Ontogenetic allometry of the postcranial skeleton of the giraffe (Giraffa camelopardalis) with application to giraffe life history evolution and palaeontology

Abstract

Giraffes (Giraffa camelopardalis) have evolved into a unique and extreme shape. The principle determinant of its shape is the skeleton and the overarching theme of the study was to describe how this shape is achieved throughout ontogeny. Accordingly, the study had three main objectives: 1) To describe the growth of the giraffe postcranial skeleton allometrically, 2) To interpret the allometric patterns described in an evolutionary and functional sense and 3) To reconstruct the size and shape of the extinct Giraffa sivalensis using, if feasible, allometric equations obtained in this study. Secondary objectives were to a) establish if sexual dimorphism was evident in G. camelopardalis and b) determine if growth patterns in the foetus differed from those in postnatal G. camelopardalis. Data were collected from giraffes culled as part of conservancy management in Zimbabwe. The sample included 59 animals from which vertebral dimensions were taken in 48 animals and long bone dimensions in 47 animals. Body masses ranged from 21 kg to 77 kg in foetuses and 147 kg to 1412 kg postnatally, representing 29 males and 30 females. In addition to body mass, external body dimensions were recorded from each animal. Each vertebra and unilateral long bone was dissected from the carcasses and cleaned, after which dimensions were measured with a vernier calliper, measuring board or measuring tape. Vertebral dimensions measured included body (centrum) length, height and width as well as vertebral spinous process length. Long bone dimensions included length, two midshaft diameters and circumference. Allometric equations (y=bxk) were constructed from the data, with special interest in the scaling exponent (k) to illustrate regions of positively allometric, isometric or negatively allometric growth. In the first series of analyses the growth patterns of the components of the postcranial axial skeleton were analysed. The adaptations in vertebral growth to create and maintain extraordinary shape were identified as disproportionate elongation of the cervical vertebrae after birth, increasing cross sectional diameters of the cervical vertebrae from cranial to caudal and positively allometric spinal process growth. The theory of sexual selection as a driver for neck elongation in giraffes was brought into question by showing that male and female vertebral elongation rates are similar relative to increases in body mass. The second series of analyses described the growth pattern of the long bones of the appendicular skeleton. The allometric exponents seemed unremarkable compared to the few species described previously, and it was shown that the giraffe appendicular skeleton does not elongate in the dramatic way the neck does. Limbs at birth, after lengthening with positive allometry in utero, are already elongated and slender in shape and a further increase in the gracility of the bones is either not possible or not desirable. This result implies that it is neck elongation rather than leg elongation that is the dominant factor in the evolution of the giraffe shape. Nevertheless, the front limb bones and especially the humerus may show responsiveness to increasing high loads and/ or bending moments, which may be caused by the neck mass which increases with positive allometry, or with behaviours such as splaying the forelegs during drinking. In the third component of the study ontogenetic allometric equations in extant giraffes were applied to the remains of an extinct giraffid, G. sivalensis. The procedure was unusual as it employed ontogenetic regressions instead of the more commonly used interspecific regressions. The appropriateness of each equation to estimate body mass was evaluated by calculating the prediction error incurred in both extant giraffes and okapis (Okapia johnstoni). It was concluded that, due to body shape, ontogenetic equations were adequate and perhaps preferable to interspecific equations to estimate proportions in Giraffa species. This analysis showed that G sivalensis was smaller than extant giraffes and weighed around 400 kg (range 228 kg 575 kg), with a neck length of about 147 cm and a height of 390 cm. There may be evidence of sexual dimorphism in this species, with males being about twice the body weight of females. However, if sexual dimorphism was not present and all the bones were correctly attributed to this species, then G. sivalensis had a slender neck with a relatively stocky body. In conclusion, this study established ontogenetic regression equations for the skeleton of an animal of which the body shape seems to be at the extreme limits of mammalian possibility. The value of the current study lies especially in its sample size and quality, which included an unprecedented number of giraffe body masses, vertebral and long bone dimensions. This dataset had applications in the giraffe s evolutionary biology, palaeontology and even ecology. Future studies still need to compare the findings from giraffe growth with similar data from other taxa, especially those with long legs and necks. Specifically, it would interesting to determine if positively allometric neck growth combined with isometric leg growth is found in other mammalian species. In addition, the strength of giraffe long bones and vertebrae needs to be investigated with more accuracy using parameters like second moment of area. Lastly, further palaeontological studies on other giraffid sizes are necessary to validate the current and future interpretations of fossil giraffid findings.