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Woolly mammoths nursed babies through long winters -- and it may have wiped them out

Doug O'Harra

Talk about a tragic flaw: A case of elephantine mothering gone too far.

Woolly mammoths of prehistoric Alaska and Yukon nursed their babies for two and possibly three years -- far longer than modern elephants, according to an ingenious new study that analyzed fossil teeth for elemental clues into Ice Age life.

Woolly mammoth hunt
Overhunting may have led to the extinction of the woolly mammoth -- but a new study finds the animals' nursing habits could have played a role, too.
This extended lactation was a Far North adaptation that sustained the hairy little beasties during the long, dark Arctic winters when there wasn't much out there to eat.

Just as important, by keeping the vulnerable young animals close to momma for several extra years, they protected their offspring against predation by the saber-toothed tigers and Beringian lions that prowled the dim steppe under sinister bars of Pleistocene auroras.

But then, these devoted mothers somehow lost their edge.

When the northern climate began to warm and grow wetter about 10,000 years ago, this prolonged period of nursing transformed from being sort of a woolly mammoth superpower into an evolutionary liability, according to researcher Jessica Metcalfe and two other scientists in a study published this month in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

Locked into a life cycle that depended on producing gobs of nutritious milk year after year all winter long, these Ice Age icons would have found it more and more difficult to nourish and protect their offspring in the face of changing vegetation and the arrival of human hunters.

Within a few thousand years, the mammoths -- supremely suited to life on the arid, frozen steppe -- had all disappeared.

"Woolly mammoths may have been more vulnerable to the effects of climate change and human hunting than modern elephants," wrote Metcalfe and co-authors professor Fred Longstaffe of Western and Grant Zazula of the Yukon Paleontology Program, "not only because of their harsher environment, but also because of the metabolic demands of lactation and prolonged nursing, especially during the longer winter months."

The study adds insight into the causes one of the world's most mysterious animal die-offs, the extinctions 40 large mammal species between 13,000 and 10,000 years ago when the last Ice Age ended. Sorting out the causes has long been one of Arctic paleontology's hottest controversies, with some scientists blaming overhunting by people and others arguing that it was caused mostly by climate change, or even a cataclysmic impact by an asteroid.

Metcalfe zeroed in on fossil chemistry.

"The modern relevance of this kind of work is that the past is the key to the present," Metcalfe says in this video about her project. "By learning about the climate in the past and animal behavior in the past -- including things like weaning and nursing -- we can better understand the changes that are happening now."

Among the most common of Alaska's prehistoric giants was the multi-ton, tusk-swinging woolly mammoth -- an intelligent and hairy species of elephant that roamed a prehistoric steppe that stretched from Europe across Siberia and over the 500-mile wide Bering Land Bridge. The animals thrived in Alaska and the Yukon for tens of thousands of years. Their remains are so common in the dirt of Interior Alaska river valleys that the creature was named the state's official fossil.



Metcalfe investigated the fate of these northern behemoths by conducting high-tech chemical analysis of the stable isotopes found in their fossilized teeth.

Seeking prehistoric secrets inside pearly whites

Stable isotopes are naturally occurring versions of common elements like carbon, oxygen and nitrogen with different atomic weights. They build up in bones and teeth while an animal is alive. Analyzing the ratio of one to another can offer clues to what an animal ate, where it lived, when it was weaned and more. (Check out Wikipedia's archaeological isotopic primer.)

"You can just do so many different things with stable isotope analysis," Metcalfe explains in her video. "Basically any material in the world, you can look at the isotope values within that material and find out something about how the world works."

Metcalfe began with a trove of at least 22 teeth and jaw material dug from the ground near the Old Crow River, just east of Alaska in the Yukon Territory. Eight of the teeth were at least 40,000 years old, based on carbon dating.

Metcalfe ground off samples using carbide and diamond drill bits, cooked the stuff in a sophisticated multi-step process and then analyzed the results for trace elements and the ratio of among the isotopes.

What the scientists found suggests these Yukon mammoths didn't start scarfing down plants and other solid food before age two or three. Their long period of nursing made a peculiar sense -- given that big, mammoth-chomping cats were a-lurking in the polar night.

"In modern Africa, lions can hunt baby elephants, but not adults," Metcalfe explained in a release about her work. "They can't kill adults. But they can kill babies and by and large, they tend to be successful when they hunt at night because they have adapted night vision.

"In Old Crow, where you have long, long hours of darkness, the infants are going to be more vulnerable, so the mothers nursed longer to keep them close."

As the climate changed, the vegetation would have shifted. Mammoth mothers may have started producing poorer quality milk for their infants, leading to much lower survival of the offspring. Throw in the presence of human hunters, and the species may have been given a significant push toward the downward spiral of extinction.

Again, modern elephant ecology offers some clues.

"Today, a leading cause of infant elephant deaths in Myanmar (in Southeast Asia) is insufficient maternal milk production," Metcalfe said.

Nutritionally stressed mothers, weakened or sickly calves, habitat growing brushier with less mammoth food and more trees, sophisticated human hunters taking advantage -- all could have contributed to their demise.

The techniques used in the study, which has been widely reported across the web, could be applied to find out whether nursing and weaning were important factors in the fate of other prehistoric animals, the scientists said.

"We have shown that the combination of stable carbon, nitrogen, and oxygen isotope and trace element analysis of skeletal remains from multiple individuals of varying age can significantly contribute to our understanding of the timing and duration of nursing and weaning in mammoths," they wrote. "This combination of multiple lines of evidence offers great promise."

Doug O'Harra is an Anchorage writer.