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.
