At the 2015 Society of Vertebrate Paleontology meetings, researcher Michael Cherney of the University of Michigan, presented findings about weaning age (i.e. when a calf stops nursing) in fossil mammoths. By studying modern African elephants at the Toledo Zoo, Cherney was able to characterize the isotopic effects of weaning in a close relative of mammoths. Decreased nursing causes predictable changes in the isotopic composition of elephant tail hairs sampled over time.
The key to Cherney's research is that these same nitrogen isotopes are preserved in fossil mammoth tusks, which grew throughout life. Records of early life history in tusks from juvenile mammoths can be used to determine the age at which individuals were weaned. He combined this with the knowledge that climate stress has been associated with delayed weaning in modern elephants, while overhunting of can lead to accelerated maturation in populations. His results suggest that weaning age in Siberian woolly mammoths decreased leading up to extinction. This is inconsistent with climate change being the cause of extinction and provides evidence for overhunting shortly before they went extinct.
"I think analysis of life-history data from fossil proboscidean tusks is a tool that could resolve questions concerning the late Pleistocene extinctions of various fossil elephant species. These insights also give context for understanding other contemporaneous extinctions and the impact of past human populations on their environments," said Cherney.
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AUTHOR CONTACT INFORMATION
University of Michigan
Ann Arbor, MI, USA
SHIFT IN WEANING AGE SUPPORTS HUNTING-INDUCED EXTIRPATION OF SIBERIAN WOOLLY MAMMOTHS (MAMMUTHUS PRIMIGENIUS)
CHERNEY, Michael D., University of Michigan, Ann Arbor, MI, USA
Woolly mammoths disappear from the fossil record of mainland Siberia around 10 ka BP and then are lost globally around 4 ka BP. The causes of this continental extirpation and eventual extinction are still obscure, but current competing hypotheses implicate detrimental effects from hunting pressure and climatic changes during the Pleistocene-Holocene transition. Age of final weaning is a life-history landmark that is expected to change differently in response to predation and climate-related nutritional stress. African elephants (Loxodonta africana) wean calves later during multi-year droughts. This phenomenon provides a way to investigate nutritional status and environmental quality for fossil proboscidean populations. If reduced quality or quantity of food, the usual proximate causes of climate- induced stress, were forcing the decline of Siberian woolly mammoths, we would expect their average weaning age to increase toward the end of the Pleistocene. On the other hand, overhunting can accelerate maturation in populations. This suggests we might see the opposite pattern (decreased weaning age) if exploitation from humans was the primary pressure reducing proboscidean populations. Thus, the pattern of weaning age through time can be used to evaluate whether extirpation was more likely due to hunting or to climate change.
To establish a 'weaning signature' that could be detected in fossil tusk records, I serially analyzed nitrogen stable isotope composition (δ15N) of tail hairs from a mother-calf pair of African elephants at the Toledo Zoo, Ohio, to document changes associated with nursing and weaning. Data from this modern analog provided context for interpreting isotope profiles obtained from young woolly mammoth tusks. Using serial records of tusk collagen δ15N, I estimated weaning age for nine Siberian specimens from Taimir and Chukotka with accelerator mass spectrometry dates between 10 and > 41 thousand radiocarbon years BP. Most records display a gradual multi-year decrease in δ15N values followed by an abrupt increase that appears to reflect short-term nutritional stress during the first year after being fully weaned. Using the spike in δ15N as an indirect indication of final weaning, this analysis shows a decrease in weaning age from 7 to 4 years over this interval. This result corroborates hypotheses that implicate hunting by humans as the primary cause of woolly mammoth extinction.