Objectives Previous studies suggest that the postures habitually adopted by an animal influence the mechanical loading of its long bones. Relatively extended limb postures in larger animals should preferentially reduce anteroposterior (A‐P) relative to mediolateral (M‐L) bending of the limb bones and therefore decrease A‐P/M‐L rigidity. We test this hypothesis by examining growth‐related changes in limb bone structure in two primate taxa that differ in ontogenetic patterns of joint posture. Materials and Methods Knee and elbow angles of adult and immature vervets (Chlorocebus aethiops, n = 16) were compared to published data for baboons (Papio hamadryas ursinus, n = 33, Patel et al., ). Ontogenetic changes in ratios of A‐P/M‐L bending rigidity in the femur and humerus were compared in skeletal samples (C. aethiops, n = 28; P. cynocephalus, n = 39). Size changes were assessed with linear regression, and age group differences tested with ANOVA. Results Only the knee of baboons shows significant postural change, becoming more extended with age and mass. A‐P/M‐L bending rigidity of the femur decreases during ontogeny in immature and adult female baboons only. Trends in the humerus are less marked. Adult male baboons have higher A‐P/M‐L bending rigidity of the femur than females. Conclusions The hypothesized relationship between more extended joints and reduced A‐P/M‐L bending rigidity is supported by our results for immature and adult female baboon hind limbs, and the lack of significant age changes in either parameter in forelimbs and vervets. Adult males of both species depart from general ontogenetic trends, possibly due to socially mediated behavioral differences between sexes. Am J Phys Anthropol 161:72–83, 2016. © 2016 Wiley Periodicals, Inc.