Giraffine giraffes (that is, the giraffid clade that includes Giraffa and its closest relatives) are famous for being long necked, with the usual explanation for the neck being that it evolved to enable these animals to avoid competing with other browsers.
But for this assumption to be experimentally supported you’d have to show that giraffes use their long necks to forage high up, and show that giraffes have a competitive advantage over shorter browsers. Surprisingly, it has been argued that these assumptions don’t hold up. In some areas, over 50% of all giraffe browsing is done below 2 m, and thus well within the browsing height of potential competitor species (Simmons & Scheepers 1996), and only dominant males living in groups otherwise consisting of females were found to feed at the sort of height expected for an animal with such a long neck. In view of these counterintuitive findings, Simmons & Scheepers (1996) wondered if the primary function of the neck was not to increase browse height [adjacent pic from here].
Significantly perhaps, the long neck doesn’t just have a role in foraging and feeding: it also has an important sociosexual function, given that male giraffes battle with their necks in a sort of ritualized combat (termed necking. Yes really). They swing their thick-roofed skulls like sledgehammers, landing blows on the neck and head of a competitor. These battles can be vicious and result in fatalities (the photo below, borrowed from here, depicts a male giraffe killed in 1999 by another male). Unsurprisingly, males therefore have thicker and bigger ossicones, a more strongly reinforced skull, and thicker and tougher neck skin, than females. But that’s not where the sexual dimorphism ends. It turns out that males have proportionally bigger, deeper, and more muscular necks than females. Furthermore, the neck of a male giraffe continues growing throughout life and the neck increases in size allometrically. Larger-necked males are also socially dominant and preferred by females.
It would seem from these pieces of evidence that the large neck of the giraffe is maintained by sexual selection, and does not serve to increase foraging range. This heretical idea was proposed by Robert Simmons and Lue Scheepers in their 1996 paper ‘Winning by a neck: sexual selection in the evolution of giraffe’. If you’re wondering why females have long necks as well, it could be argued that these have arisen as a ‘neutral by-product of genetic correlation between the sexes’ (Simmons & Scheepers 1996, p. 783).
This ‘necks for sex’ hypothesis has become reasonably well known among hoofed mammal researchers, and it has even recently inspired a similar hypothesis proposed to explain the evolution of the long neck in sauropod dinosaurs (Senter 2007). But it would be going too far to say that it has been universally accepted, largely because a great deal of uncertainty has remained over the foraging ecology and browsing preference of giraffes. In a new study on giraffe feeding behaviour, Cameron & du Toit (2007) have provided a good empirical test of giraffe feeding behaviour, and their data strongly contradicts some of the findings reported by Simmons & Scheepers (1996). Amusingly, Cameron & du Toit’s paper uses the same clause in the title as Simmons & Scheepers’ (‘Winning by a neck’).
Essentially, they show that feeding competition from smaller browsers so depletes the amount and quality of browse available low down in vegetation (below 2.5 m) that giraffes really are forced to forage at height (over 4 m), and that giraffe feeding efficiency is strongly reduced when giraffes have to feed within the foraging range of shorter browsers. This study strongly suggests that the long neck is significantly advantageous in terms of feeding ecology, and that its evolution is therefore plausibly well explained by resource competition with other browsers. This is significant for the simple reason that no previous study experimentally tested the exact feeding preference of giraffes. If it surprises you that no-one had done such a blindingly obvious bit of research, then I have to tell you that there are a million other really obvious bits of research on living animals that have yet to be performed and published.
While it’s not ‘case closed’, we at least now have some good data confirming that, as conventionally predicted, the long neck of Giraffa is, after all, useful in terms of avoiding competition with other browsers.
Incidentally this isn’t the only controversial thing about giraffe necks. Dissent remains over how many vertebrae they have: they were most often thought to have seven (like most other mammals) until Solounias (1999) argued that they actually had eight. Note also that the several* giraffine species are unusual among giraffids in being long-necked: the vast majority of giraffids (virtually all of which are extinct) were short-necked [adjacent pic of necking male giraffes from here].
* There are six named fossil species of the extant genus Giraffa, and while there is only supposed to be one living Giraffa species (G. camelopardalis), there might be two (go here for more on this subject). At least one close relative of Giraffa, Bohlinia from the Miocene, was also long-necked (but otherwise more archaic).
Vampire bats coming next I promise, unless I get distracted (which I will).
Refs – –
Cameron, E. Z. & du Toit, J. T. 2007. Winning by a neck: tall giraffes avoid competing with shorter browsers. The American Naturalist 169, 130-135.
Senter, P. 2007. Necks for sex: sexual selection as an explanation for sauropod dinosaur neck elongation. Journal of Zoology 271, 45-53.
Simmons, R. E. & Scheepers, L. 1996. Winning by a neck: sexual selection in the evolution of giraffe. The American Naturalist 148, 771-786.
Solounias, N. 1999. The remarkable anatomy of the giraffe’s neck. Journal of Zoology 247, 257-268.