The variability of giraffe skull morphology

Giraffe (Giraffa spp.) are among the most unique and charismatic extant animals, largely due to their distinct morphology. The evolutionary and developmental origin of ossicones is a key factor in giraffid biology, yet other features building their characteristic head physiognomy remain poorly studied. Here, we analysed the cranial diversity of extant Giraffa using a continent-wide sample with a balanced sex ratio and ontogenetic growth series, using shape analysis (3D geometric morphometrics). Detailed comparisons showed significant differences in cranial morphology across the four Giraffa species, and in some cases also between subspecies of the same species. The analyses revealed that females reach adult cranial form at an earlier age (around 4 years old), while males continue growing until the eighth year, aligning with the onset of sexual maturity for each of the sexes. Multivariate methods unambiguously prompt significant covariation between skull parts. Such high covariation patterns indicate that Giraffa skull morphology largely evolves as a whole (i.e., morphologically integrated), even though parts of it (such as the ossicones) develop in different magnitudes. Considering their role in territorial and mating behaviours, we suggest that ossicone morphology influences the parapatric evolution of Giraffa through sexual selection. Palaeontological evidence shows that development of larger ossicones is a relatively recent trait for members of the genus Giraffa, with the cranial morphology of the southern giraffe (Giraffa giraffa) being the closest to that of the ancestral forms, possibly through plesiomorphy or paedomorphosis.