Subspecific Variation in Life-History Traits of Hispid Cotton Rats (Sigmodon hispidus)

I investigated postnatal growth and sexual maturation, food processing, and reproduction in four subspecies of the hispid cotton rat (Sigmodon hispidus). These traits were compared among subspecies and with previously published data. Subspecific differences in postnatal growth rate, energy digestion and assimilation rates, thyroxin use rate, and mass of neonates were determined in laboratory-reared cotton rats. Sizes of field- and laboratory-conceived litters also were measured. Variability in size of adults and in postnatal growth rate was low among subspecies. The body mass of neonates and mass at vaginal opening and at first estrus differed significantly, however, among most subspecies. Significant differences in sizes of field-conceived litters existed among subspecies, but no significant differences persisted under a single set of laboratory conditions. No significant subspecific differences in energy digestion or assimilation occurred. These results indicate that much of the reported subspecific variation in size of adults, growth, and size of litters that occurs among populations in the field is due to environmental factors.