Meeting Abstract
Wing bones of adult birds may have microscopic adaptations for flight. In birds that use continuous flapping flight, circumferential vascular canals are abundant in the humerus and ulna, where they form laminar bone presumably to resist twisting loads during flight. In contrast, the paucity of laminar bone in the radius presumably reflects reduced twisting loads in that part of the wing. Here, we test this biomechanical hypothesis with ontogenetic data. If wing bone laminarity is an avian adaptation to resist flight-induced torsion, then it should increase with age until maturity. Alternatively, if wing bone laminarity reflects allometry, then laminarity should vary directly with growth rate and decrease with age. We collected 19 pigeons of known age (0 – 9 weeks post-hatching). Transverse sections were cut from the midshaft of the humerus, ulna, and radius. Midshaft bone circumference was measured and used to plot growth curves for each element. From these growth curves, we calculated the mean circumferential growth rate for the elements of each sampled specimen. In elements containing cortical bone, the proportion of circumferential to total canals (laminarity index) was calculated. Linear regression was used to assess the correlation between laminarity and mean circumferential growth rate. Our results reveal that wing bone laminarity is greater in juveniles than in adults. Moveover, as mean circumferential growth rate decreases with age, so does wing bone laminarity. Linear regression analysis suggests that growth rate explains approximately 70% of the variation in wing bone laminarity. We conclude that wing bone laminarity, at least in the pigeon, is not a flight adaptation but instead an expression of ontogenetic allometry. Evolutionary shifts in ontogenetic allometry may explain interspecific variation of wing bone laminarity across birds.
