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

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Abstract
Body-size trends were examined for populations of mammals occurring on islands bordering Baja Calfornia, Mexico. Most rodent species (Peromyscus, Neotoma) display gigantism on the islands, a pattern consistent with observations reported elsewhere. Species of Perognathus are characteristically dwarfed. 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 compeptition for available seeds is intense. Large body size should be favored among generalist species (Peromysus, 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 competitors and predators and large body size itself may allow generalist species to exploit a grater range of food sources. The effect of depressed resource levels is greater for specialists than it is for generalists on islands. A review of body-size trends in other insular mammals indicates that the present hypothesis may have wide applicability.