Rabbit and Hare Body Size Evolution

By: Meredith Kime, Courtney Marcos, Sean O’Leary, and Nick Scolaro (Stonehill College, BIO323: Evolution, Spring 2021)

Eastern cottontail: © Gareth Rasberry, shared under the CC BY-SA 3.0 license
Ungulates: © XAttlexattle, shared under the CC BY-SA 4.0 license

The rabbit, commonly known as a visitor in backyards, has evolved in a way to maintain a relatively small body size. In the Evolution article, “Why aren’t rabbits and hares larger?”, researchers Susumu Tomiya and Lauren Miller from UC-Berkeley sought to determine why rabbits and hares retained a small body size. Rabbits and hares are in the order Lagomorpha, along pikas. Compared to its sister clade, which includes rodents, rabbit body size evolution has been limited. This study does not include rabbits domestically bred, only wild populations of rabbits. Rabbits are an ideal group to study because of their limited diversity and extensive range across all continents, except for Antarctica.

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With a great range of factors, including climate, predation, and species competition, all of which could contribute to body size evolution, researchers looked to bridge the gap between these and the retention of rabbit body mass and their competitor’s evolution. Ungulates, large mammals with hooves, are the main competition for the rabbit as they once shared the same niche. The theory of evolution incorporates both living and non-living factors to explain an organism’s adaptations, but, in this case, the researchers assumed that body size evolution was due to environmental, rather than genetic, factors to draw their hypothesis. They predicted competition with other species in the same niche constrained the potential of the rabbit’s maximum body size.

To test the role of competition in optimal body size, the researchers used various models to determine how body mass has evolved. These tests included comparing the body mass of rabbits and ungulates in multiple locations worldwide, using local-population energy values to form an equation used for calculations, and collecting fossil data. The researchers primarily used fossils from different periods, between 23 to 55.8 million years ago, to collect their data and determine why rabbit body size did not change much, but the ungulates did.

The researchers searched for these fossils across various ecoregions and compared the body masses of rabbit fossils to fossils of artiodactyls (including cows, sheep, and pigs) and perissodactyls (which includes horses and rhinos). The researchers performed many statistical tests to determine the relationship between  body mass of rabbits and ungulates. They predicted that rabbits are competitively dominant over similar-sized ungulates if they share resources. The researchers found that at one time, the body mass ranges of rabbits and artiodactyls overlapped, but as extinctions occurred, probably due to competition, the body size ranges of these groups segregated.

Using statistical data and historical patterns, the researchers identified a predictor of the maximum rabbit body mass and called it the “competitive ceiling body mass.” This is because a body mass restraint was placed on rabbits by ungulates, meaning most rabbits did not surpass a certain body mass. After all, the smaller body mass is more advantageous for rabbits, whereas ungulates evolved a larger body mass because it was more beneficial for them. The researchers used the data obtained to determine that of all the statistical predictors, the”competitive ceiling body mass” had the most predictive strength.

This strong predictor adds to more and more evidence that phenotypic evolution in mammals and speciation are not as closely related as previously believed. Researchers studied the relationship between the two by observing rabbits and ungulates because of their simple relationship. The association was easy to observe because they have extensive and well-recorded fossil records. Their ecological changes have been relatively steady over time, and the chance of inter-clade competition is great because they have similar diets. The researchers’ analysis of the rabbits’ body mass shows their maximum body mass is directly related to the minimum body mass of ungulates.

Two basic expectations arose after analyzing these body mass discoveries. The first was that rabbitsweighing more than about 6.3 kilograms (~14 pounds) are at a competitive disadvantage to ungulates of all sizes and are unlikely to coexist with them. The second was that larger rabbits are also at a disadvantage to smaller rabbits, and they also cannot stably coexist with each other. The missing presence of rabbits larger than 6.3 kg where ungulates are present reinforces this first expectation. Meanwhile, habitat-level separation of sympatric large and small rabbits and hares into open and closed habitats supports their second expectation. These scientists were able to find relationships between the patterns pointing to the evolutionary constraints on rabbits placed by herbivore ungulates.

Using the “competitive ceiling body mass” predictor to analyze these relationships found in the data, the researchers concluded the evolution of ungulate size prevented rabbits from evolving to be larger. The researchers found that due to the competition between rabbits and ungulates, the ungulates’ body size grew larger to avoid competition for resources. Since the ungulates evolved to have a greater body mass, the rabbits did not need to evolve to be different, as the ungulates no longer shared their niche. This study has high importance due to its application of competitive exclusion on body size evolution that scientists can apply to relationships between other species within an ecosystem.

Article: Tomiya, S., and L.K. Miller. 2021. Why aren’t rabbit and hares larger? Evolution 75(4): 847–860. https://doi.org/10.1111/evo.14187

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