For 26 years, Earthwatch teams have been helping Dr. Larry Agenbroad (Northern Arizona University) unearth the remains of 52 mammoths from a prehistoric sinkhole at Hot Springs, South Dakota. Most of the mammoths found have been young males, suggesting inexperience and a solitary lifestyle contributed to their entrapment. Now samples of dentin layers from the tusks of several mammoths at the site are changing perceptions of how these Pleistocene mammals lived and died.
Agenbroad and colleagues Daniel Fisher (University of Michigan) and David Fox (University of Minnesota) reported on their findings from the tusk samples in Advances in Mammoth Research, Proceedings of the Second International Mammoth Conference in 1999, published in May 2003. Their analysis suggests that mammoths trapped in the prehistoric sinkhole frequented sinkhole environments for several months prior to death, and that most died in the early spring or early autumn.
"The method was developed for mastodonts, and this was the first time a large sample of mammoth tusks was attempted," said Agenbroad, principal investigator of the Earthwatch-supported Mammoth Graveyard project. "Tusks are ever-growing, and record life events as well as environmental data. Ivory contains a cyclic series of increments, roughly analogous to tree 'growth rings' and can reflect 'good' and 'bad' environmental conditions as well as stress periods in the individual's life."
Agenbroad and colleagues took samples of dentin layers from the base of several tusks excavated at Mammoth Site of Hot Springs, focusing on the layers directly adjacent to the pulp cavity that recorded the last two or three years of life. They report on the complete results from four of these samples, yielding valuable clues to the behavior of mammoths at this site.
Measuring the presence of oxygen-18, a rare isotope of the oxygen atom, in the mammoth dentin, the scientists found annual cycles with highest isotope levels correlating with the end of the growing season, or late summer. These data indicated that two of the mammoths sampled died in the early spring, while two of them died in the early autumn, contrasting with earlier predictions that the mammoths at the site were mostly winter mortalities.
"The original hypothesis was that the teenage and young adult males represented in the Mammoth Site population would exploit the green vegetation around the edge of a thermal pond in the winter, rather than sweeping 2-3 ft of new, wet snow off last season's dead grasses," said Agenbroad. "A revision of the hypothesis is that there are two periods of time, early spring and early autumn, where the intervals of wet wallrock prevented mammoths from ascending the sinkhole walls."
The data also showed that the growth rate of dentin layers in the mammoth tusks lost its seasonal variation in the last year or so prior to death, and oxygen isotope profiles show a narrower range of variation. This suggests to the scientists that the mammoths were spending time consistently at sinkhole environments, which offered a more consistent source of food and water than the more seasonal temperate environments that surrounded the sinkholes.
The findings of Agenbroad and his colleagues are unique in that they represent a local population of prehistoric mammoths, rather than an arbitrary collection of samples. Continued investigation of mammoth remains at this singular site, supported by Earthwatch teams, will help tease apart how these animals interacted with their changing environment, with lessons for our own.
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The Mammoth Site is now an exciting in situ museum, near the Black Hills of South Dakota and Mt. Rushmore, where more than 100,000 visitors per year come with their families to see first-hand how scientists are answering the unsolved questions of Pleistocene extinction. Learn more at: http://www.mammothsite.com
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