By Stacey Venzel
Mammals rely on X and Y chromosomes to determine the sex of an embryo. Most turtle species (and a few other reptiles) rely on the environment, not genetics, to decide if the baby will be a boy or girl.
A few turtles are genetically sexed at the time of fertilization, but the rest of the species are sexed based on the egg temperature, a characteristic that is influenced in many ways by its immediate surroundings.
THE ROLE OF TEMPERATURE
During a turtle egg’s time below ground, a small but unique and precise temperature range in a specific part of the incubation period is influential in distinguishing male and female turtles, with females tending to develop in warmer temperatures. The key temperature in olive ridley turtles is 84.434 degrees Fahrenheit, but just 83.732 degrees in loggerheads; slight deviances above or below result in girls or boys.
OTHER ENVIRONMENTAL FACTORS
It is important to note that temperature-dependent sex determination is individually based, not in relation to a nest at large. Regions within the same nest can have different temperatures, such as the bottom of a deep nest hole being cooler or part of the nest residing in the shade or being exposed to less sunny days, in which instance more males might generate.
When a turtle nest has an approximate equal ratio of males to females, it suggests the presence of genetics in sex determination. For this reason, softshell turtles are believed to be chromosome-dependent. Genetics are also responsible for the development of boys and girls in giant musk turtles.
Karen Eckert, David Gulko, Sea Turtles: An Ecological Guide (Honolulu, Mutual Publishing, 2004), 68.
Carl J Franklin, Turtles: An Extraordinary Natural History 245 Million Years in the Making (St. Paul, Voyageur Press, 2007), 34.
Whit Gibbons, Judy Greene, Turtle: The Animal Answer Guide (Baltimore, The John Hopkins University Press, 2009), 67-68.