Experiments which measure seed removal rates of ants, birds, and mammals were conducted in the Sonoran Desert of North America and the Monte Desert of South America. Rodents were the primary consumers in North America, but ants also took large amounts of seeds there. Ants were the major granivores in the Monte, where mammal seed consumption was insignificant. Birds were relatively unimportant consumers in both deserts. Total seed removal rates were more than an order of magnitude larger in the Sonoran Desert. These results are much in contrast with the great overall similarity of the 2 systems in climate and flora. The extinction, 1 or 2 million yr ago, of a marsupial family, Argyrolagidae, which appears to have been the ecological equivalent of the rodent granivores, Heteromyidae, may have cause the differences in granivory either because the Argyrolagidae were mutualistic with the plants or, in an evolutionary sense, with the granivorous ants.