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What factors determine how many eggs a turtle lays?

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An Olive Ridley turtle laying eggs in a nest on the beach.
An Olive Ridley turtle laying eggs in a nest on the beach.

By Stacey Venzel

Sea turtles lay hundreds of eggs in a season compared to only a dozen in box turtles, but evolutionarily, they are contributing to their species’ survival at the same rate. When it comes to evolution, it’s all about trade-off.

The number of eggs a turtle lays is determined by the size of the eggs, the size of the female and the offspring’s survivability. Natural selection of egg production evolved to prevent species populations from extinction, which means even though sea turtles lay hundreds of eggs while box turtles only lay a few, they both produce the same amount of mature offspring in a lifetime.

EGG SIZE
An individual turtle can hold more marble-sized eggs than golf ball-sized eggs simply because more of the smaller eggs will fit. The same concept can be applied to a jar. You could fit hundreds of marbles into a jar but only a few golf balls and maybe one orange. With the exception of two-inch leatherback eggs, sea turtle eggs look just like ping pong balls whereas elongated box turtle eggs are closer in size to a quarter. The size difference in eggs, however, is insubstantial when compared to female body sizes between these turtles.

FEMALE BODY SIZE
Similar to egg size, a larger body makes room for more eggs. You could fit more marbles into a 1 cup measuring scoop than a 1/4 cup scoop just like more eggs fit inside a sea turtle than a box turtle or an itty bitty mud turtle. Sea turtles weigh hundreds to thousands of pounds while box turtles are lucky if they hit double digits and mud turtles won’t even tip the scale. Even within the same species, larger females lay more eggs than smaller females because the size increase allows for added egg space.

OFFSPRING SURVIVAL RATE
On an individual level, the chances of a hatchling making it into adulthood—and therefore being able to pass on its genes to further the species population—varies between species. In sea turtles, though 90 percent of the eggs hatch, less than 1 percent reach sexual maturity. These are the worst odds of any turtles, which is a strong indication of why sea turtles lay the most number of eggs. They have to in order to survive as a species.

OTHER FACTORS
In addition to size and survival rate, some other evolutionary adaptations are noteworthy for their role in the number of eggs. Energy requirement for the female’s survival as well as how much she can expend to her egg must be taken into account. For sea turtles, a lot of hatchlings also ensures that the little turtles can rely on each other to dig out of the deep nest, an evolutionary marvel known as protocooperation. A shallower nest would increase vulnerability while fewer eggs would mean suffocation upon hatching from an inability to reach the beach surface.

SOURCES:
Karen Eckert, David Gulko, Sea Turtles: An Ecological Guide (Honolulu, Mutual Publishing, 2004), 26, 66, 72-73.

Whit Gibbons, Judy Greene, Turtle: The Animal Answer Guide (Baltimore, The John Hopkins University Press, 2009), 58-59, 66-67.

Jennifer Viegas, “Sea Turtle Baby Boom Breaks Records,” Discovery News, February 1, 2012, http://news.discovery.com/animals/sea-turtle-120201.htm

 
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