Stone salamanders preserve climate record
Yellowstone Park fossils show subtle effects of temperature.
Fossil hunters in Yellowstone National Park have discovered an unusual way to record the effects of climate change. Specimens from the past 3,000 years suggest that salamanders have grown bigger as the climate has warmed, and may continue to change as temperatures rise and lakes dry up.
During development, tiger salamanders (Ambystoma tigrinum) can metamorphose and head for land rather than staying in the water. And warmer climes have made salamanders on land outgrow their water-based relatives, says Elizabeth Hadly of Stanford University in California. Hadley and her colleagues examined almost 3,000 salamander vertebrae from the park's Lamar Cave in Wyoming.
The difference is particularly pronounced in the warmest period of Yellowstone's history, between 1,150 and 650 years ago, the researchers add. Hotter conditions allow for more abundant food and faster growth rates, they suspect, and such effects are expected to be less marked in the water, where temperature changes are smaller.
The researchers, who publish their results in BMC Ecology1, analysed fossils from 15 different layers of rock in Lamar Cave. They dated the deposits and divided them into five time intervals corresponding to five periods with different climates over the past 3,000 years.
Warm weather is indeed likely to encourage rapid growth in land-based salamanders, says Susan Evans, who studies anatomy and developmental biology at University College London. Food is likely to be more abundant, and the cold-blooded creatures would have a higher metabolic rate and therefore speedier growth.
In water, on the other hand, it is cold weather that encourages the salamanders to beef up: their level of thyroid hormone falls, so they keep on growing for longer.
Some experts argue, however, that it is not climate that has caused the land-based salamanders to grow larger. Other factors, such as overall population numbers, may also influence size by influencing the intensity of competition for food, notes Evans.
To really understand how different organisms respond to rapid climate change, Hadly and her colleagues say they need to look at other prehistoric time periods.
In hot water
Although the fossils of land and water salamanders differ in size, Hadly's team found no difference in the overall number of those living on the land, relative to the number in the water, suggesting that climate did not influence the decision to metamorphose.
"This demonstrates that species will respond to climatic change in ways we're not always able to anticipate," says Hadly.
Still, she predicts that if the climate continues to get hotter, the ponds will start to dry out in Yellowstone, forcing more salamanders to live on land. If the number of water-based salamanders slumps too much, she warns, the food chain of the lakes could be disrupted.
- Bruzgul J. E., Long W. & Hadly E. A. BMC Ecol., 5. 7 (2005).