Mammoth Kill
Mammoth is anyspecies of the extinct genus Mammuthus, proboscideans commonly
equipped withlong, curved tusks and in northern species, a covering of long hair. They
lived from thePtiocene epoch from around 5 million years ago, into the Hotocene at
about 4,500 yearsago, and were members of the family Elephantidae, which contains,
along withmammoths, the two genera of modern elephants and their ancestors.
A
Like their modernrelatives, mammoths were quite large. The largest known
species reachedheights in the region of 4m at the shoulder and weights up
t0 8 tonnes, whileexceptionally large males may have exceeded 12 tonnes.
However, mostspecies of mammoth were only about as large as a modern
Asian elephant.Both sexes bore tusks. A first, small set appeared at about
the age of sixmonths and these were replaced at about 18 months by the
permanent set.Growth of the permanent set was at a rate of about l t0 6 inches
per year. Based onstudies of their close relatives, the modem elephants,
mammoths probablyhad a gestation period of 22 months, resulting in a single
calf being born.Their social structure was probably the same as that of African
and Asianelephants, with females living in herds headed by a matriarch, whilst
hulls livedsolitary lives or formed loose groups after sexual maturity.
B
MEXICOCITY-Although it’s hard to imagine in this age of urbansprawl and
automobiles, NorthAmerica once belonged to mammoths, camels, ground
sloths as large ascows, bear-size beavers and other formidable beasts. Some
11,000 years ago,however, these large bodied mammals and others-about 70
species inall-disappeared. Their demise coincided roughly with the arrival
of humans in theNew World and dramatic climatic change-factors that have
inspired severaltheories about the die-off. Yet despite decades of scientific
investigation, theexact cause remains a mystery. Now new findings offer
support to one ofthese controversial hypotheses: that human hunting drove
this megafaunalmenagerie ( 巨型动物兽群)to extinction. The overkill model
emerged in the1960s, when it was put forth by Paul S. Martin of the
University ofArizona. Since then, critics have charged that no evidence exists
to support theidea that the first Americans hunted to the extent necessary to
cause theseextinctions. But at the annual meeting of the Society of Vertebrate
Paleontology inMexico City last October, paleoecologist John Alroy of the
University ofCalifornia at Santa Barbara argued that, in fact, hunting-driven
extinction is notonly plausible, it was unavoidable. He has determined, using
a computersimulation that even a very modest amount of hunting would have
wiped theseanimals out.
C
Assuming aninitial human population of 100 people that grew no more than 2
percent annually, Alroydetermined that if each band of, say, 50 people killed
15 to 20 largemammals a year, humans could have eliminated the animal
populations within1,000 years. Large mammals in particular would have been
vulnerable to thepressure because they have longer gestation periods than
smaller mammalsand their young require extended care.
D
Not everyoneagrees with Alroy’s assessment. For one, the results dependin
part onpopulation-size estimates for the extinct animals-figures that are not
necessarily reliable.But a more specific criticism comes from mammalogist
Ross D. E. MacPheeof the American Museum of Natural History in New York
City, who pointsout that the relevant archaeological record contains barely a dozen examples ofstone points embedded in mammoth bones (and none, it should be noted, are knownfrom other megafaunal remains)-hardly what one might expect if hunting drovethese animals to extinction. Furthermore, some of these species had huge rangesthe giant Jefferson’s ground sloth, for example, lived as farnorth as the Yukon and as far south as Mexicowhich would have made slaughteringthem in numbers sufficient to cause their extinction rather implausible, he says.
E
MacPhee agreesthat humans most likely brought about these extinctions (as well as othersaround the world that coincided with human arrival), but not directly. Rather
he suggests thatpeople may have introduced hyperlethal disease, perhaps through their dogs orhitchhiking vermin, which then spread wildly among the immunologically naivespecies of the New World. As in the overkill model, populations of largemammals would have a harder time recovering. Repeated outbreaks of ahyperdisease could thus quickly drive them to the point of no return. So farMacPhee does not have empirical evidence for the hyperdisease hypothesis, andit won’t be easy to come by: hyperlethal diseasewould kill far too quickly to leave its signature on the bones themselves. Buthe hopes that analyses of tissue and DNA from the last mammoths to perish willeventually reveal murderous microbes.
F
The thirdexplanation for what brought on this North American extinction does not involvehuman beings. Instead, its proponents blame the loss on the weather. ThePleistocene epoch witnessed considerable climatic instability, explainspaleontologist Russell W. Graham of the Denver Museum of Nature and Science. Asa result, certain habitats disappeared, and species that had once formedcommunities split apart. For some animals, this change brought opportunity. Formuch of the megafauna, however, the increasingly homogeneous environment leftthem with shrinking geographical ranges-a death sentence for large animals,which need large ranges. Although these
creatures managedto maintain viable populations through most of the Pleistocene, the final majorfluctuation-the so-called Younger Dryas eventpushed them over the edge, Grahamsays. For his part, Alroy is convinced that human hunters demolished the titansof the Ice Age. The overkill model explains everything the disease and climatescenarios explain, he asserts, and makes accurate predictions about whichspecies would eventually go extinct.“Personally,I’m a vegetarian,” he remarks, “and I findall of this kind of gross but believable.”