105 學年度學測英文 第 54 題

Many marine animals, including penguins and marine iguanas, have evolved ways to get rid of excess salt by using special salt-expelling glands around their tongue. However, the sea snake’s salt glands cannot handle the massive amounts of salt that would enter their bodies if they actually drank seawater. This poses a serious problem when it comes to getting enough water to drink. If seawater is not an option, how does this animal survive in the ocean?

An international team of researchers focused on a population of yellow-bellied sea snakes living near Costa Rica, where rain often does not fall for up to seven months out of the year. Because yellow-bellied sea snakes usually spend all of their time far from land, rain is the animals’ only source of fresh water. When it rains, a thin layer of fresh water forms on top of the ocean, providing the snakes with a fleeting opportunity to lap up that precious resource. But during the dry season when there is no rain, snakes presumably have nothing to drink. Thus, the team became interested in testing whether sea snakes became dehydrated at sea.

The researchers collected more than 500 yellow-bellied sea snakes and weighed them. They found that during the dry season about half of the snakes accepted fresh water offered to them, while nearly none did during the wet season. A snake’s likelihood to drink also correlated with its body condition, with more withered snakes being more likely to drink, and to drink more. Finally, as predicted, snakes captured during the dry season contained significantly less body water than those scooped up in the rainy season. Thus, it seems the snake is able to endure certain degrees of dehydration in between rains. Scientists believe that dehydration at sea may explain the declining populations of sea snakes in some parts of the world.