Shrinking giants: Unraveling the evolution of animal size

Researchers have proposed a new theory explaining the puzzling trend of animal size evolution, a topic that has intrigued scientists for decades.

Led by Dr. Shovonlal Roy from the University of Reading, the research provides insights into why some species, such as Alaskan horses, Cryptodira turtles, and island lizards, have exhibited a decrease in size over time.

Evolutionary pressures

The study hinges on the interplay of two pivotal ecological factors: the intensity of direct competition for resources between species, and the risk of extinction due to environmental factors. 

In scenarios where direct competition is less fierce, species tend to grow larger. However, this increase in size can make them more susceptible to extinction, as observed in the case of dinosaurs. This intricate balance highlights the dynamic nature of evolutionary pressures shaping animal sizes.

“Just like how we try to adapt to hot or cold weather depending on where we live, our research shows animal size can get bigger or smaller over long periods depending on the habitat or environment,” said Dr. Roy.

Direct competition 

“In places and times where there’s lots of competition between different species for food and shelter, animal sizes often get smaller as the species spread out and adapt to the distribution of resources and competitors. For example, small horses that lived in Alaska during the Ice Age rapidly shrank due to changes in the climate and vegetation.”

“Where direct competition is less, sizes tend to get bigger, even though being really big and few in number can make animals more vulnerable to dying out – such as what happened with the dinosaurs.”   

“Changes in ecological factors help explain why fossil records shows such confusing mixes of size evolution patterns, with some lineages shrinking over time and others growing.” 

Challenging previous assumptions 

Using sophisticated computer models to simulate evolution, the research team uncovered patterns in size evolution that challenge previous assumptions, particularly those related to “Cope’s rule.”

This rule, named after 19th-century paleontologist Edward Cope, suggests a general trend in the evolution of animal groups towards larger body sizes over millions of years. 

For instance, the evolution of horses from small, dog-sized ancestors to their current larger forms exemplifies this rule. However, the fossil record presents a more complex picture, with some species growing larger while others become smaller.

Cope’s rule

“Cope’s rule states that lineages evolve toward larger body sizes over evolutionary time,” wrote the study authors. 

“Recognizing that larger body sizes often improve an animal’s ability to capture prey, avoid predators, fight competitors, maintain metabolism, raise thermal inertia, accommodate climatic variation, withstand starvation, extend longevity, attract mates, and enhance reproductive success, researchers have hypothesized that Cope’s rule applies to all animals – and to all mammals in particular. However, empirical evidence concerning body-size evolution is remarkably conflicting.”

Three distinct patterns 

The researchers identified three distinct patterns of body-size change:

• Gradual size increase over time, typically observed in marine animals, where competition is primarily based on body size rather than niche differences.
  
• Size increase followed by extinction events, where large-bodied apex predators are most vulnerable, leaving room for other species to evolve into larger forms.
  
• Gradual size decrease over time, occurring in situations of high competition and overlapping habitat and resource use, leading to a divergence into distinct niches and subsequent size reduction.

Study implications 

“The three different macroevolutionary patterns our analyses have revealed have potential to reconcile and refine the debate on the validity of Cope’s rule by providing a richer conceptual framework for assessing and understanding phyletic patterns,” wrote the study authors. 

“In moving beyond merely documenting a trend toward larger body sizes by distinguishing between a gradual increase to an evolutionary equilibrium and a perpetual increase driven by extinctions, it may be possible to focus paleontological efforts on taxa and systems that are known to have higher extinction risks and in which selection pressures for higher body sizes may be larger.”

“Thus, our study is opening up possibilities for mapping ecological determinants and process-based mechanisms onto the contrasting patterns in body-size evolution observed in nature. In this way, several testable hypotheses can be formulated based on the findings we have reported here.”

The study is published in the journal Communications Biology.

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