Complex amphibian responses to past climate change

Eastern tiger salamander: Ambystoma tigrinum, courtesy of Wikimedia Commons

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.

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Columbus Day post: Failed Columbus settlement found no silver

Christopher Columbus

Columbus may have made his name for the history books by “finding” the Americas, but his real claim to fame may have been his world-class snake oil salesman ability. After returning from his famed voyage of 1492, Columbus bowed before his patrons, Spain’s King Ferdinand and Queen Isabella, and spun a tale of precious metals in the new world. His fairy stories about a land of silver and gold lit a fire under the Spanish royals, leading them to finance a much larger second expedition to the New World Columbus had stumbled on.

Tall tales of silver and gold

His tall tales also attracted a huge crowd of settlers with gold in their eyes and riches on the brain. This hapless group, 1500 strong, landed on the north shore of today’s Dominican Republic in 1494, only to find a land of sand and hardship, not the streets of gold they’d expected. Catastrophe piled on top of catastrophe in the form of hunger, disease, hurricanes, and conflicts with the indigenous people. Mutiny not unnaturally followed, and four years after the settlement was established, the few hundred remaining settlers abandoned the effort and returned home.

This failed first European town in the New World left behind a treasure trove for archaeologists, however. Among the finds was about 100 pounds of galena, a lead ore containing silver. Medieval people used galena to determine how much silver a substance contained. They could create galena with a known amount of silver, or galena could be found and its silver content compared with galena containing the known amount of ore.

Why is all this galena lying around?

For many years, scholars had thought that the galena found at the settlement, which its founders had called La Isabela after the financier queen, was the result of the settlers’ discovery of silver in the area. The odd thing was, no settler ever left a written record of such a discovery. Given the 100 pounds of it and the detailed records people on such expeditions tended to leave, such a find usually would have earned a mention.

In addition to the pounds of galena, researchers also found several hundred pounds of slag, which turned out to be lead silicate with flecks of silver in it. This slag also could have been the result of settlers’ attempts to extract silver from a local discovery. But again, there was no mention in the record of what would have been a significant operation, given the 400-plus pounds of slag. The galena and slag were discovered near a building that was used to store royal property.

Starving and desperate for silver

To address the discrepancy between the written record and archeological inference, a group of researchers applied chemical analysis to the galena and slag and compared it to known samples of Caribbean ores. They found that the galena and slag did not come from a Caribbean location and instead probably traveled with the settlers from Spain. The researchers speculate that the settlers, starving and desperate, were frantically trying to extract silver from items in the royal coffers.

Supporting this idea is the fact that the lead in the galena would normally have been more useful to them, for things like musket balls or supports for a ship. The waste, in the form of the lead silicate slag, was of no use to them at all.

An expert in medieval chemistry told the researchers that people of that era commonly mixed galena with ores they thought might contained a precious metal, using galena with a known quantity as a marker for how much gold or silver their discovered ore contained. This explanation may provide the reason the settler had galena with them in the first place.

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