MORPHOLOGICAL RELATIONSHIPS AMONG COEXISTING HETEROMYIDS - AN INCISIVE DENTAL CHARACTER

We studied morphological relationships among coexisting heteromyid rod ents from two North American deserts. Because limb morphology affects foraging behavior, we divided species into bipedal and quadrupedal gui lds. 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 intersp ersed regularly between bipedal and quadrupedal guild members. Size ra tios for pouch volume tended toward equality among Great Basin heterom yids both within each guild and among combined guilds. For no other tr aits did size ratios approach equality. Incisors of heteromyids are us ed for husking, some of which occurs above ground where predation risk is high. Therefore, husking speed may be critical. For each species t here 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 pr edation risk, and this seed size class may be the object of evolutiona ry specialization. A strong correlation between pouch volume and incis or width supports this hypothesis. Specializing on different seed size s may result from competition.