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Woolly mammoths WERE wiped out by hunters: Tusks suggest that calves were forced to wean earlier because of human predators

Researchers from University of Michigan have added to evidence that hunting caused the Ice Age creature to disappear 10,000 years ago
Found chemical clues about weaning age - when a calf stops nursing - embedded in the tusks of juvenile Siberian woolly mammoths
Evidence suggests animals were forced to mature earlier over time
Matches signs of hunting pressure, not climate change

Published: 04:26 EST, 16 October 2015 | Updated: 08:55 EST, 17 October 2015

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Whether humans were responsible for wiping out the woolly mammoth is hotly debated.

Now two US palaeontologists have added to evidence that hunting caused the Ice Age creature to disappear from Siberia and North America 10,000 years ago.

They found chemical clues about weaning age - when a calf stops nursing - embedded in the tusks of juvenile Siberian woolly mammoths to conclude that climate change was not the primary case of the animal's extinction.

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US palaeontologists have added to evidence that hunting caused the Ice Age creature to disappear from Siberia and North America 10,000 years ago. They found chemical clues about weaning age - when a calf stops nursing - embedded in the tusks of juvenile Siberian woolly mammoths (some shown above)

Two researchers at the University of Michigan discovered an isotopic signature in 15 tusks to show their weaning age decreased by about three years over a span of 30,000 years leading to the mammoth's extinction.

Hunting pressure is known to force animals to mature faster and could likely result in earlier weaning, they said.

In contrast, climate-related nutritional stress is associated with delayed weaning in modern elephants, suggesting climate change was not to blame.

Doctoral student, Michael Cherney, explained: 'This shift to earlier weaning age in the time leading up to woolly mammoth extinction provides compelling evidence of hunting pressure and adds to a growing body of life-history data that are inconsistent with the idea that climate changes drove the extinctions of many large Ice Age mammals.'

'These findings will not end the debate, but we hope they will show people the promise of a new approach toward solving a question that, so far, has just led to divided camps.'

Cherney and Daniel Fisher, Director of the Museum of Palaeontology at the university, studied specimens Dr Fisher has collected over the past 20 years.

'We have known for about a decade that valuable information about weaning age could be extracted from these tusks,' Dr Fisher said.

'But this is the first time we've had data from enough individuals, and covering a wide enough range of geologic ages, to show a pattern through time,'

'This is a milestone in the development of our approach, and it shows that the extinction problem is solvable.'

The duo analysed 14 tusks from mammoths ranging in age between three and 12, between 10 inches and 30 inches long.

As part of the study, Cherney measured the isotopic composition of tail hairs from a mother-calf pair of African elephants at the Toledo Zoo in Ohio.

Daniel Fisher, Director of the Museum of Palaeontology at the university (left) and Cherney (right) studied specimens Dr Fisher has collected over the past 20 years. One of the tusks is shown above

LYUBA THE BABY MAMMOTH

A young mammoth was discovered in Siberia in May 2007 by reindeer herder Yuri Khudi and his sons who were searching for wood along the Yuribei River.

Since then, Lyuba, whose body is 42,000 years old but perfectly preserved, has become famous.

The baby mammoth measured 33inches (85cm) tall and 51 inches (130cm) long when she died at one month old.

X-rays revealed she fell into a sticky mud hole and choked to death, leaving her mother to grieve for her.

Daniel Fisher was behind the research, which revealed Lyuba’s trunk, mouth, oesophagus and trachea are all clogged with sediment, suggesting she choked to death or was asphyxiated.

Professor Adrian Lister, a mammoth researcher at the Natural History Museum, said: 'CT scans show her upper respiratory tract is packed with mud, which corresponds to our model of her sinking into soft mud.’

Pip Brewer, curator of fossils, mammals, at the museum told MailOnline that because Lyuba was so small, she would not have had the strength to pull herself out of the mud hole. And her fate befell other small mammoths too.

‘We already know of a couple of examples of other mammals that died this way. They didn’t have the bodyweight to extract themselves from the mud in the summer months,’ she said.

The elephant calf was in the process of being weaned from mother's milk, which enabled him to observe the isotopic effects of nursing and the long transition to a fully solid diet for a close relative of mammoths.

Cherney compared the ratio of the two stable isotopes of nitrogen, nitrogen-14 and nitrogen-15, from proteins in elephant tail hairs.

He found that as the proportion of solid food in the elephant calf's diet increased, the ratio of nitrogen-15 to nitrogen-14 steadily dropped. This pattern had been previously documented in other mammals, including humans, but never in elephants.

Cherney then used CT scans to identify annual growth increments, which resemble a tree's annual growth rings, in the tusks.

Samples for each year of growth were collected, and nitrogen isotopes from collagen proteins were measured.

The isotopic ratios from the mammoth calves' early years of life consistently displayed a trend toward lower nitrogen-15 values, reflecting the decreased contribution of milk to the overall diet, Cherney explained.

'It was the same pattern we saw in the Toledo Zoo elephant calf,' he said.

The gradual decrease in nitrogen-15 was followed, in most cases, by an abrupt increase that Cherney and Dr Fisher interpreted as a sign of short-term nutritional stress during the first year after being fully weaned.

The duo showed that over the span of 30,000 years, the average weaning age of the young mammoths decreased from age eight to age five.

The current weaning study is part of a much larger, decades-long effort by Fisher and a series of graduate students to extract 'life history' information preserved in fossil tusks.

Dr 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.'

Over the years, he has shown that mammoth tusks hold information about growth rates, age of sexual maturation, spacing of pregnancies, and weaning.

Cherney explained that because the timing of those life-history milestones can be affected by various environmental pressures, the tusks provide a way to 'look directly at how the animals themselves were impacted by, and responded to, changes in their environment'.

Daniel Fisher was involved in research that showed a baby mammoth now named Lyuba (shown above) choked to death in a muddy hole around 42,000 years ago. He showed her trunk, mouth, oesophagus and trachea are all clogged with sediment, suggesting she was asphyxiated in a muddy hole

Cherney then used CT scans to identify annual growth increments, which resemble a tree's annual growth rings, in the tusks. Samples for each year of growth were collected, and nitrogen isotopes from collagen proteins were measured. One part of the complex process is shown above