The elephant is the largest of all land mammals living today. Photo: Brian McNab
Reading Dinosaurs were lazy animals: The herbivores among them could become so big because they did not move much, reports an American researcher. In addition, they were able to convert their energy five times more efficiently than land mammals living today. The combination of these factors with a wide range of food made the dinosaurs the largest land animals ever to live on Earth. Only some marine mammals such as the blue whale reach even larger body masses. The energy that an animal consumes relative to its mass determines its maximum size, Brian McNab of Florida shows in his new study. When two animals consume the same amount of energy, the animal becomes larger and uses its energy more efficiently. McNab has developed a model that compares the energy consumption of dinosaurs with that of today's mammals and reptiles. He used data from a study that determined the energy expenditure of free-living reptiles, birds and mammals. The giants consumed only about one-fifth of the energy that is consumed by living mammals of the same mass, McNab reports. Sauropods? four-legged herbivorous dinosaurs like Apatosaurus? So they were probably so big because they could use their energy very efficiently. Apatosaurus is the correct name for the colloquially known in the vernacular under the name Brontosaurus.

Although dinosaurs seem to have had lower energy expenditure than mammals, they still have a higher than most reptiles living today: at least they seem to have consumed more than three times as much energy as lizards in relation to body size. If dinosaurs had an energy turnover like lizards, they would probably have weighed 330 tons, the study shows. Thus, dinosaurs seem to have had an energy turnover intermediate that of most reptiles and mammals, McNab concludes.

Extinct giants such as Apatosaurus have their mass probably due to the large supply of food and a very efficient energy utilization through little movement, the results show. How these two factors are related, is also clear from the example of marine mammals: their energy turnover is about twice as high as the land-living mammals. Animals like whales need so much energy to keep their body temperature constant. That they can actually consume so much energy is probably due to the great food supply of the sea.

Brian McNab (University of Florida, Gainesville): PNAS, doi: 10.1073 / pnas.0904000106 ddp / Stefanie shrub display


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