Mammals vary enormously in size, from weighing less than a penny to measuring more than three school buses in length. Some groups of mammals have become very large, such as elephants and whales,【C1】______others have always been small, like primates. A new theory developed by an international team, led by Jordan Okie,【C2】______an explanation for why and how【C3】______groups of organisms are able to evolve【C4】______sizes, whereas others are not. The research team【C5】______information on how quickly an individual animal grows and used it to【C6】______how large it may get【C7】______evolutionary time. Their research was published recently. The new theory developed from the observation that some animals【C8】______fast and die young, while others take their time and mature much later. 'Fast' animals-such as mice—breed very quickly, while humans mature slowly and are relatively older when they first have children. The theory【C9】______that those species that are relatively faster are more likely to evolve a large size quicker than slow species, and that their maximum size will be greater. The research team tested their theory using the【C10】______records of mammals over the last 70 million years, examining the maximum size of each mammal group【C11】______that time, including whales, elephants, seals and primates. They found that their theory was very well【C12】______. 'Primates have evolved very slowly, and never got bigger than 1,000 pounds,' said Okie, 'The【C13】______was true of whales, which evolved their large size at the fastest【C14】______recorded.' The theory also makes predictions about the relative risks of extinction for large animals【C15】______small. The maximum size of an animal is limited by the rate of mortality in the【C16】______. Because larger animals【C17】______breed less frequently than smaller animals,【C18】______the mortality rate doubles, the maximum size is predicted to be 16 times smaller. 'This is a really surprising【C19】______,' said Okie. 'It points to another reason why many of the large animals went【C20】______after the last Ice Age, and their high risk of extinction in modern environments.' 【C1】