Growth patterns and masses of the heads and necks of male and female giraffes

Abstract

We have analyzed the growth patterns of the head and neck of 65 male and 71 female giraffes from two different populations of giraffes, and also the dimensions of 19 different components of the head and neck in 8 female and 13 male giraffes, to establish if they showed sexual dimorphism and if sexual selection for a weapon was a possible origin of the long neck of giraffes. We found that in both genders the rate of increase in head mass was hypoallometric with respect to body mass. The rate of increase in neck length was similar in both genders and faster than the rate of increase in body mass. Increases in neck mass tend to be isometric relative to increases in body mass in both genders before puberty (ca 650kg body mass in males and 700kg in females) but in giraffes of greater body mass increases in neck mass are iso-to hyperallometric in both genders, with final neck, body and head mass being greater in males. The only significant gender difference we found for the dimensions of the 19 different head and neck components was that ossicones and skulls were heavier in mature males than in mature females, but increases in skull mass did not alter the growth pattern of head mass significantly. These data suggest that the morphology and growth patterns of the heads and necks of male and female giraffes are similar, that sexual dimorphism of the head and neck is minimal and can be attributed to secretion of sex steroids. We have concluded that there is no evidence that sexual selection was a factor in the evolution of giraffe morphology and that the long neck of giraffes did not evolve as a weapon 2 in males. The more likely selective advantage of a long neck was improvement of access to high level browse.

The digestive morphophysiology of wild, free-living, giraffes

“…Masseter muscle mass has been reported in a single captive giraffe by Clauss et al (2008) and by us in wild giraffes in the context of the anatomy of their head and skull and not digestion (Mitchell et al, 2013B). Similarly, we have reported mandible mass, and tongue length and mass (Mitchell et al, 2013B).…”

“…They also have a unique tapetum lucidum which enhances vision in mesopic light (Coimbra et al, 2013). Moreover, with age their visual field becomes more binocular and near distance stereoscopic visual acuity increases (Mitchell et al 2013B). Stereoscopic vision, together with their ability to discriminate between closely spaced objects that the large number of photoreceptors confers, enhances their ability to discriminate between sizes and shapes of browse and thus selectivity.…”

“…Most browsing occurs during daylight hours when their ability to distinguish colour (Backhaus 1959), and their visual acuity facilitates browse selection. Visual acuity is related to the size of the surface area of the retina and thus the number of photoreceptors present, both of which are larger in giraffes than in all other ungulates (Schiviz et al 2008;Mitchell et al 2013B) and to the types and the way in which the photoreceptors are distributed in the retina (Schiviz et al, 2008;Coimbra et al, 2013). In giraffes the characteristics and distribution of their photoreceptors confer an ability to distinguish fine detail and allow visual guidance of the tongue and lips (Coimbra et al, 2013).…”

Fossil evidence and stages of elongation of theGiraffa camelopardalisneck

“…It has been popularly demonstrated that this adaptation permits giraffes to use vegetation at higher levels from the ground, therefore outcompeting the shorter-necked herbivores, and allows for a specialized mode of fighting for male dominance in mating, termed ‘necking’ [1,2]. Analysis of rate of neck mass increase found little difference between male and female giraffes, suggesting a smaller role of sexual selection in the elongation of the neck [3]. It has been proposed that both foraging and male combat probably contributed to giraffe neck lengthening; observation of giraffes browsing from tree heights up to 5 m favours the ‘competing browsers’ hypothesis, and the direct selection of larger-necked males by oestrous females supports the sexual selection theory [4].…”

Scaling of the appendicular skeleton of the giraffe (Giraffa camelopardalis)

“…We did not expect to find sexual dimorphism between equivalent body mass giraffes, and we did not find any. Our expectation was based on the lack of dimorphism in giraffe neck length and cervical vertebrae growth patterns (Mitchell et al, ; van Sittert et al, ). The results we report here further confirm that sexual selection has not played a major (if any) role in the evolution of tallness in giraffes.…”