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Island Lizards Shrink and Horses Get Bigger Because of These Evolutionary Patterns

Why do certain animals get bigger and others get smaller over time? Learn why the size of an animal depends on food availability and extinction risk.

By Elizabeth Gamillo
Jan 18, 2024 10:15 PMJan 19, 2024 2:15 PM
(Credit: GoodFocused/Shutterstock)


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A new computer model might explain why some animals shrink over time while others grow. It has to do a lot with their habitat and resource availability.

"Where direct competition is less, sizes tend to get bigger, even though being big and few in number can make animals more vulnerable to dying out – such as what happened with the dinosaurs," said Shovonlal Roy, study lead author who specialized in ecosystem modeling at the University of Reading, in a press release. The study was published in Communications Biology.

Why Do Animal Sizes Change Overtime? 

The dinosaurs most likely were their gigantic size because they had less competition. However, their size was also their downfall. As animals get bigger, they are more susceptible to dying out, explained Roy in a statement.

Once the dinosaurs were wiped out, other species (like mammals) could roam Earth freely, have access to food and other resources, and could evolve bigger bodies.

Animals can also evolve smaller bodies if competition for resources is high and other species share habitats. Examples of some species shrinking include island lizards and cryptodira turtles.  

Read More: How Did Some Animals Survive the Asteroid that Killed the Dinosaurs?

What Is Cope's Rule?

In biology, Cope's rule explains that animals will increase in body size as time and generations pass. One example of this is the ancestors of horses. During the Eocene Epoch, the oldest known horse, Hyracotherium, was about the size of a small dog, but horses look much bigger today.

On the fossil record, paleontologists have seen mixes of evolution patterns. Animals that were once giant shrank, and those that were small grew bigger. The fossil records also line up with ecological changes.

Roy and his team used a computer model and tested Cope's rule to see how and why body sizes increased or decreased across the evolutionary timeline. Two of three evolutionary patterns and predictions tested were consistent with Cope's rule.

Gradual size increased over time, and species increased in size following an extinction. The only prediction that did not follow Cope's rule was that animals also could decrease in size over time. Cope's rule may not be applicable to all evolutionary predictions. The study's authors note that to fully understand how body sizes evolve, the rates of evolution and the levels of interference between competitions must be considered.

Read More: How Sauropods Evolved to Their Enormous Size

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