How did they count them?
Using a technique called 'mark-recapture'. Captured snakes are marked with a tiny, harmless microchip tag, which is injected under their skin. Each tag has a nine digit number, which can be read by passing it over a scanner, a bit like a food item in a supermarket. By recording how often the tagged snakes are recaptured, they can also estimate the number of snakes that haven't been caught, using a complicated calculation called Begon's Weighted Mean.

What else did they find out?
Over half of the snakes had scars as a result of rat bites, and 43% of them had lost part of their tails.

In 1995, female snakes outnumbered males by almost two to one. This meant that there was a high risk of inbreeding, because the females would not have enough males to choose from.

What's the latest news?
The next survey in early 1997 found that the snake population had doubled since the rats were removed, although numbers fell slightly two years later, after a severe hurricane flooded the island (see Natural Disasters). Recent studies have found that most snakes are now growing larger, looking healthier, and have fewer injuries.

For more detailed facts and figures, see Vital Statistics