by Whit Gibbons

June 8, 2014

Global climate change is being tracked and ecological changes have been confirmed by ecologists in many ways. Researchers have documented premature flowering in some plants, earlier nesting by wood ducks, and some reptiles emerging from hibernation in the winter. I have written about the following phenomenon before. It is yet another way in which gradually rising temperatures could affect the world's organisms over a few decades.

The importance of environmental temperatures on animals throughout the world is incontestable. One way is by altering sex ratios in some species. For example, in some turtles and alligators one sex is produced at high incubation temperatures and the other at low temperatures. Jim Spotila of Drexel University has even suggested that the influence of temperature on sex determination may have caused the extinction of some dinosaurs. At the end of the Mesozoic era, major temperature changes occurred on a global scale. If incubation temperatures determined sex in some of these ancient reptiles, when average temperatures throughout the world rose or dropped several degrees over a short period of geologic time, some species may have begun producing young of only one sex. If so, whether male or female, the final result would be no more mating. And no more offspring.

A small coastal fish, the Atlantic silverside, also becomes male or female depending on temperature. Although genetics has an influence, the sex of these fish is determined to some degree by the temperature of the water during the larval period. Along the South Carolina coastline, silversides born during the cool temperatures of spring are predominately female. Those born during summer are mostly male. Because silversides are born during both spring and summer, over the course of a year approximately equal numbers of males and females are produced. If ocean temperatures continue to rise, the silversides in South Carolina would presumably shift toward being all males. Not a good strategy for any species. But one study with silversides suggests that the species can adjust its sex ratio in response to rising temperatures.

Scientists studying Atlantic silversides in Nova Scotia found that at that latitude, temperature has no effect on sex determination. Instead, sex is determined genetically. In between South Carolina and Nova Scotia, along the New York coast, cold water still results in a shift toward more females, but not as dramatically as further south. This suggests that an intermediate state exists between genetic and environmental influences.

A 50:50 sex ratio is characteristic of most animal populations, with one female being born on average for every male, although temperature clearly can affect the sex ratio in silversides. In laboratory experiments, researchers raised thousands of Atlantic silversides at constant high or low temperatures, simulating those temperatures most likely to produce males or females in South Carolina and New York. After each generation, the scientists determined the number of each sex. Some of the experimental populations started with many more of one sex than the other. But, sure enough, after several generations each experimental population had reached a balanced sex ratio of the same number of males as females, regardless of the water temperature.

Remember, if left at a constant temperature, silversides from South Carolina and New York should have an excess of one sex or the other. Yet the sex ratio gradually became balanced during the experiments. The explanation is a complex relationship between genetics and environmental conditions, and how the adjustment is made is not completely understood. The findings with Atlantic silversides emphasize the subtlety of the response of organisms to environmental change and also confirm the complex and sensitive balance in which the earth's ecosystems rest.

But just because Atlantic silversides can adjust their sex ratio with changing temperatures should not make us complacent about what might happen if we experience the global warming trend predicted by many scientists. Will all life on earth be able to adjust to global warming the way silversides can, or will some go the way of the dinosaurs?

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