Evolution Intelligent Design

Claim: Turtle embryos can choose their sex

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In the sense of deciding which part of the egg to spend time in. But not everyone is convinced:

Developing reptile embryos that have temperature-dependent sex determination were long thought to passively accept their weather-based fate. But now, new research suggests that at least in M. reevesii, embryos can move around to find slightly cooler or warmer regions within their eggs, and thereby have some control over the sex they develop into. The authors propose that this ability may help offset drastic, population-wide shifts in sex ratios that are occurring due to climate change. The findings appear today (August 1) in Current Biology.

“I find it fascinating that [embryonic movements] may be an important contributor to the production of equal numbers of males and females at intermediate conditions in the wild,” writes evolutionary biologist Nicole Valenzuela of Iowa State University to The Scientist in an email. “It is quite a novel finding.” …

Fredric Janzen, an evolutionary biologist at Iowa State University, is yet to be convinced that embryos can shift around the egg and influence their sex. As he and others have argued previously, the idea is implausible for several reasons. “In a real nest, they’ve only moved basically a millimeter—a millimeter in a thirty-four-millimeter egg . . . there’s no thermal gradient in a millimeter,” he explains. “And really, there’s no way for them to move because the yolk sac is large, it’s surrounded by an impenetrable membrane—so it’s not like these things can just swim.


Katarina Zimmer, “Turtle Embryos May Have a Say in Deciding Their Sex” at The Scientist

No claim is being made that the turtles in the eggs are actually weighing the options, just that their individual pre-hatching movements would make a difference. Some have made such claims though:

… there is the weird habit of attributing impossible thought processes to animals who are supposedly thinking in Darwinian terms about how to ensure the survival of their selfish genes. For example, the mares who supposedly cause an abortion because they perceive that the stallion will not accept another stallion’s offspring. The mare presumably knows all this and can act on it—even though she cannot otherwise perform the simplest reasoning tasks.

And there’s the Darwinbird of pop science who “is optimising the likelihood of her offspring mating and rearing young (so ensuring the continuation of her genes into future generations)” by controlling the sex of her offspring: “There is some evidence she can bias the sex ratio by controlling hormones, particularly progesterone.” So we are really to believe that a bird who is too stupid to evade a giant wind turbine is planning her offspring’s future? Does “science-based thinking” really require that of us?

In real life, what’s fascinating is how complex it all turns out to be, yet there always seem to be enough turtles of each sex.

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