PEOPLE HAVE MORE QUESTIONS ABOUT TURTLES

by Whit Gibbons

July 26, 2009

Last week I began answering questions about turtle reproduction. Here are more turtle questions and their multipart answers.

Q. Do all turtles and other reptiles lay eggs, and do they come in different shapes and colors?

A. Turtles, like birds and crocodilians, lay eggs with a shell. Most, but not all, snakes and lizards also lay eggs; some species give live birth. Two unusual mammals, the duckbill platypus and the spiny anteater, also lay eggs. Turtle eggs do not vary in color the way bird eggs do; the eggs of all turtles and other reptiles are white or cream colored. Turtle eggs do vary in shape, with some species laying completely spherical eggs while the eggs of others are elongate.

In North America, the largest species of turtles, such as snappers, sea turtles, and softshells, lay spherical eggs. Smaller species, such as mud turtles, painted turtles, and spotted turtles, lay eggs that can be twice as long as they are wide. However, as is often true in biology, trying to identify a pattern can be difficult. Thus, to say that all big turtles lay round eggs and all little ones lay oblong ones works for U.S. turtles but not worldwide. For example, the small flap-shelled turtles of Asia lay round eggs whereas the large river terrapins of India and Southeast Asia lay elongated eggs.

Egg shell texture varies considerably among different families of turtles The softshell and mud turtles lay eggs that are hard and brittle; they feel like a bird's egg. The eggs of slider and painted turtles, as well as many others, have leathery shells. Some species are intermediate between the brittle texture and the leathery.

Q. How long do female turtles hold eggs in their body?

A. How long a female turtle can hold shelled eggs in her body and still have them be viable is known for only a few species. One source of information is from studies of loggerhead and green sea turtles. The time interval between nesting bouts of individual females was determined on nesting beaches in Georgia and Costa Rica. Each female sea turtle was given an ID tag and could be recorded each time it came to the beach to nest. The shortest average period that has been documented under natural conditions is about two weeks, although some laid eggs and returned to nest again in only eight days.

Researchers at the Savannah River Ecology Lab (SREL) reached a similar conclusion-- two weeks to a month--for sliders and for eastern mud turtles in South Carolina. The determinations were based on x-ray photography. Female turtles were collected and x-rayed as they left a lake to nest and then again when they returned to the water. Eggs are quite apparent on a radiograph, and a full complement of eggs in a female was visible on the day of her departure.

When the female reentered the wetland one to several days later, if no eggs were visible on the radiograph it meant the female had nested. Some females nested twice during the same season and so were again found to be carrying eggs. In this way a minimum time period was established for how long a particular female had held eggs. The simple calculation was based on the day on which no eggs were present until the day she was caught again as she returned to lay a second time. The time period for holding eggs could be estimated when she reentered the lake again with no eggs present. Based on hundreds of captures of slider and mud turtles with eggs, a range of time for how long shelled eggs are held could be established for these two species.

The most dramatic example of female turtles carrying eggs for long periods is the chicken turtle. Kurt Buhlmann at SREL used x-ray photography to determine that some females held their eggs for up to six months without laying them in a nest. Yet the eggs produced healthy young turtles, proving that some turtles can hold eggs for long periods and still have viable offspring.

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