Scientists have long tried to figure out what could have caused the disappearance of the woolly mammoth, one of the largest mammals to ever walk the Earth, during the last glacial period around 10,000 years in the past.
While some experts believe that drastic changes in the Earth's climate could have caused the giant creatures to become extinct thousands of years ago, evidence unearthed in recent studies now suggests that overhunting on the part of early human civilizations could have triggered the mammoths' mass die-off.
Researchers at the University of Michigan have discovered chemical clues found in the tusks of juvenile woolly mammoths that point to a sudden decrease in the animal's weaning age, the period when a mammoth calf stops nursing.
Michael Cherney, a graduate student at Michigan, and Daniel Fisher, director of the Museum of Paleontology and Cherney's adviser, found isotopic evidence in 15 mammoth tusks from Siberia that reveal a weaning age reduction of up to three years over the span of around 30,000 years leading up to the extinction of the prehistoric mammals.
Climate Change Factors vs. Human Hunting
According to the researchers, nutritional stress brought on by changes in the climate are often linked to a delay in the weaning of present-day elephant species, while pressure from hunting is associated with an acceleration in the animals' maturation, which would likely contribute to early weaning in elephants.
Cherney pointed out that their findings are not aimed at ending the debate on the possible cause of the mammoth's extinction but rather they hope that these results would provide other scientists with a new approach to help solve the question.
To carry out their investigations, the researchers studied tusks from Siberian mammoths, which Fisher has collected throughout the course of 20 years.
The samples, which included around three dozen tusks from juvenile mammoths, were transported with the permission of the Russian government as well as assistance from fellow researchers in France, the Netherlands and Russia.
Despite knowing the possibility of extracting clues regarding the mammoth's weaning age from tusks for years, Fisher said the recent study was the first instance that they were able to collect enough individual data from different geologic ages to allow them to establish a pattern through time.
He said that this breakthrough in their scientific approach shows that the question about the mammoth's extinction is indeed solvable.
Extracting Isotopic Evidence
The researchers examined the isotopic composition of hairs recovered from a pair of mother and calf African elephants. The elephant calf was in the middle of its weaning from its mother's milk, allowing Cherney to study the isotopic impact of nursing on a close relative of woolly mammoths and the animal's transition to a primarily solid diet.
By comparing the ratio of stable nitrogen isotopes from proteins extracted from the elephant tail hairs, Cherney discovered that the solid food proportions included in the diet of the elephant calf increased, while some nitrogen isotopes decreased. This was the first time this pattern has been observed in elephants.
Cherney then used the findings of the isotopic composition analysis from the elephant calf to study the weaning signature found on the mammoth tusks. Various CT scans of the specimens helped Cherney to determine the yearly growth increments in the tusks, which resemble the yearly growth of rings in trees.
The ratio of isotopes observed by the researchers from the early years of the calves consistently showed a pattern toward lower values of nitrogen-15. This suggests that the contribution of milk to the overall diet of the animals decreased.
Cherney said that this trend was similar to the one they saw in the African elephant calf.
In most cases, the steady decrease in nitrogen-15 values was followed by a sudden increase that the researchers believe could be a sign of temporary nutritional stress, particularly within the first year of the animal having fully weaned.
The radiocarbon dating the researchers conducted on the Siberian tusks revealed that the nitrogen-15 decrease occurred from approximately 40,000 years ago to 10,000 years ago.
The findings point to a weaning age reduction from eight years to five years over a period of 30,000 years.
The ongoing study on mammoth weaning is included in an overarching and decades-long research by Fisher and several other graduate students. Their goal is to extract information on the "life history" of the prehistoric mammals locked in fossil tusks.
"I started studying tusks 30 years ago and realized early on that life histories are the key," Fisher said.
"Nobody else has used tusks, which are after all a record of life and growth, as a source of data in this way."
The term life history is used by biologists to refer to the complete range of changes a living organism goes through during its development and growth.
The results of the University of Michigan study are being presented at the Society of Vertebrate Paleontology's meeting in Dallas, Texas.