Imagine that you're a beaver: you might find enough small prey to easily survive. Now, imagine that you're a polar bear weighing half a ton, and things should become more difficult: you'll need to catch bigger preys. In Why are lions not as big as elephants?, a short press release from the Public Library of Science, you'll discover that as it takes more energy to catch a large prey than a small one, and that a carnivore size cannot really exceed one ton. Apparently, life was easier for herbivores who were able to reach 30 tons or more...
Here is a short excerpt from this news release.
Smaller species less than 15-20 kg in weight specialize on very small vertebrates and invertebrates, which weigh a small fraction of their own weight, whereas larger species (>15-20 kg) specialize on large vertebrate prey near their own mass. While carnivores around the size of a lynx or larger can obtain higher net energy intake by switching to relatively large prey, the difficulty of catching and subduing these animals means that a large-prey specialist would expend twice as much energy as a small-prey specialist of equivalent body size.
You can read the full research paper published by PLoS Biology, The Costs of Carnivory (Vol. 5, No. 2). This study was led by Chris Carbone, of the Institute of Zoology in London. Here is the author's summary.
Carnivores fall into two dietary groups based on the energetic requirements of their feeding strategies: small-bodied species, which feed mostly on prey smaller than themselves, and large-bodied species, which prefer prey around their own size. While carnivores around the size of a lynx or larger can obtain higher net energy intake by switching to relatively large prey, the difficulty of catching and subduing these animals means that a large-prey specialist would expend twice as much energy as a small-prey specialist of equivalent body size. Analyzing the balance between energy intake and expenditure across a range of species, we predict that mammalian carnivores should have a maximum body mass of one ton.
For more information, you can check Why lions are smaller than elephants (New Scientist, January 16, 2007) or How big can a meat-eater get? (John Whitfield, Nature, January 15, 2007), which explains why the metabolic costs of living have stopped carnivores from growing too huge. And this article looks at the -- not so bright -- future of polar bears.
The result gives a fresh perspective on why large carnivores are particularly vulnerable to extinction, and brings more bad news for polar bears. Not only is the arctic ice melting beneath them, but today's largest land carnivore also lives on a metabolic precipice, barely able to catch enough food to support its bulk, say Chris Carbone of the Institute of Zoology, London, and his colleagues.
I've never thought before that there was a upper limit for the weight of carnivores. But imagine a 30-ton lion living in Africa. It will have to chase and to eat a thousand antelopes a day. What a boring life!
Sources: Public Library of Science news release, January 15, 2007; and various other websites
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