Sex differences in bone growth of broiler chickens.

Females are often supposed to have a lighter skeleton than males, even in avian domestic species. However, in broiler chickens, females are less susceptible to bone deformities than males. In order to better understand these conflicting facts, male and female broilers were compared for the growth of cortical bone. Morphology, histomorphometry, composition and biomechanical properties of the tibiotarsi were analysed in both sexes at 1, 12, 26 and 42 days of age. The quantity of bone tissue of the tibiotarsus (weight, volume, diameter of the diaphysis, area of the cortex) was smaller in females, although the occurrence of varus-valgus deformations of the intertarsal joint was largely reduced in female chickens (8.8% versus 19.9% in males at 42 days of age). The tibia became significantly lighter in females from 26 d of age. Differences in tibia length and volume became significant at 42 d of age only, while cross-sections of the diaphysis were smaller in females from the hatching, leading to thinner bones in females. The percentage of dry matter of tibiae was higher in females from hatching. From 12 days old, tibiotarsi of females tended to be less porous and were more mineralized (higher ash/dry matter ratio). In females, mineralization proceeded at a higher rate (MAR) until 26 d of age and MAR became higher in males afterwards. The stiffness of the tibia diaphysis was similar in males and females all along the growth. In conclusion, the growth of cortical bone is very different in male and female broilers. In female broilers, the thinness of bone diaphysis is counter-balanced by modifications in the composition of the matrix and in the porosity of the cortex, leading to equal biomechanical characteristics of tibiotarsi in both sexes.