Shift in weaning age supports hunting-induced extinction of Siberian woolly mammoths

October 15, 2015
mammoth

Chemical clues about weaning age embedded in the tusks of juvenile Siberian woolly mammoths suggest that hunting, rather than climate change, was the primary cause of the elephant-like animal's extinction.

Woolly mammoths disappeared from Siberia and North America about 10,000 years ago, along with other giant mammals that went extinct at the end of the last glacial period. Current competing hypotheses for the mammoth's extinction point to human hunting or climate change, possibly combining in a deadly one-two punch.

Despite decades of study, the issue remains unresolved and hotly debated. But two University of Michigan paleontologists may have found an ingenious way around the logjam.

U-M doctoral student Michael Cherney and his adviser, Museum of Paleontology Director Daniel Fisher, say an isotopic signature in 15 tusks from juvenile Siberian woolly mammoths suggests that the weaning age, which is the time when a calf stops nursing, decreased by about three years over a span of roughly 30,000 years leading up to the woolly mammoth's extinction.

Climate-related nutritional stress is associated with delayed weaning in modern elephants, while hunting pressure is known to accelerate maturation in animals and would likely result in earlier weaning, according to Cherney and Fisher.

"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," said Cherney, who is conducting the work for his doctoral dissertation in the U-M Department of Earth and Environmental Sciences.

"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," said Cherney, who is scheduled to present his findings Oct. 15 at a meeting of the Society of Vertebrate Paleontology in Dallas.

The study was made possible by the extensive collection of Siberian mammoth tusks that Fisher has amassed over the past 20 years. The specimens—collected and exported under permits from the Russian government with the help of colleagues in Russia, France and the Netherlands—include about three dozen juvenile tusks.

"We have known for about a decade that valuable information about weaning age could be extracted from these tusks," said Fisher, a professor in the Department of Earth and Environmental Sciences and the Department of Ecology and Evolutionary Biology. Fisher also led the team that recovered the partial remains of a mammoth this month near Chelsea, Mich.

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

Fifteen tusks from individuals ranging in age from 3 to 12 were analyzed. The 3-year-old's tusk is about 10 inches long, while the 12-year-old's tusk is about 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. The elephant calf was in the process of being weaned from mother's milk, which enabled Cherney 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.

Armed with this isotopic weaning signature, he then turned to the mammoth tusks. CT scans enabled Cherney 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 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 said.

"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 Fisher interpret as a sign of short-term nutritional stress during the first year after being fully weaned.

Radiocarbon dating of the 15 Siberian tusks showed they span the period from about 40,000 years ago to about 10,000 years ago.

Cherney and Fisher showed that over the span of 30,000 years, the average weaning age decreased from age 8 to age 5.

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. Biologists use the term life history to refer to the full range of changes an organism experiences in the course of its growth and development.

"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."

Over the years, Fisher and his students have shown that mammoth tusks hold life-history information about growth rates, age of sexual maturation, spacing of pregnancies, and weaning.

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," Cherney said.

Often, environmental changes have predictable effects on life histories. By analyzing evidence from mammoth , Fisher and his students can test those predictions.

"The strength of life-history analyses for resolving the extinction debate rests in the knowledge that the age of final weaning is a life-history landmark that is expected to change differently in response to predation and climate-related nutritional stress," said Cherney, who will speak during the Romer Prize Session at the paleontology meeting. "Our analysis sets up a test of competing hypotheses, and our preliminary results are consistent with expectations under hunting pressure."

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13 comments

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Returners
2 / 5 (4) Oct 15, 2015
Nothing new under the Sun.

Just look around.

Modern human idjits hunting modern elephants to extinction in Africa and Asia.

rgw
3.7 / 5 (3) Oct 15, 2015
Hunting five ton beasts with a personal howitzer, armored trucks and airplanes is a far removal from tracking them on foot with a spear and across endless frozen wastes.
SteveGinGTO
3.4 / 5 (5) Oct 15, 2015
This is horse crap.

They say, "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," while saying they studied 15 tusks.

There are many THOUSANDS of tusks out there to test. This sampling is FAR FAR too small to make any sweeping "suggestions".

In addition, Metzler, Grayson, Surovell and others have shown that the Overkill theory has essentially no support in the literature. In "Clovis Hunting and Large Mammal Extinction: A Critical Review of the Evidence" (2002), Grayson and Metzler show that out of all the purported "Clovis mammoth kill sites", only 12 unequivocal sites in North America exist (plus 2 for mastodons). And that there are NO such sites at ALL for any of the other 30+ genera that went extinct during the same short period in N. America. Surovell asks if that is enough.

Such articles are crap is because they don't mention the evidence that is contrary.
plasmasrevenge
1 / 5 (5) Oct 15, 2015
Re: "Such articles are crap is because they don't mention the evidence that is contrary."

Yes, agreed, 100%. These researchers make no mention of the 7 instances of mammoth tusks purchased from the tusk market which very clearly show meteorites embedded in tusks; nor any mention of the mammoth carcass which was observed partially covered in a black radioactive mat with exotic isotopes.

That work by Firestone is far more compelling than this junk, and the fact that we are even still having this conversation after those discoveries shows how dysfunctional this discipline is. They look at the Firestone dates, and they say, "Well, this does not fit into our chronology for the extinction." Yeah, um, what were you expecting? These are highly radioactive events. You should not be expecting the dates to line up, to begin with.
B Fast
5 / 5 (2) Oct 15, 2015
In Carmacks, Yukon Territory, Canada the first nations people have a display demonstrating the technique they used to used for subduing mammoths.

Their technique is about as follows:
> They find a set of 4 pairs of trees that form a straight line. Each of the pairs of trees are about 10 feet apart.
> They partially cut down one of each pair of trees just like a beaver would.
> They place a large snair made of vines between each pair of trees.
> They then would antagonize the mammoth, get it to chase them, and they would run through the trap.
> If all went well, the snairs would snare the mammoth, and it would be held from going anywhere by the 8 trees. Then they would be left to their spears and arrows to deal with the disabled mammoth.

They also report that they would dress in their finest clothes to perform this task, as they figured that they would probably die, and they may as well get one more use out of their finest attire.
huckmucus
1 / 5 (3) Oct 15, 2015
Resolution of the debate will be found in the study of bison. Just sayin'.
AGreatWhopper
1 / 5 (3) Oct 15, 2015
I think that one of the reasons this one is difficult to work out- besides all the obvious ones- is that the mechanism wasn't necessarily the same in NA as the rest of the world. In NA there is good evidence that nitrogen fixing cover plants, like alfalfa and clover didn't survive the last glaciation. Megafauna need nitrogen and some new studies are suggesting that their extinction in NA could have happened in as little as 50-100 years. Clovis man was not that good, nor that numerous. Those plants were re-introduced later, by human colonists, but if the statement is accurate, it's hard hard imagine how much of the megafauna could have failed to die out. If true the mystery isn't what killed them, but how they lived that long. The gaping hole in the theory is what they did during the previous glaciations. Enough in frost protected valleys? Something that only lived during the glaciations and was a supplement?
jljenkins
1 / 5 (3) Oct 15, 2015
huckmucus1 / 5 (1) 58 minutes agoResolution of the debate will be found in the study of bison. Just sayin'.


Hey, check the gen X BS speak at the door. This is a science site. "Just sayin'" is gen X's favorite invention. Don't have the frustration tolerance to keep your damned mouth shut, but don't want responsibility for the statement, so you say that and it means "this is what I think, but you can't hold me responsible/debate me for it".

Responsiblity is a science core value. Go lie to your gen X friends, defraud each other, call anyone that ever questions behavior a "hater" and slouch through life together, but stay out of the lab. You don't have the mind for it.
huckmucus
not rated yet Oct 15, 2015
huckmucus1 / 5 (1) 58 minutes agoResolution of the debate will be found in the study of bison. Just sayin'.


Hey, check the gen X BS speak at the door. This is a science site. "Just sayin'" is gen X's favorite invention. Don't have the frustration tolerance to keep your damned mouth shut, but don't want responsibility for the statement, so you say that and it means "this is what I think, but you can't hold me responsible/debate me for it".

Responsiblity is a science core value. Go lie to your gen X friends, defraud each other, call anyone that ever questions behavior a "hater" and slouch through life together, but stay out of the lab. You don't have the mind for it.


Typical moron focusing on form over substance. If you knew squat about latifrons, pricus, antiquus, etc then you'd have seen the substance instead of getting your eyes caught on the shiny object. P.S. talk about a lack of frustration tolerance; pot, kettle, black.
EnricM
not rated yet Oct 16, 2015
Hunting five ton beasts with a personal howitzer...


Plus, the human population back then wasn't so big and a single kill would have been able to sustain a well sized group for quite some time, specially if we consider that the group were also gathering, fishing and hunting other prey.

There is however a point that nobody has yet taken into consideration that may confirm the "hunting" theory: Non-human predation.

I have no idea qhy everybody gives for granted that the only predator back then was Homo sapiens, but this is actually far from true as there were quite a lot of huge super predators that could perfectly do the job.

What if there was a population explosion of these other predators? For instance due to a better weather and higher survival rate of cubs, or that the melting glaciers made that more super predators had to migrate into the mamoths territory?
NealWV
1 / 5 (2) Oct 20, 2015
Nothing on this whole site is worthy of being a story unless it says something about how humans are destroying everything around us! Don't forget people we are part of and from this planet as well and it will be here long after we are gone.
huckmucus
not rated yet Oct 20, 2015
Nothing on this whole site is worthy of being a story unless it says something about how humans are destroying everything around us! Don't forget people we are part of and from this planet as well and it will be here long after we are gone.


Christopher Stone's "ontological problem."
jsdarkdestruction
5 / 5 (2) Oct 26, 2015
Nothing on this whole site is worthy of being a story unless it says something about how humans are destroying everything around us! Don't forget people we are part of and from this planet as well and it will be here long after we are gone.

1.they've regularly put up the argument it was a comet or meteor or climate change for the most part many times here.
2.yes, we are a part of this planet and its ecosystem.
3. So since the earth will be here long after us we should just ignore the environment and do whatever we'd like? I want future generations to have at least adecent planet to live on, preferably better than decent but I'm a realist, not one previous generations destroyed.

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