Reptile theory turns turtle

The reproductive behaviour of a turtle species overturns a theory that held that reptiles always lay eggs on land

 
Last Updated: Saturday 04 July 2015

Exception tests rule: a long-n Millions of years ago, some adventurous amphibians left their watery abode, forayed on to beaches and evolved into reptiles that started laying eggs on dry land, breaking away from their ancestors, who laid eggs in fresh water. The reptile eggs were shrouded by a special fluid-membrane called the amniotic membrane, which kept the embryo moist, a function earlier served by water.

Till recently, scientists believed that reptiles always laid their eggs on land. This, they said, was essential, because the developing embryos need oxygen and would drown under water. But this belief turned turtle when scientists, tipped off by Australian aborigines, discovered a not-so-well-known turtle, the Northern Long-necked Turtle, that laid its eggs in water (BBC Wildlife, Vol 12, No 5).

Despite 2 years of combing areas where pregnant turtles gather, scientists had failed to track down the nesting sites of this species. Then they surgically implanted radio-transmitters in dummy eggs within the oviducts or egg-sacs of the gravid females. The idea was that the electronic spies would get deposited in the nest along with the real eggs, signalling the nest's location. That did the trick -- of the 11 females released, 2 deposited the transmitters, and the cover was blown.

Although contrary to common perception, this turtle's egg-laying habits follow a logic of their own. The climate in this part of Australia swings wildly between monsoonal and severe drought, when all waterholes dry up. So this turtle produces eggs that can somehow survive under water in a state of arrested development until the floods recede. Only then do the eggs start developing, under the hardened mud, emerging about 6 months later with the return of the rains. By using this unique trick, females find time before the dry season to lay 3 or 4 clutches, rather than just the 1 or 2 they could have fitted in if they tarried till the rains stopped.

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