Abstract
Evolution of the giraffe neck was originally proposed as an adaptation
to foraging at the tops of acacia trees, but this theory has been
overshadowed by the “necks for sex” hypothesis that proposed that long
necks evolved via sexual selection associated with male neck fighting.
The necks for sex hypothesis predicted that males would have longer
necks than females and that their necks would continue to grow
throughout their lives. Because adult giraffe males are much larger than
adult females, male giraffe necks are indeed longer but this is also
true for all the core anatomy. We measured and analyzed the relative
anatomical proportions of the neck, legs, and body trunk of the Masai
giraffe (G.c. tippelskirchi) in captivity in North America and from wild
populations in Tanzania. In contradiction to the necks for sex
hypothesis, female giraffe have proportionally longer necks compared to
their forelegs than males. Moreover, the female body trunk is
proportionally longer whereas male forelegs are proportionally longer.
We speculate that the proportionally longer female neck is to compensate
for female’s overall shorter stature in foraging and their longer trunk
is to accommodate fetal growth. Male’s longer forelegs may be an
adaptation for mounting females during mating. Mean differences in these
major body components define sex phenotypes, but several male and female
giraffe display opposite-sex phenotypes with a significantly higher
level of discordancy is seen in captive males. We speculate that the
sex-differential phenotype is maintained by mate choice selection in the
wild, and this selection is relaxed in captivity where mates are
arranged by humans.