Scientists have long wondered why some animal species seem to gradually get smaller over long periods of time, as shown by the fossil record. A new study proposes it may depend on two key ecological factors – competition for resources between species and the risk of extinction in their environment.

Using computer models that simulate evolution over thousands or millions of years, researchers from the University of Reading in England identified three patterns of body size change. Their study, published in Communications Biology, helps explain the puzzling mix of size evolution trends seen in fossils.

Lead researcher Dr. Shovonlal Roy said animals’ sizes can increase or decrease like we adapt to hot or cold climates. Where competition for food and shelter between species is high, animal body sizes tend to decrease as species spread out and adapt to resource distribution and competitors, he explained.

For example, small horses that lived in Alaska during the Ice Age got smaller quickly due to climate and vegetation changes. With less direct competition, sizes tend to increase. But being very large with few numbers makes animals more vulnerable to extinction, like dinosaurs.

The simulations predicted a decrease in size over time can happen when competition is high and species’ habitat and resource use overlap some. As species evolve into different niches, there’s pressure to reduce size. Shrinking sizes were already seen in vertebrates, bony fish, cryptodire turtles, Pleistocene Alaskan horses and island lizards.

The team questioned contradictions fossils posed to “Cope’s rule”, which is the tendency of some animal groups to evolve toward larger body sizes over thousands to millions of years. For instance, early horse ancestors were small dog-sized animals that increased in size over evolution to modern horses.

However, fossils show notably contradictory trends, with increased size in some groups but decreased size in others. The simulations identified three body size change patterns arising under different conditions:

  • Gradual increase in size over time occurs when competition is mainly determined by relative body sizes rather than niche differences, seen in some marine invertebrate genera.
  • Size increase followed by recurrent extinctions, where very large animals regularly die out, opening opportunities for other species to fill their place and develop even larger bodies continuing the cycle, especially affecting large predators. Mass extinctions particularly impact large mammals and flying birds like dinosaurs and giant flying reptiles.
  • Gradual decrease in size over time, contradicting Cope’s rule, was also predicted and helps explain observations in vertebrates, bony fish, cryptodire turtles, Alaskan horses and island lizards. As species spread into different niches, there is evolutionary pressure to reduce size when competition is high.

The study sheds new light on why some animals shrink over generations according to their ecological circumstances, resolving longstanding mysteries seen in the fossil record.


University of Reading | Roy, S., Brännström, Å. & Dieckmann, U. Ecological determinants of Cope’s rule and its inverse. Commun Biol 7, 38 (2024).

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