Complex amphibian responses to past climate change
November 18, 2010 1 Comment
We were like gophers, but now we’re like voles
Timeline, 2005: There is a cave in Yellowstone packed with fossils from the late Holocene, from about 3000 years ago. We can glean from this trove of stony bone how different taxa respond to climate change at the morphological and genetic levels and define and make predictions about the current response of the world to such changes.
The cave, which is in a sensitive area of the park off limits to visitors, houses the fossilized bones of rodents, wolves, amphibians, bears, coyotes, beavers, and elk, among others. This fossil cornucopia has yielded so much in the way of stony evidence that sorting it all is in itself a mammoth task. But two climatic stories have emerged from the samples it has yielded.
A global warming story…from the Middle Ages
The first story is about salamanders and climate change. No, it’s not a 21st-century story about global warming, but a Middle Ages story about a hotter planet. From about 1150 to 650 years ago, the earth underwent a brief warming period known as the Medieval Warming Period. During this time, the sea surface temperature was about a degree warmer and overall, the planet was much drier. This climatic anomaly was followed by what many climatologists call the Little Ice Age, a period that ended around 1900.
During the warm and dry period, animals in what would become Yellowstone National Park responded in ways that left clues about how animals may respond today to our warming planet. Amphibians make particularly sensitive sentinels of environmental change, alerting us to the presence of pollutants or other alterations that affect them before larger manifestations are detectable. And they even provide us evidence in their fossils.
Hot times, smaller paedomorphic salamanders
A group from Stanford excavated the fossils of Ambystoma tigrinum (the tiger salamander) from 15 layers at the Yellowstone site and divided them into five time periods based on their estimated age. They then divided the fossils again based on whether they represented the tiger salamander in its larval, paedomorphic, early adult, or later adult stages. The tiger salamander exhibits paedomorphism, in which the animal achieves reproductive capacity or adulthood while still retaining juvenile characteristics. In the case of the tiger salamander, this translates into remaining in the water, rather than becoming a terrestrial adult, and into retaining characteristics like frilly gills. The molecular determinant of whether or not an amphibian undergoes complete metamorphosis from juvenile to adult is thyroid hormone; when levels of this internal signal are low, the animal will remain juvenile.
The researchers found that during the medieval warming period, the paedomorphic salamanders became smaller than they were during cooler times. This outcome would be expected because when water is cooler, thyroid hormone levels will be lower, and the animal will continue growing as a juvenile.
Hot times, larger adult salamanders
On the other hand, the terrestrial adult salamanders were much larger during the warm period than during cooler periods. Again, this outcome would be expected because the heat on land would encourage faster metabolism, which would result in faster growth. The researchers found no difference in actual numbers between groups at cool vs. warm periods, but express concern that drying in Yellowstone today as a result of global warming might reduce the number of aquatic paedomorphs, affecting aquatic food webs.
From amphibians to gopher teeth
The same group also studied DNA from fossilized teeth of gophers and voles discovered in the cave. They found that during the dry period, gophers, who were stuck underground and isolated, experienced genetic bottlenecking, a reduction in diversity that persists today. However, the mobile, above-ground voles sought mates far and wide during the dry, warm period and actually experienced an increase in diversity. The lead researcher in the group compares early groups of isolated humans to the gophers, saying that they would have experienced a loss of diversity. But today’s population, with our ability to travel the globe with ease, is probably undergoing an increase in diversity since we’re able to mate with people a hemisphere away.