Research: A New Understanding of Animal Size Changes Over Time: Competition, Ecological Pressures, and Cope's Law
A recent study revealed the factors influencing changes in animal size over time, identified three evolutionary patterns based on competition and environmental pressures, and provided an understanding of the inconsistencies in the fossil record.
New research has revealed key drivers of the changing sizes of certain animals over time, challenging traditional evolutionary theories with its findings on changes in species size.
The mystery behind why Alaskan horses, cryptodiran deer and island lizards shrank over time has been solved in a new study. The new theoretical research suggests that the size of animals over time depends on two key ecological factors: the intensity of direct competition for resources between species, and the danger of extinction from the environment.
The research findings and ecological factors
Using computer models that simulate evolution, the research, published in the journal Biology Communications, explains why certain species become progressively smaller, as evidenced by the fossil record.
Dr Shobunal Roy, Ecosystem Modeler from the University of Reading who led the research, said: "Just as we try to adapt to hot or cold weather depending on where we live, our research shows that the size of animals can grow or shrink over long periods of time, depending on the home the growth or environmental conditions."
"In places and times where there is a lot of competition between different species for food and shelter, animal sizes often get smaller as species spread and adapt to the distribution of resources and competitors. For example, small horses that lived in Alaska during the Ice Age shrank rapidly due to changes in climate and vegetation.”
"In places where direct competition is less, sizes tend to increase, although being very large and few in number can make the animals more vulnerable to extinction."
"Changes in ecological factors help explain why the fossil record shows such confusing mixtures of size evolutionary patterns, with some lineages getting smaller over time and others getting bigger."
The research team challenged the contradictions that fossil evidence posed to "Koop's Law". Cope's rule refers to the tendency of certain groups of animals to develop larger body sizes over thousands and millions of years. The rule is named after Edward Cope, a 19th-century paleontologist who first noticed this pattern in the fossil record. For example, the ancestors of horses were Small animals the size of a dog which grew over evolutionary time, eventually developing into the modern horse.
However, fossil evidence for some lineages shows strikingly contradictory trends, with increasing size in some groups but decreasing size in others.
Using computer models that simulate evolution, the study identified three different patterns of change in body size that occur under different conditions:
- Gradual increase in size over time: This occurs when competition between species is determined primarily by their relative body size rather than by niche differences. For example, some types of marine animal species (e.g. invertebrates) gradually increased for millions of years.
- Increase in size followed by extinction: Here the largest animals go extinct again and again, which opens up opportunities for other species to take their place and develop even larger bodies, continuing the cycle. Mass extinctions have hit large carnivores the hardest. Very large mammals and birds particularly vulnerable to extinction – For example, dinosaurs and giant flying reptiles.
- Gradual decrease in size Over time: the simulations also predicted the opposite of Kopp's law: small species over time. This happens when competition is high and there is some degree of overlap in habitat and resource use. As species evolve from each other into separate niches, they face evolutionary pressure to reduce their size. A decrease in size has previously been reported for Vertebrates, Bony fish, cryptodiran deer, Pleistocene horses of Alaska and island lizards.
For the scientific article: "Ecological factors of Cope's law and its opposite law" January 18, 2024, Biology of communication.
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