Animal size

Why do some species get bigger and others smaller?

Species are known to adapt to environmental pressures by changing size. Sometimes they get bigger, other times they grow smaller.

Elephants, rhinos and hippos have all reduced in size when living on small islands. In his guide to prehistoric mammals, Donald R. Prothero outlines the possible causes for this. He writes:

Ecological theory suggests that once they no longer had to cope with large predators, and also had to live on the much smaller base of plant resources of an island, large mammals shrank in body size because there were few advantages to being huge.

Prothero also outlines the reasons why it might benefit an animal to grow large. These reasons are:

  • It’s easier to fight off predators when you’re a big animal
  • Large size is a competitive advantage when fighting with other males for mates
  • Big predators can capture big prey
  • Larger animals can travel longer distances in search of food and water

Marine animals got bigger over 542 million years

In 2015 scientists from Stanford University shared the results of their research into animal body size. The team studied data on 17,208 marine animal genera from across the ages. Some of the creatures investigated lived 542 million years ago.

The researchers discovered that marine animals have increased in size over the last 542 million years. The increase was not seen in all species and some animals grew in size far more than others. However, when all marine animals were considered as a group, the researchers found a 150-fold increase.

This mean biovolume increase occurred because groups of animals that were larger early on, evolved into a greater number of new species than did animals that started out smaller.

Stanford palaeontologist Jonathan Payne said:

“That’s also something we didn’t know before … For reasons that we don’t completely understand, the classes with large body size appear to be the ones that over time have become differentially more diverse.”

Is bigger always better for animals?

The truth is that scientists are still working to understand everything about animal size. Size is no simple matter. Even within species, some individuals are larger than others because of social and environmental pressures.