TOEFL Reading Practice #024 — The Temperature That Decides

The Temperature That Decides

¶1 In most animals, an offspring's sex is fixed the instant an egg is fertilized, locked in by chromosomes inherited from each parent. Sea turtles and many other reptiles work differently. A hatchling's sex is decided weeks later, not by any gene it carries, but by how warm the sand around its egg happens to be. For decades this looked like little more than a curiosity. It has since become a closely watched process in climate science. A shift of just a degree or two in nest temperature can push a whole generation of hatchlings toward one sex.

¶2 Reptiles complicate the usual genetic story. Most turtle species have no sex chromosomes a microscope can tell apart, and a newly hatched turtle offers no external clue to its sex either. Confirming a hatchling's sex means directly examining its gonads, the internal organs that develop into ovaries or testes. That difficulty delayed the discovery for decades. In the early 1970s, the French biologist Claude Pieau ran one of the first controlled tests, incubating eggs of the European pond turtle at fixed temperatures. Eggs kept above thirty degrees Celsius hatched almost entirely as females; eggs kept below twenty-five degrees hatched almost entirely as males. Because every other condition in the incubator was identical, temperature alone explained the split, and biologists named the pattern temperature-dependent sex determination, or TSD.

¶3 The mechanism proved narrower than those early experiments suggested. Temperature does not influence sex throughout incubation; it matters only during a brief window in the middle third of development, called the thermosensitive period. Outside that window, temperature has no effect on sex at all, however extreme. Inside the window, warmth speeds up an enzyme that converts one sex hormone into another, tilting the balance that guides the embryo's gonads toward ovaries or testes. Each population has a pivotal temperature that produces an even fifty-fifty split; within a degree or two on either side of it, the outcome swings almost entirely to one sex or the other. A beach running just a degree or two warmer than its historical average does not add a few more female hatchlings; it can produce nearly all of them.

¶4 Two independent lines of evidence pinned down how sharply this switch operates. The first came from experiments that moved eggs between warm and cool incubators partway through development, rather than holding each egg at one constant temperature. [A] By shifting eggs a few days earlier or later each time, researchers mapped the thermosensitive period almost day by day. [B] The second line of evidence came from the nesting beaches, where researchers buried data loggers in real nests to record sand temperature and later confirmed each hatchling's sex. [C] Nests that stayed within a degree or two of the local pivotal temperature produced a mix of males and females. Nests that drifted outside that band hatched almost entirely as one sex, matching what the lab had already predicted. [D] Because a controlled experiment and an uncontrolled beach agreed, biologists could rule out other explanations, such as nest moisture.

¶5 The same sharp switch that makes TSD easy to map in a lab is what makes it risky under climate change. In 2018, the biologist Michael Jensen and colleagues sampled green sea turtles from the northern Great Barrier Reef. They found that more than ninety-nine percent of juveniles from the warmest beaches were female. A species like this needs only a small number of males each generation. Yet decades of nests this far past the historical pivotal temperature threaten to erase the male half of the population within a single generation. TSD matched hatchling sex to conditions that once changed slowly across centuries; now it faces a temperature shift arriving faster than any pivotal temperature can evolve to track.