The size and types of the largest local land animals vary greatly from place to place, prompting scientists to question what controls the success of animals of certain sizes over others. Now a report published in the current issue of the Proceedings of the National Academy of Sciences shows that the size of a landmass limits the maximal body size of its top animal.

Gary Burness and Jared Diamond of the University of California School of Medicine, together with Timothy Flannery of the South Australian Museum in Adelaide, examined the body size and food requirements of top terrestrial animals from the past 65,000 years. The list included herbivores and carnivores from 25 oceanic islands and five continents, ranging from the woolly mammoth of Eurasia to the dwarf hippopotamus of Cyprus. The researchers found that the maximal body size of land animals relates to the size of the landmass on which they live: larger animals require larger individual territories to obtain sufficient food. And because more food is available to herbivores from a given area, they tend to be larger than carnivores inhabiting the same range.

According to the report, this relationship between land area and animal size is strong enough to induce evolutionary change over long time periods. The authors cite examples of animals that migrated from mainland environments to colonize an island for which they were too large and those species that grew in response to a new, relatively colossal home range. The Wrangel Island mammoth, for one, declined approximately 65 percent in body size in the 5,000 years after the severing of the land bridge linking the island to Eurasia.

The new findings, the authors conclude, can explain the size of the largest-ever extinct mammals. Unfortunately, however, "the never-since-surpassed size of the largest dinosaurs remains unexplained."