What Became of the Megafauna?
Twilight of the Mammoths: Ice Age Extinctions and the Rewilding
of America. Paul S. Martin. xx + 250 pp. University of
California Press, 2005. $29.95.
Between 50,000 and 10,000 years ago, during the final millennia of
the Pleistocene Epoch, roughly 100 genera of megafauna (animals
weighing more than 100 pounds) became extinct worldwide. Among them
are such well-known creatures as mammoths and saber-toothed tigers
and the more obscure, though no less significant,
Diprotodon (an Australian marsupial the size of a
hippopotamus) and Coelodonta (a woolly rhinoceros found in
Europe). Whether their disappearance was caused by changes in
climate or by "overkill" (being hunted to extinction by
humans) has been hotly debated for the past 40 years. In
Twilight of the Mammoths: Ice Age Extinctions and the
Rewilding of America, Paul S. Martin reviews the
end-Pleistocene extinction, arguing that overkill is the more likely explanation.
In the first chapter Martin introduces the reader to the
Pleistocene, evaluating the taxa that were present on each continent
and summarizing what we know about those that became extinct. He has
helpfully included figures containing scaled drawings of
reconstructed skeletons and sketches of some of the extinct
megafauna. The second chapter summarizes the overkill hypothesis and
notes that accurate dating is needed to further develop the concept.
In the next few chapters Martin provides an autobiographical account
of his fieldwork and of the evidence for overkill he has gathered
during his long career. He recounts his experiences working with
colleagues and friends in various caves in North and South America.
We learn about the vast deposits of ground-sloth dung found both in
Rampart Cave in Arizona and in Cueva del Milodón in Chile.
Martin also describes the evidence from cave deposits that ancient
condors and mountain goats were typical inhabitants of the Grand
Canyon. He highlights the synchronous extinction of large mammals in
North and South America around 12,000 years ago and discusses the
ecology of many of the extinct mammals—explaining, for
example, how pollen and microfloral analyses of dung from Rampart
Cave reveal that the globe mallow plant was the preferred food of
the extinct Shasta ground sloth.
Succeeding chapters lay out the dynamics of the extinctions,
including the effects of human dispersal into new areas, the
chronological relation between the arrival of humans and the
disappearance of large mammals, and the evidence at kill sites that
megafauna were used by humans. Martin makes a strong case that
anthropogenic influences caused the demise of megafauna in North
America, citing evidence that the continent's first human
inhabitants, the Clovis people, arrived about 13,000 years ago, just
before the extinction episode. Although the North American data may
not be entirely convincing on its own, when Martin adds to it a
discussion of the arrival of humans and subsequent extinction of
megafauna in Australia, Madagascar, New Zealand, South America and
the West Indies, his argument in favor of the overkill hypothesis
A separate chapter deals with the possibility that the extinctions
were induced by climate change. Martin dismisses the idea that
climate could on its own provide a viable explanation, arguing that
shifts in temperature at the end of the Pleistocene were not much
different from those that had occurred earlier in the epoch, when
few if any megafauna became extinct. Martin explains his position
sufficiently. However, scientists are aware that climate change does
affect mammals (by changing population size, for example), and the
chapter would have benefited from a more detailed discussion of
this. Recent studies have analyzed ancient DNA from
radiocarbon-dated fossils of both large and small mammals,
determining how genetic diversity has changed over time. In some
cases, a great decrease in genetic diversity, suggesting a severe
reduction in population size, has been shown to have taken place at
about the same time as a shift in climate. A sharp reduction in the
population size of a species could in turn play a role in its
ultimate disappearance. Investigating these types of effects at the
end of the Pleistocene would greatly enhance our understanding of
exactly how the extinction proceeded.
Conservation is a theme throughout the book, and Martin closes with
an overt call for drastic conservation efforts on behalf of extant
fauna. He also shows the importance of fossils—whether bones,
sediment or dung—as a nonrenewable resource. They contain
valuable information that can easily be lost, as when sloth dung in
Rampart Cave was inadvertently burned.
However, Martin's suggestion that we should create Pleistocene parks
in North America seems rash and extreme. Introducing nonnative
animals such as camels, elephants and lions to establish a prehuman
ecological state could have dire consequences. Many of these
mammals—elephants, for example—diverged evolutionarily
from their extinct North American counterparts millions of years
ago, claiming their own niches and coevolving with other non-North
American species. Although we speculate that at times the living
animals behave like extinct forms, introducing these species will
not recreate the "wild" state that was lost when the
megafauna died. Interactions between the native and introduced
species would be unfamiliar and would have to evolve anew. A better
solution would be to focus on preserving remaining fauna and environments.
Martin's ill-conceived ideas about Pleistocene parks
notwithstanding, Twilight of the Mammoths is well written
and can be easily understood by nonscientists. It provides a fine
introduction to the end-Pleistocene extinction through the eyes of a
proponent of the overkill hypothesis. I particularly enjoyed the
personal accounts of fieldwork and of interactions with colleagues.
Also, the subsections that close some of the chapters are very
insightful. In fact, the material on radiocarbon dating is so
succinct and informative that I plan on using it for teaching.
Natural history enthusiasts will certainly enjoy this book.