Survival of the Smaller

A researcher holds a tooth that came from a mammoth that weighed between six and eight tons.

Felisa Smith

During the Pleistocene, or last Ice Age, large mammals such as the saber-toothed cat, woolly mammoth, and giant ground sloth were especially prone to extinction. New research shows that this extinction bias toward larger mammals correlated with the global spread of humans and our hominin relatives.

Researchers led by biologist Felisa Smith at the University of New Mexico used fossil data to analyze the body size and extinction time periods of over 6,000 non-flying, terrestrial mammal species in Africa, Australia, Eurasia, North America, and South America. They compared extinction rates in the late Quaternary Period (125 thousand years ago to present) to those of the rest of the Cenozoic Era (sixty-five to one million years ago).

They found that, through most of the Cenozoic and prior to hominin prevalence, bigger animals were no more likely to go extinct than smaller ones. In fact, average mammalian body size increased. During the late Quaternary, however, when humans started to spread around the globe, mammal size began to decrease. Larger mammals were selectively eradicated from each continent at times that correspond to human colonization.

In the Americas, for example, average body size decreased by over 90 percent in North and South America, between twenty and ten thousand years ago. This encompasses the time when Homo sapiens is thought to have arrived in the New World, around fifteen to thirteen thousand years ago. In Africa, however, average mammalian body size was smaller than expected 125 thousand years ago. This suggests that humans and other hominins had already hunted many large mammals to extinction. Future work, Smith says, may look more closely at the thousands of years leading up to that point to find exactly when average mammalian body size first took a dive.

In addition to the historic extinction data, the researchers used conservation statuses from the International Union for Conservation of Nature Red List of Threatened Species to make predictions about the fate of present-day mammals. If all mammals currently in one of the Threatened, Vulnerable, or Endangered categories becomes Extinct, there will be nearly one-third fewer non-flying, terrestrial mammal species than there were 125 thousand years ago. (Science)

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