Diverging selection on body size in specialist terrestrial mammals

Body size is a fundamental organismal trait, affecting a wide variety of physiological and ecological functions. Such relationships are often interactive and nonlinear, forming complex feedbacks. In terrestrial mammals, larger bodies are associated with higher mobility in trade-off with larger absolute resource demand. Here we propose a hypothesis, with support from empirical patterns and a mathematical model, that this trade-off interacts with diet specialization to drive diverging selection on body size because specialists are more efficient resource users and have lower mortality risks at extreme sizes. Our analysis of a global terrestrial mammal species dataset found significantly lower proportions of specialists at intermediate sizes, but higher proportions towards extreme sizes; this pattern also applies to species assemblages in zoographic realms. Our mathematical model of coexistence between equal-sized specialists and generalists shows that specialists of extreme sizes have higher equilibrium frequencies and likelihood of coexistence with generalists at quasi-stability. The combined results support dietary specialization as a key factor for shaping body size diversity. Our work highlights the value of connecting ecology and evolution in understanding the diversity of key traits like body size, and calls for further investigations on the related history of resource distribution and lineage diversification.