Sexual dimorphisms in body proportions of Masai giraffe

Abstract

Abstract Giraffes exhibit a large sexual dimorphism in stature and body mass. Whether sexual dimorphisms also exist in relative body proportions of the axial and appendicular skeleton has been debated, particularly regarding the evolution of the giraffe’s iconic long neck. We measured and analyzed the relative anatomical proportions of the neck, legs, and body trunk of the Masai giraffe (G.c. tippelskirchi) in captive and wild populations. We found that female Masai giraffes have proportionally longer necks relative to their forelegs than males. Moreover, the female body trunk is proportionally longer whereas male foreleg length and neck width are proportionally greater. The sexual dimorphisms in body proportions were found in both captive and wild Masai giraffes suggesting that these differences are genetically determined. We speculate that the proportionally longer female neck is to compensate for females’ overall shorter stature to expand access to forage and their longer trunk is to accommodate fetal growth. Males’ longer forelegs, which contribute to the overall anterior body stature, likely provides some advantage in physical intrasexual competitions. Differences in the proportions of major body components define sex phenotypes, but several male and female giraffes display opposite-sex phenotypes with a significantly higher level of discordancy seen in captive males. We speculate that body proportion sexual dimorphisms are maintained in the wild by natural and/or sexual selection, but in captivity selection is relaxed because of human-altered mating and feeding behavior resulting in a higher proportion of sexual dimorphism discordances.