猛犸象灭绝Mammoth Kill
Mammoth Kill
Mammoth is any species of the extinct genus Mammuthus, proboscideans commonly
equipped with long, curved tusks and in northern species, a covering of long hair. They
lived from the Ptiocene epoch from around 5 million years ago, into the Hotocene at
about 4,500 years ago, and were members of the family Elephantidae, which contains,
along with mammoths, the two genera of modern elephants and their ancestors.
A
Like their modern relatives, mammoths were quite large. The largest known
species reached heights in the region of 4m at the shoulder and weights up
t0 8 tonnes, while exceptionally large males may have exceeded 12 tonnes.
However, most species 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 six months 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 on studies of their close relatives, the modem elephants,
mammoths probably had 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 Asian elephants, with females living in herds headed by a matriarch, whilst
hulls lived solitary lives or formed loose groups after sexual maturity.
B
MEXICO CITY-Although it’s hard to imagine in this age of urban sprawl and
automobiles, North America once belonged to mammoths, camels, ground
sloths as large as cows, bear-size beavers and other formidable beasts. Some
11,000 years ago, however, these large bodied mammals and others-about 70
species in all-disappeared. Their demise coincided roughly with the arrival
of humans in the New World and dramatic climatic change-factors that have
inspired several theories about the die-off. Yet despite decades of scientific
investigation, the exact cause remains a mystery. Now new findings offer
support to one of these controversial hypotheses: that human hunting drove
this megafaunal menagerie ( 巨型动物兽群）to extinction. The overkill model
emerged in the 1960s, when it was put forth by Paul S. Martin of the
University of Arizona. Since then, critics have charged that no evidence exists
to support the idea that the first Americans hunted to the extent necessary to
cause these extinctions. But at the annual meeting of the Society of Vertebrate
Paleontology in Mexico City last October, paleoecologist John Alroy of the
University of California at Santa Barbara argued that, in fact, hunting-driven
extinction is not only plausible, it was unavoidable. He has determined, using
a computer simulation that even a very modest amount of hunting would have
wiped these animals out.
C
Assuming an initial human population of 100 people that grew no more than 2
percent annually, Alroy determined that if each band of, say, 50 people killed
15 to 20 large mammals a year, humans could have eliminated the animal
populations within 1,000 years. Large mammals in particular would have been
vulnerable to the pressure because they have longer gestation periods than
smaller mammals and their young require extended care.
D
Not everyone agrees with Alroy’s assessment. For one, the results depend in
part on population-size estimates for the extinct animals-figures that are not
necessarily reliable. But a more specific criticism comes from mammalogist
Ross D. E. MacPhee of the American Museum of Natural History in New York
City, who points out that the relevant archaeological record contains barely a dozen examples of stone points embedded in mammoth bones (and none, it should be noted, are known from other megafaunal remains)-hardly what one might expect if hunting drove these animals to extinction. Furthermore, some of these species had huge ranges the giant Jefferson’s ground sloth, for example, lived as far north as the Yukon and as far south as Mexicowhich would have made slaughtering them in numbers sufficient to cause their extinction rather implausible, he says.
E
MacPhee agrees that humans most likely brought about these extinctions (as well as others around the world that coincided with human arrival), but not directly. Rather
he suggests that people may have introduced hyperlethal disease, perhaps through their dogs or hitchhiking vermin, which then spread wildly among the immunologically naive species of the New World. As in the overkill model, populations of large mammals would have a harder time recovering. Repeated outbreaks of a hyperdisease could thus quickly drive them to the point of no return. So far MacPhee does not have empirical evidence for the hyperdisease hypothesis, and it won’t be easy to come by: hyperlethal disease would kill far too quickly to leave its signature on the bones themselves. But he hopes that analyses of tissue and DNA from the last mammoths to perish will eventually reveal murderous microbes.
F
The third explanation for what brought on this North American extinction does not involve human beings. Instead, its proponents blame the loss on the weather. The Pleistocene epoch witnessed considerable climatic instability, explains paleontologist Russell W. Graham of the Denver Museum of Nature and Science. As a result, certain habitats disappeared, and species that had once formed communities split apart. For some animals, this change brought opportunity. For much of the megafauna, however, the increasingly homogeneous environment left them with shrinking geographical ranges-a death sentence for large animals, which need large ranges. Although these
creatures managed to maintain viable populations through most of the Pleistocene, the final major fluctuation-the so-called Younger Dryas eventpushed them over the edge, Graham says. For his part, Alroy is convinced that human hunters demolished the titans of the Ice Age. The overkill model explains everything the disease and climate scenarios explain, he asserts, and makes accurate predictions about which species would eventually go extinct.“Personally, I’m a vegetarian,” he remarks, “and I find all of this kind of gross but believable.”
答案
填空题
14. Mammoth inhabited North America some 11,000 years ago.
15. Mammoth died out due to human hunting. (theory one)
16. An overkill model was raised.
17. A deadly disease was introduced by human. (theory two)
18. There is little empirical evidence.
19. The extinction was caused by considerable climate instability. (theory three)
20. Their habitats were ruined.
人名理论配对题
Professor A
Professor B
Professor C
21. Hunting is not the reason why mammoth died out. A
22. The bigger the animals are, the more room they need. B
23. Human was an indirect cause of their extinction. B
24. Climate changes destroyed their habitats. C
25. Calculation estimate of the population could help the control of animal species. C
26. B
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发表于 2019-1-7 10:38:03
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