inches from us, a magnificient predator was staring at us with its dark beady eyes. It was a gecko! Though Tikku had seen geckos before, he found this close encounter fascinating. He banged the photoframe and the gecko darted out. He and I chased the gecko and it again disappeared behind the frame.

All of us have seen geckos in and around our homes. It is fascinating to watch a gecko on the hunt. It waits patiently, its eyes fixed on the moth. Suddenly, when the moth is not paying attention, it scurries forward. At the corner of the wall, it will leap across with a flick of its tail. Wait, move, wait, move. Often, its prey will fly off, but occasionally you will see the gecko catch its prey a gruesome sight.

While we find this gruesome, it is actually a great education on how nature works. Tikku and I have discussed it in length, and now he understands that the gecko is helping the insect population by catching the weaker ones; only the stronger ones will have babies in the next generation. Without this evolutionary push from predators, humans may never have evolved and you would not be reading this article today.

Have you heard the gecko's repetitive call? Indonesians hear its call as "ge-kok-ge-kok" and call it gekok, which is how it got its English name. That's also why Bengali's call it "tik-tiki". Tikku was surprised to learn that the gecko is the only lizard that makes a sound. Many people believe geckos are poisonous, but the fact is that the gecko is not poisonous at all, though it can carry many parasites.

The next day, Tikku came and banged the photoframe many many times. Then we saw an amazing spectacle. After one such bang, suddenly there was wriggling mass on the floor. It was the gecko's tail! This is also a common gecko behaviour, it drops its tail when a predator (e.g. a snake) may catch it from behind. The tail kept wriggling on the floor for nearly a minute. Although its tail will grow back, I must look after this gecko, for it looses its fat reserve when its tail falls off, and it can't jump as effectively now.

Two years ago, an important discovery was made about geckos. It was known that the gecko can support upto 40 times its own weight (about 4 kg) even on smooth glass. This could be explained either by wet capillary adhesion (as in frogs), or by a molecular force called van der Waals. It was found that the gecko's clinging force did not change on materials of different wetness, so van der Waals was the only explanation.

The gecko's toes have fine hairs, called setae, each about 5 microns wide, ending in upto a thousand even finer spatula, each about 0.2 microns. When these are very very close to the wall, the electrons on the spatula interact with electrons in the wall, and statistically, more are aligned than are not, resulting in an attractive force. The equation for this force is A÷(6πD3) where D is the distance between the surfaces, and A is a very small constant -- around 10-19 for most materials. Only when D is extremely small -- about 1/2000th millimeter, does this force become significant. When the gecko unrolls the tiny fibers on its feet along the wall surface very large forces can be generated.

Tikku was fascinated by all these details and crawled around for hours imagining he was a gecko. Once he bit Chikku, with very bad consequences. Today Tikku and Chikku have gone to see Spiderman 2. What I am dying to tell them after they get back is that the spider's grip on the wall is also due to van der Waals, and in fact, scientists are now trying to create the setae structure in nanomaterials, so that someday you can walk on walls as efficiently as a gecko.

Still, many questions remain unanswered e.g. if the forces are so high, how does the gecko lift its leg so easily? I am sure someone in Tikku's generation will finally find these answers.