Effects of phylogeny and locomotor style on the allometry of body mass and pelvic dimensions in birds.

The pelvic girdle provides physical support and attachment for the hind limb musculature. In birds there is variability in pelvic morphology across different orders and this has been used as evidence for various types of locomotion. However, the morphological variation of pelvic bones has yet to be studied systematically in birds. Therefore, we investigated basic allometric relationships among female body mass (as a size proxy) and various pelvic measurements in a phylogenetic context. We also examined in detail the inter-relationships among various pelvic measurements. Also considered were the effects of broader taxonomic relationships at the level of order, with further examination of the influence of style of terrestrial locomotion on the allometric relationships. Positive relationships were initially found among all pelvic linear measurements and female body mass (FBM). The relationships among measures of pelvic width and FBM were isometric, whereas those between pelvic length and FBM showed positive allometry. Also, FBM explained more of the variance in pelvic length than in width. Similarly, the angle of the pelvis had a significant negative relationship, but FBM only explained a very low proportion of the variation in pelvic angles. In general terms, ancova showed that the effect of FBM was smaller than the effect of locomotor style in this species set. Both the synsacrum and pelvic girdle play roles in weight support and therefore scale in proportion to body weight accordingly. All three parts of the pelvis (ilium, ischium and pubis) are attached around the acetabulum and also provide muscle attachment points, so it might be expected for them to scale in a similar manner. Increased angulation of the pelvis is linked to orders which employ their hind limbs in feeding, perching and running, although FBM also explains a very low proportion of the variation in pelvic angle. Muscle attachment and the confines on morphology presented by locomotion explain much of the variation exhibited by the relationships among pelvic measurements.