Move over, Leo. Give me more elbow room

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 Sciences.

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.

For example, elephants that reached the Mediterranean island of Crete from the European mainland evolved into two dwarf species. Conversely, on the Indian Ocean island of Mauritius, a small pigeon evolved into the flightless, 19-kilogram dodo.

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.