MATERNAL TRAITS AND REPRODUCTION IN RICHARDSONS GROUND-SQUIRRELS

Differences among conspecifics in body mass result from underlying dif ferences in structural size and physiological condition. To determine whether the structural or physiological component of body mass has a s tronger influence on reproductive traits at parturition, we studied th e body composition (lean dry mass and fat content), structural size (1 st principal component scores computed from 10 skeletal measurements), and body condition (residuals from regression of body mass on structu ral size) of yearling and older female Richardson's ground squirrels ( Spermophilus richardsonii). At parturition, differences among yearling s in body mass primarily reflected differences in structural size asso ciated with continuing structural growth. Older females appeared to re ach a deterministic adult structural size, and body mass of older fema les was most strongly associated with body condition. Structural size of yearling females and body condition of older females had significan t positive effects on litter mass. For older females, date of parturit ion had a significant negative influence on litter size. Both yearling and older females exhibited significant positive effects of parturiti on date and body condition on neonate mass. In older females, a trade- off resulted in smaller litters of heavier neonates as the breeding se ason progressed, perhaps reflecting the need for rapid growth of offsp ring before hibernation. Maternal size, maternal condition, and season al timing were important aspects of reproduction in Richardson's groun d squirrels, but these characteristics are seldom considered concurren tly in studies of life histories.