In animal evolution, a classical yet controversial idea posits that bigger is better. Bigger animals can reign supreme as hunters, fighters, and mates by simply out-bulking the competition. Living large enables easier survival, so the long-held theory goes. Called “Cope’s Rule,” after paleontologist Edward Drinker Cope, the concept first emerged among paleobiologists in the 19th century. Initially, Cope’s Rule seemed like a good explanation for the mega mammals like wooly mammoths and saber tooth tigers that once roamed North America. For dinosaurs, too, Cope’s Rule appears to fit. But exceptions to the trend soon became apparent.
Over millennia, some animal species and populations have grown smaller, not larger, according to the fossil record. Ice Age horses that occupied Alaska in the distant past, some anole lizards living on Caribbean islands, many types of turtles, and even dragonflies all shrunk through time, research suggests. These observations run counter to prevailing expectations of animal size, and have gone largely unexplained for decades. A newly proposed evolutionary hypothesis, tested via computer modeling, could help solve the mystery.
Intense levels of competition between species and a high risk of extinction leads animals to trend smaller, according to the theory outlined in a study published January 18 in the journal Communications Biology. The researchers built a complex computer model of community evolution with 20 different mathematical parameters including things like predation rate and baseline mortality. They tracked a modeled group of interacting organisms through a time progression across multiple scenarios and explored how body size, species abundance, and ecological niche shifted under different circumstances. Through their model, they explored various resource and risk dynamics and found size outcomes changed depending on the scenario.
In some set-ups, Cope’s Rule does indeed appear to hold…
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