Large animals may be predisposed to extinction

By Bjorn Carey

LiveScience staff writer

updated 9:14 p.m. ET July 18, 2006

Once upon a time, a 2-ton wombat lumbered across the Australian Outback. Around the same time, mammoths and saber-toothed tigers had the California coastline all to themselves.

Millions of years before any of these animals existed, Tyrannosaurus rex and other colossal dinosaurs ruled the world.

These and some of the other largest and most fantastic creatures ever to walk the planet are long gone, victims of mass extinctions of large beasts. And for reasons poorly understood, often the animals to fill the voids were tiny by comparison.

Predisposed to extinction
Scientists generally accept that a giant asteroid slammed into the Gulf of Mexico some 65 million years ago, setting off a chain of catastrophic events that ultimately led to the extinction of dinosaurs. Whether or not an asteroid is to blame, the so-called KT boundary in the fossil record displays a mass extinction of dinosaurs and other large animals around the world.

Small scavenging mammals and birds survived the event, and scientists can't say for sure why dinosaurs did not.

Since bigger beasts couldn't take shelter in small protected burrows, perhaps they were done in by fierce environmental conditions. Or maybe with so many plants dying off, big herbivores simply had nothing to eat, and as they died out, so did the big carnivores.

Or perhaps with all the stress, dinosaurs simply couldn't reproduce quickly enough to keep up with sexually nimble mammals and were soon outnumbered.

"If a disease or climate impact is severe enough to kill off most of the young in a generation, it will take a very long time to replace them," zoologist Alex Greenwood of Old Dominion University told LiveScience. "Smaller mammals like rodents, for example, would not be as severely affected since they have multiple young and a very short birth cycle."

Along with mammals, turtles and crocodiles, which can lay hundreds of eggs at a time, managed to survive the mass extinction. Also, because they could take shelter in water, these reptiles probably weren't competing with land mammals for resources.

A mammoth lesson
Then mammals got bigger. And eventually they paid the price.

Several mammoths and other big mammals died off during the Pleistocene/Holocene extinction event, which started around 50,000 years ago and continued through the end of the last major ice age about 10,000 years ago.

Today's large mammals— often with small populations, long gestation periods and late weaning ages—are similarly predisposed to sudden mass extinction, scientists say. For big beasts, taking care of offspring is typically a time sink and an energy drain, and the whole setup makes the young highly susceptible to predation.

Large mammals are also slow to reach sexual maturity, and mortality rates before that age are generally high. The slow pace at which new individuals are introduced to the population presents an obvious challenge to enduring tough times when population numbers drop precariously low. 

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