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Dinosaurs, Size, and Land Area
Very interesting article in the current Science News (Dec 1).
The reference is at
but you have to have a subscription to get at it.
The average size of the largest modern-day land animals on each of 25
oceanic islands and five continents strongly depends on the land area
there, a new study shows. The formula holds across diverse animals and
habitats, from the iguanas and owls that live on wave-washed outcrops
in the Galpagos Islands to the lions and elephants that populate the
plains of Africa.
The research looked at the largest carnivores and herbivores found in
each of the areas during the past 65,000 years. That restricts the
analysis to the period for which there's a fairly complete fossil
record, says Jared Diamond, an evolutionary biologist at University
of California, Los Angeles. It also enables the researchers to
consider large animals that went extinct only recently, including
mammoths and lions in North America, saber-toothed tigers in
South America, and elephant birds on Madagascar.
For a given land area, the largest warm-blooded herbivores were
about 14 times as heavy as the biggest warm-blooded carnivores.
Among cold-blooded animals, plant eaters were around 33 times
as heavy as meat eaters. Diamond and his colleagues report their
findings in the Dec. 4 Proceedings of the National Academy of
The team's results are "compelling" and can be explained in terms
of the food requirements of individual animals, says Stuart L.
Pimm, a conservation biologist at Columbia University. Ecosystems
can support larger populations if species evolve to smaller sizes
or if their members' metabolisms become more efficient. Successive
generations of animals that survive tend to grow or shrink according
to the availability of food and living space.
The low metabolic rates of cold-blooded animals may explain why a
reptile can be the largest meat eater or vegetarian in an ecosystem.
On the Indonesian island of Flores, the 70-kg Komodo dragon rules
the jungle, even preying on wild horses. The island's food supply
would support a self-sustaining population of warm-blooded carnivores
only if they weighed at most 5 kg, says Diamond. Predators that small
would be no match for a horse, he notes.
The formula derived from modern-day animals also holds for
Indricotherium, an 11,000-kg rhino relative that roamed southern Asia
about 30 million years ago. Diamond's team notes, however, that
dinosaurs don't seem to follow the rule. Even if the largest
dinosaurs had the metabolism of a cold-blooded animal, they were much
heavier than the new study would predict. One possible explanation for
the anomaly, the researchers suggest, is that increased concentrations
of carbon dioxide in the air during the dinosaurs'
reign could have boosted the food supply available to them.
Burness, G.P., J. Diamond, and T. Flannery. 2001. Dinosaurs, dragons,
and dwarfs: The evolution of maximal body size. Proceedings of the
National Academy of Sciences 98(Dec. 4):14518-14523. Abstract
Now this is something I've been curious about for a long time, the size
dinosaurs reached and the land area required to support them.
The abstract says
These patterns explain the size of the largest-ever extinct mammal, but
the size of the largest dinosaurs exceeds that predicted from land areas
and remains unexplained.
This also makes me wonder about the KT impact event. Changes in land
area? Atmosphere composition? The animals that survived, how can they be
viewed in this context?