by Whit Gibbons

January 27, 2008

Global climate change is being tracked by ecologists in many ways, including documentation of earlier flowering in some plants, earlier nesting by wood ducks, and earlier emergence from hibernation by some reptiles. What other ways could gradually rising temperatures over a few decades affect the world's organisms?

One way could be 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 temperatures of the ocean continue to rise, the silversides in South Carolina would presumably shift toward being all males. Not a good strategy for any species. But a study with silversides suggests that the species can adjust its sex ratio in response to even relatively short-term environmental change such as rising temperatures.

In understanding the complete biology of the Atlantic silversides, scientists studied the same species in Nova Scotia. 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, water temperature partially affects the sex ratio, but not as strongly as in South Carolina. That is, cold water still results in a shift toward more females, but the ratio is less skewed than in South Carolina. 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 might happen if we experience the global warming trend predicted by many scientists. If we artificially raise the earth's temperature by several degrees in only a few decades, the temperature change could have dramatic effects on other systems that have not evolved to make adjustments. 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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