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How are turtles born?

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An eastern box turtle laying eggs.
An eastern box turtle laying eggs.

By Stacey Venzel

Every turtle hatches from an egg. Each group of eggs is called a clutch, and each clutch is laid in a nest dug by the female.

Similarities and differences exist in the reproduction rituals among species from the interaction between males and females to the number of eggs in a nest.

COURTSHIP
Males begin looking for females as the weather warms up, with the majority of nesting occurring in the spring and summer. The chicken turtle is an exception, nesting mainly in the fall and winter.

In some species, the male attempts to attract the female with a display. Male river cooters, painted turtles and sliders vibrate their webbed claws near the female’s head, presenting themselves to her. Some turtles change color during mating season to attract mates, like the bright color shift in the body of the male painted terrapin. Other species, like snapping turtles, battle for the female in male combat. Berlandier’s tortoises are even known to bump their shell repeatedly against a female until she gives in.

A loggerhead turtle laying eggs.
A loggerhead turtle laying eggs.

MATING
Eggs are fertilized internally when a male climbs onto the back of a female and curls his tail toward her cloaca, inserting his penis. Mating can last anywhere from minutes to hours, varying between species. Males of a species that mount on land, such as tortoises and box turtles, have a noticeable indentation on their plastron, the term for their belly. For turtles that mount in the water, mating is made easier by the longer tails exhibited by males. Aquatic males also often have lengthier claws, like those of a green sea turtle that help him hook onto the female’s shell.

DIGGING NESTS
Except for the northern Australian snake-necked turtle and Central American river turtle, all females dig nests exclusively on land. The females can be very picky about the nesting location, looking for ideal substrate and temperature. Snapping turtles can travel a couple miles to lay eggs while sea turtles, known to return to the same beach where they were born, swim thousands of miles.

NUMBER OF EGGS
The number of eggs laid by a female varies between species and individuals. As a general rule, smaller turtles lay less eggs than larger individuals simply because there is less space to hold the eggs inside the body. Tiny mud turtles, for example, only lay an average of three eggs in a nest. Larger snapping turtle females can hold anywhere from 30 to 100 eggs. Sea turtles hold the record, though, not only because of their large size but also because of a hatchling’s low survival rate. Essentially, a large clutch size is an evolutionary trade-off to ensure at least some survivability when the odd’s are not in a hatchling’s favor due to high predation and heavy energy expenditure. Research on loggerhead and green sea turtles projects a female can lay upwards of 400 eggs in a nesting season, but few will survive to adulthood.

SIBLINGS
All hatchlings have the same mother, but some can have different fathers. Females often mate with different males before laying a clutch. Therefore, eggs within the same clutch can be fertilized by different males. While this phenomenon has not been observed in every species, it has been noted in many including sea turtles and gopher tortoises.

Sources:

J C Avise, D E Pearse, “Turtle mating systems: Behavior, storage, and genetic paternity,” Journal of Heredity, 2001, https://jhered.oxfordjournals.org/content/92/2/206.full.

Karen Eckert, David Gulko, Sea Turtles: An Ecological Guide (Honolulu, Mutual Publishing, 2004), 58-59.

Carl J Franklin, Turtles: An Extraordinary Natural History 245 Million Years in the Making (St. Paul, Voyageur Press, 2007), 31-33.

Whit Gibbons, Judy Greene, Turtle: The Animal Answer Guide (Baltimore, The John Hopkins University Press, 2009), 56-57, 59, 65-67.

 
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