Why aren't rabbits and hares larger?

Area of Science:

• Macroevolutionary biology
• Paleontology
• Ecology

Background:

• Competition among large clades is a known factor influencing lineage diversification.
• Mechanistically clear examples of macroevolutionary competition effects are rare.
• Lagomorphs exhibit limited phenotypic diversity and insular gigantism, suggesting potential evolutionary constraints.

Purpose of the Study:

• To investigate the hypothesis that competition with ungulate-type herbivores (UTHs) constrained lagomorph evolution.
• To explain the limited phenotypic diversity and insular gigantism observed in lagomorphs.

Main Methods:

• Analysis of modern ecoregion data to correlate UTH body mass with lagomorph body mass.
• Examination of local-population energy use scaling patterns to identify competitive disadvantages.
• Modeling of North American lagomorph body mass trajectories from the late Eocene to Pleistocene.

Main Results:

• Minimum UTH body mass significantly predicts maximum lagomorph body mass in modern ecoregions.
• Lagomorphs over ~6.3 kg show a competitive disadvantage against artiodactyls, aligning with their observed size limits.
• Fossil data indicate lagomorph maximum body mass was capped by the smallest contemporary perissodactyl or artiodactyl.

Conclusions:

• Competition with UTHs, alongside intra-clade competition and predation, regulated lagomorph body size evolution.
• Phenotypic boundaries among clades within an adaptive zone can be conditionally coupled.
• Biotic and abiotic factors synergistically drive diversity patterns.