Scaling of the appendicular skeleton of the giraffe (Giraffa camelopardalis)

Abstract

Giraffes have remarkably long and slender limb bones, but it is unknown how they grow with regard to body mass, sex and neck length. In this study we measured the length, medio-lateral diameter (ML), cranio-caudal diameter (CC) and circumference of the humerus, radius, metacarpus, femur, tibia and metatarsus in 10 fetuses, 21 females and 23 males of known body masses. Allometric exponents were determined and compared. We found the average bone length increased from 340±50mm at birth to 700±120mm at maturity, while average diameters increased from 30±3mm to 70±11mm. Fetal bones increased with positive allometry in length (relative to body mass) and in diameter (relative to body mass and length). In postnatal giraffes bone lengths and diameters increased iso- or negatively allometric relative to increases in body mass, except for the humerus CC diameter which increased with positive allometry. Humerus circumference also increased with positive allometry, that of the radius and tibia isometrically and the femur and metapodials with negative allometry.Relative to increases in bone length, both the humerus and femur widened with positive allometry. In the distal limb bones ML diameters increased isometrically (radius, metacarpus) or positively allometric (tibia, metatarsus) while the corresponding CC widths increased with negative allometry and isometrically respectively. Except for the humerus and femur, exponents were not significantly different between corresponding front and hind limb segments. We concluded that the patterns of bone growth in males and females are identical. In fetuses the growth of the appendicular skeleton is faster than it is after birth which is a pattern opposite to that reported for the neck. Allometric exponents seemed unremarkable compared to the few species described previously, and pointed to the importance of neck elongation rather than leg elongation during evolution. Nevertheless, the front limb bones and especially the humerus may show adaptation to behaviors such as drinking posture.