Endless Forms

Cutting edge news about discoveries in ecology, evolution and conservation.

Giraffe Necks: A Tall Tale

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Photo by Anne-Marie Hodge

When the giraffe was first discovered by Europeans, some speculated that it was the result of a cross between a camel and a leopard — hence the Latin name of the northern giraffe: Giraffa camelopardalis. The giraffe is a unique creature in many ways, but the origin of its elongated neck has long served as fodder for many “just-so” stories. Rudyard Kipling used it as a metaphor for self-improvement. Countless biology textbooks use it as the default example for Lamarck’s theory of evolution via inheritance of acquired characteristics. The lengthy neck has provided endless opportunities for comic relief in cartoons and illustrations.

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Photo by Anne-Marie Hodge

Amongst all of speculation as to why any reasonable species would develop such an extreme feature, two hypotheses predominate: 1) Giraffe necks elongated in a co-evolutionary dynamic with Acacia trees, their main food source, as each tried to gain the upper hand in the herbivore-plant war; or 2) long necks are the result of sexual selection, akin to peacock tails, that result in more successful mate acquisition for longer-necked individuals—either because female giraffes think long necks are sexy or because males use them as weapons to compete for mates. It also could have been a combination of both, or long necks could have enhanced capacities for thermoregulation, vigilance for predators . . . or something else. The debate continues.

The giraffe’s neck actually contains the same number of cervical (neck) vertebrae found in nearly all other mammals. What has not been thoroughly explored, however, is the rate at which these vertebrae develop relative to other parts of the giraffe’s body, including its head and overall body mass.

Giraffes are swift, strong, and stunning creatures, but they still look comically disproportionate to many humans. They don’t start out like that, however: giraffes are not born with exceptionally long necks, and their cervicals don’t begin to extend rapidly until later in life. But just how rapidly, relative to the rest of the body? Is there a difference between male and female growth patterns? What can the answers to these questions tell us about the evolutionary roots of the giraffe’s long neck?

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Photo by Anne-Marie Hodge

These questions motivated a group of biologists from the University of Wyoming and the Centre for Veterinary Wildlife Studies at the University of Pretoria to investigate growth patterns of the necks and heads of giraffes (Mitchell et al. 2013). The scientists analyzed numerous morphological measurements in order to determine whether neck growth scaled “allometrically” with head and body growth—ie, whether the neck remained at a consistent size ratio with the rest of the giraffe’s parts, or whether a giraffe goes through an “awkward teen phase” with proportions that are skewed relative to those of adults. The researchers also compared growth patterns between males and females and examined sexual dimorphism (differences in appearance, size, etc between the sexes) to determine whether developmental patterns differ between males and females.

It turns out that there is no significant difference between male and female giraffes in neck growth rate (measured by both neck mass and neck length), but that in both sexes the neck length did increase at a faster rate than body mass. The rates of neck and body growth rates are similar in youngsters, but the necks outpace body mass once puberty hits, elongating out of proportion to the growth of the rest of the body.

In other words, there is now scientific evidence that giraffes go through a gangly teenage stage.

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Young giraffe. Photo by Anne-Marie Hodge

Interestingly, a giraffe’s head actually grows at a slower rate than its overall body mass. Males did end up with slightly heavier heads and necks, due to thicker ossicones and skull bones. The researchers attribute this to basic differences in sex steroid levels, and males have a much longer total growth period during their lives than do females. These factors underlie the slight male:female body size differential found in most mammals.

Giraffe head and neck growth rates and the ratio of neck mass to total body mass both remained similar between the sexes, meaning that they differ a bit in size but not proportion. This low degree of sexual dimorphism in skull and neck proportions has important implications for our understanding of giraffe evolution and biology. If giraffe necks had developed due to pressures from one form of sexual selection, we would expect to find one sex (amongst mammals, it would typically be the males) with profoundly more extreme features. In one form of sexual selection, these extreme features would provide the basis for the other sex (typically the females, sorry guys) to choose a mate from their array of suitors based on display characteristics of some sort. In the other type of sexual selection, male giraffes would use their massive necks to battle for access to mates. The females themselves would exhibit little aesthetic choosiness and would just mate with the males that were the most successful at winning fights.

Many people have probably seen documentary footage of giraffes engaging in dramatic bouts of “necking”, in which they appear to fight by swinging their long necks and heavy heads at each other like weapons. On the surface, this seems like it may indicate sexual selection via male competition, and yet Mitchell et al. point out that studies have shown that large males rarely participate in these activities, and it’s more often immature males with female-like head and neck sizes that do all of the “fighting” in order to establish a dominance hierarchy. Most importantly, the winners of the fights don’t go on to acquire more mates (Pratt & Anderson 1982).

The takeaway message from this study is that although giraffes possess an extremely exaggerated feature, the lack of difference between males and females tells us that it’s unlikely this was due to sexual selection. This makes it much more likely that some form of enhanced resource utilization or physiological efficiency was the mechanism for neck elongation over evolutionary time.

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Photo by Anne-Marie Hodge 

The issue is far from resolved, however. There have been noteworthy criticisms of the main two theories regarding the evolution of giraffe necks. Some studies have suggested that giraffes rarely browse at the full height that their necks can reach (Young & Isbell 1991). Also, in order to “out-reach” other browsers, they would only need a neck that was two meters long, not five meters, and being that high off the ground actually makes it harder to see predators such as lions that slink along the ground (Cameron & Du Toit 2005). Conversely, although Cameron & Du Toit (2007) found that 57% of the forage consumed by giraffes grew below 2 meters, i.e. in the height range of other herbivores, they suggest that the giraffe’s long neck allows it exclusive access leaves in the center of shrubs and bushes by reaching down from above. All of this will require broader and deeper investigation, of course, and in the end it is likely that the selective pressure that have produced such long necks is attributable to more than one precise factor.

Although it is most famous for its long neck, the giraffe exhibits an array of highly specialized features, and Mitchell et al. review a few fun facts about giraffe morphology. For example, relative to total cranial mass, the sinus cavities of giraffes are massive relative to those of other artiodactyls, allowing the development of such an enormous head without a debilitatingly heavy skull. In addition, the cervical vertebrae close to the head are lighter than those lower in the neck, further decreasing the mechanical stress of holding a head that high on a neck that long. The occipital condyles, bony projections that allow the head to rotate on the neck, have such a wide range of motion that the giraffe can actually partially lay the top of its head along its neck while feeding, a sort of reverse flamingo strategy. Fancy, no?

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(Gratuitous photo of me “kissing” a giraffe in Nairobi).

Sources:

Cameron, E.Z. & du Toit, J.T. (2005). Social influences on vigilance behaviour in giraffes, Giraffa camelopardalis. Anim. Behav. 69, 1337–1344.

Mitchell, G., Roberts, D., van Sittert, S. & Skinner, J. D. (2013). Growth patterns and masses of the heads and necks of male and female giraffes. Journal of Zoology. 290, 49–57. doi:10.1111/jzo.12013

Pratt, D.M. & Anderson, V.H. (1982). Population, distribu- tion, and behaviour of giraffe in the Arusha National Park, Tanzania. J. Nat. Hist. 16, 481–489.

Young, T.P. & Isbell, L.A. (1991). Sex differences in giraffe feeding ecology: energetic and social constraints. Ethology 87, 79–89.

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