6.16 THIS IS HOW WE DO IT: Can the environment determine the sex of a turtle’s offspring?

Unexpected observations may be a sign that our ideas about how the world works are not quite right. As such, they provide a great opportunity for scientists to solve a problem. Figuring out how to approach the problem, however, can be like solving a puzzle, and the solution can have far-reaching implications and unexpected importance.

In 1966, Madeline Charnier reported a surprising observation in an obscure publication from West Africa. For a lizard species she observed, it seemed that the sex ratio of the offspring produced was influenced by the environment. In warmer temperatures, most of the eggs that hatched contained females. And in cooler temperatures, most of the hatched eggs contained males.

As members of a species in which our sex is determined by the chromosomes we inherit from our parents, it isn’t surprising that we’d assume that sex determination would be the same for all animals. Charnier’s observation, however, which spurred biologists to notice similar patterns of sex determination in other reptiles and some amphibian species, suggested that our understanding of sex determination wasn’t quite right.

To test whether incubation temperature really could influence offspring sex, researchers conducted a laboratory experiment. They set up incubators at two different temperatures: one cool (25° C) and one warm (30.5° C). They then collected turtle eggs. From each clutch of eggs, they put half of the eggs in the cool incubator and half in the warm incubator. As the eggs hatched, the results were as clear as could be. At 25° C, all 210 offspring produced were males. At 30.5° C, all 211 offspring produced were females. The researchers concluded that—at least in the lab—the temperature could influence the sex of the offspring.

In the initial lab experiment, the temperature was constant. In the turtles’ natural habitat, it fluctuates every day: it is cool at night and warmer in the day. To better approximate natural conditions, the researchers conducted two additional experiments. First, in the laboratory incubators they created fluctuating temperatures. In one, the temperature fluctuated from 20° to 30° C and back again each day. In the other, the temperature fluctuated similarly, but between 23° and 33° C. The results were identical to the first experiment. In the cooler incubator, 100% of the offspring hatching were male. In the warmer incubator, 100% were female.

In the final study, the eggs collected from each clutch were again divided evenly, but this time they were incubated in the turtles’ natural habitat. Half were buried at a shaded nesting site that received little sun exposure (and rarely exceeded 30° C). The other half were buried at a nesting site that was exposed to the sun (and often exceeded 30° C). Of the 100 eggs hatching at the shaded site, all were males. Of the 127 eggs hatching at the exposed site, 123 were females. Based on these results, would you be confident that incubation temperature does indeed influence the sex of the offspring? Can you think of any alternative explanations?

Based on models of climate change, it is likely that the average temperature in North America will increase by about 4° C over the next 100 years. Scientists have predicted that such a rapid change could have a significant impact on biological systems. There has been relatively little direct empirical evidence for such impacts, however. One researcher, though, decided to test this prediction using temperature-dependent sex determination in turtles.

Frederic Janzen monitored 390 nests of painted turtles over a five-year period. In each year, he identified every nest on an island in the Mississippi River. He also noted the average temperature during the month that the eggs were incubating. After the eggs hatched, he brought the turtles into the lab, where he could determine their sex.

Janzen discovered a strong linear relationship between the average temperature and the offspring sex ratio.

He concluded that an increase of even a few degrees could result in the extinction of the painted turtle as a result of producing no males. He further suggested that this was just one possible dramatic impact of rapid climate change and that it might be a bellwether for broader biological consequences.