MORPHOLOGICAL RELATIONSHIPS AMONG COEXISTING HETEROMYIDS - AN INCISIVE DENTAL CHARACTER

We studied morphological relationships among coexisting heteromyid rodents from two North American deserts. Because limb morphology affects foraging behavior, we divided species into bipedal and quadrupedal guilds. The trait for which interspecific size ratios tended most toward equality both within each guild and among combined guilds is the width (cutting edge) of the upper incisor. Sizes of this trait were interspersed regularly between bipedal and quadrupedal guild members. Size ratios for pouch volume tended toward equality among Great Basin heteromyids both within each guild and among combined guilds. For no other traits did size ratios approach equality. Incisors of heteromyids are used for husking, some of which occurs above ground where predation risk is high. Therefore, husking speed may be critical. For each species there may be an optimal size for a seed that is too large to be pouched unhusked but that can be husked efficiently enough to outweigh the predation risk, and this seed size class may be the object of evolutionary specialization. A strong correlation between pouch volume and incisor width supports this hypothesis. Specializing on different seed sizes may result from competition.