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Scientists Find World's Smallest Lizard - 2001-12-04

Two U.S. biologists have discovered the world's tiniest lizard inhabiting, appropriately enough, a very tiny Caribbean island.

The relationship between the island's diminutive area and the size of the lizard may not be a coincidence.

Beata Island is a 42 square kilometer piece of real estate off the southern tip of the Dominican Republic, part of the country's Jaragua National Park. University of Puerto Rico biologist Richard Thomas and his Pennsylvania State University colleague Blair Hedges went there to search for previously unknown species of animals.

To their delight, they found small groups of a tiny species of gecko living in a sinkhole and a cave. Mr. Thomas says they are so tiny that one could curl up on your fingernail. "The total length with the tail would be about 36 millimeters," he notes.

It ranks not only as the smallest lizard, but also the smallest of all 23,000 known species of reptiles, birds, and mammals, according the scientists' study in the December issue of the Caribbean Journal of Science. "It appears to be the smallest in the world. In actuality, it's equal in size to another species that I discovered some years ago that likewise is at the lower size limit for lizards, snakes, birds, and mammals."

In other words, don't expect to find anything much smaller in this group of animals. The tinier an animal gets, says Mr. Thomas, the larger its surface area gets as a percentage of its total volume, which stresses the body in some way.

"It could be dehydration," he says, "because the smaller an animal is, the greater its surface area is and surface area is a site for water loss. So if you get very small, you're very easily dehydrated. This could be a factor in limiting the small size of these lizards."

These lizards survive the dry Beata Island environment because their habitat is moist leaf litter. The researchers say that the animals shrivel and die from evaporation without it.

At the U.S. National Science Foundation, which funded the research, environmental biologist Larry Page says the mysteries about the physiological constraints associated with decreasing body size make this lizard an interesting discovery.

"Really tiny things can have physiological problems that we're not sure they could overcome," Mr. Page says, "just looking at the broad range of variation in organisms. Then when we find some exception like this, it tells us that 'wow!' evolution really can come up with some unique organisms, so this is a very exciting find."

But if small size is a disadvantage in some ways, it helps in others. It may make survival possible, especially on an island. The smallest species of animals tend to be found on islands. The researchers speculate that this may be because species evolve there to fill ecological voids left unfilled by other organisms that never reached the remote locations. For example, if an island lacks a species of spider, the lizards might become very small to fill the missing spider's ecological role.

Richard Thomas says it may also be that on a small area of territory, more small animals can survive than big ones. "The probability of going extinct increases as your population size decreases," he says. "So if you have a given island and the size of the individuals is very small, then the island is going to support a larger number of individuals. Your population size will be larger and therefore its probability of going extinct is less."

The researchers say their discovery of the little lizard in an area well studied by biologists illustrates just how little we know about the Earth's estimated 10 - 100 million plant and animal species. So far, 1.5 million have been discovered and described and the little gecko makes one more.