The Evolution of Body Size in Mammals: Evidence from Insular Populations in Mexico

Abstract

Body-size trends were examined for populations of mammals occurring on islands bordering Baja California, Mexico. Most rodent species (Peromyscus, Neotoma) display gigantism on the islands, a pattern consistent with observations reported elsewhere. However, species of Perognathus are characteristically dwarfed. I suggest that metabolic conservatism, expressed through reductions in body size, should be favored in insular rodents (Perognathus) which specialize on particulate resources because their food supply is distributed in a heterogeneous fashion: Seed-size diversity is reduced on islands, search costs are high for animals exploiting depleted patchy resources distributed in a rocky medium, and competition for available seeds is intense. In contrast, large body size should be favored among generalist species (Peromyscus, Neotoma) because they exploit a more homogeneous environment: Search costs are relatively low, diversity and overall levels of resources are expanded because of reduced numbers of compe...