PEOPLE HAVE MANY QUESTIONS ABOUT TURTLES

by Whit Gibbons

July 19, 2009

How do turtles reproduce? Most U.S. turtles lay their eggs from late spring to midsummer, and many people have observed nesting turtles during the past few weeks. These outdoor encounters have led to this and many other questions about turtle reproduction.

Reproduction methods among turtles follow the basic format of most other vertebrates. The mating season for most temperate zone species begins in late winter or early spring as temperatures begin to rise. During this time, males begin seeking females and engaging in courtship activities. Turtle biologists assume that all species have a ritualized courtship process that leads to the mating event itself, but courting behavior in the wild has been observed in very few turtles.

Male-male combat occurs most commonly in animal species in which males get larger than females, which includes white-tailed deer, wild turkeys, and humans. The same appears to be true for all species of turtles in which males are larger, but evidence is anecdotal and fighting is not well documented. In snapping turtles as well as some species of tortoises, adult males will engage in physical combat with other males. The purpose of such behavior in turtles as well as other animal species is for a male to become dominant by driving other males away from its territory, therefore preventing them from an opportunity to mate with local females. Male tortoises of some species will charge one another head-on and butt their shells together. The loser is the one that eventually retreats or, in rare instances, is turned over on its back. Snapping turtle males have been reported to fight in the water, creating turmoil on the surface of a lake or pond as they claw and bite each other. Observers of snapping turtle combat differ in their interpretations of the details of the process, but in larger bodies of water the loser usually retreats to another area and may even migrate to another lake or stream. In turtle species in which the males are smaller than the females, aggressive male-to-male behavior does not commonly occur.

Turtle biologists have observed complex and intriguing courtship rituals in some species of turtles. Adult male painted turtles, slider turtles (which include the common red-eared sliders that are kept as pets), and some map turtles have elongated foreclaws that are used in an elaborate courtship behavior called "titillation." During this fascinating ritual, the male turtle extends his front feet and turns them so that the backs are touching. Then he vibrates his long claws in the water in front of the female. A female interested in mating will follow the male, who slowly swims backward. Other courtship rituals undoubtedly occur in various species of aquatic turtles, but the chances are slim that someone will observe a short-term event that occurs underwater for (at most) a few days once a year. One of the best opportunities to see the titillation behavior is at a public aquarium in which freshwater turtles can be observed through glass in natural underwater settings.

Actual mating by most or all turtles occurs by the male climbing on the back of the female and curling his tail under hers. Male turtles of most species have noticeably longer tails than females. A general rule is that the male of turtles that mate on land has an indented area on the lower shell so that he is not balancing precariously on her domed upper shell. Such indentations are evident in many male tortoises and box turtles. Most aquatic turtles that mate in the water, including sea turtles, have mostly flat lower shells, as the buoyancy provided by the water allows the male to maintain his position during mating. The actual mating process may last for several hours in some terrestrial box turtles and tortoises.

Each year during turtle nesting season I receive dozens of questions about the reproduction process for turtles. This year was no exception. I have been asked numerous questions during the past few weeks. I will answer some of them, such as whether all turtles lay eggs, in next week's column.

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