How big can a land animal be?
To get really big, animals need two things: big spaces and big amounts of food. “Giants need substantial food resources, and it seems quite logical that as the planet became dominated by humans, the distribution range of these large species became increasingly limited,” says Alexandra Houssaye, research director at the National Museum of Natural History in Paris. Eventually, there was simply little space left for these giants. As a result, there are very few giant land animals today. “When resources are limited, as is the case today, it places a limit on how big a land animal can be,” adds Houssaye. “While it is possible to be big when resources are plentiful, large animals quickly become threatened when those resources begin to diminish.” Beyond a lack of resources, Houssaye says there are other factors that could impact an animal’s size. For one, it’s not always easy for big animals such as elephants, rhinos or hippos to move so much weight. “Because big animals cannot move fast or quietly, they tend to be herbivores, not predators,” she explains.
A weighty issue
For her work on the GRAVIBONE project, which was funded by the European Research Council, Houssaye and her colleagues used 3D models and tomography to better understand how the skeletons of large animals adapt to their tremendous weight. They found these animals have evolved to reduce the musculoskeletal constraints associated with heavy weight. For example, many of today’s big animals have massive bones with more developed contact areas and muscle attachments. Some animals also feature a marked increase in bone compactness, to increase resistance. “The biggest giants, including today’s elephants, have evolved columnar limbs, meaning their limbs are not flexed like in other land mammals,” notes Houssaye. “Instead, the limb bones are positioned vertically.” Houssaye says that while this positioning does help reduce the stress put on the bones, this benefit compromises the animal’s ability to move and prevents them from being able to gallop. While it might not always be easy for big animals to escape predators, often they don’t have to. “More often than not, their sheer size is enough to dissuade a predator – the exception being humans,” says Houssaye. You can read more about Houssaye’s research on bone adaptation in heavy animals here: Understanding bone adaptation in heavy animals.
Keywords
GRAVIBONE, bone, elephants, herbivores, animals, land animal, rhinos, hippos, predators, bone adaptation