Tuesday, January 16, 2007

Why Lions Are Not As Big As Elephants

The costs of carnivory. 2007. B. Carbone et al. PLoS Biol 5(2): e22.
Researchers examine the relationship of predator mass and energy expenditure in capturing prey of varying sizes and reveal how this relationship might have led to the extinction of large carnivores in the past.
By analyzing the balance between energy intake and expenditure across a range of species, the authors reveal that mammalian carnivores would not be able to exceed a body mass of one ton. Their model predictions are consistent with the data we have. Most mammalian carnivores are relatively small compared with the largest extinct terrestrial herbivorous mammals, such as the Indricothere (above), which weighed around 15 tons.

The largest existing carnivore, the polar bear, is only around half a ton, while the largest known extinct carnivores, such as the short-faced bear, weighed around one ton.

The authors also note that the largest terrestrial non-mammalian predators, such as Giganotosaurus and Tyrannosaurus, may have achieved their massive size by having a lower metabolic rate. Indeed, previous estimates of total metabolic rate for these species are similar to those of a mammal weighing about a ton.

Carnivores at the upper limits of body mass would have been heavily reliant on abundant large prey to both minimize energy expenditure and maintain high rates of energy intake. Slight environmental perturbations, man-made or otherwise, leading to lower prey availability, could readily upset this energy balance. It may have also contributed to the extinction of the largest carnivores and explain why the largest modern mammalian carnivores are so rare and vulnerable today.