The giraffe, with its towering neck and elegant stature, has always fascinated both the casual observer and the scientific community. While traditional theories credit the long neck of the giraffe to natural selection – allowing them to reach food in tall trees – a persisting theory challenges this view. Did the giraffe evolve its long neck as a weapon in mating battles rather than just to survive food competition? This provocative “necks-for-sex” theory has sparked much debate. Rob Simmons dives deeper into the evolutionary biology behind the giraffe’s iconic neck.
Giraffes are constantly in the news – either because they are under severe threat or because they are just too interesting to ignore. So, it’s no surprise that the “necks-for-sex” idea has garnered much attention in recent years and has seen pushback and support from diverse sources. Is the “necks-for-sex” idea just another tall story? Or is the giraffe’s neck long so that it can outcompete competitors for food?
The extraordinary shape of adult giraffes demands an evolutionary explanation. Charles Darwin, and many who have followed him, proposed a simple and enduring explanation: that the extra 1.8m of neck allotted to giraffes will benefit them by allowing them to out-reach shorter-necked competitors to feed on succulent green leaves in tall trees. By surviving better in times of hardship, they will pass on the long-necked genes to their offspring and facilitate the evolution of the trait. It has become a classic trope of “survival of the fittest” through natural selection.
In 1996, I went looking for data to support Darwin’s feeding competition idea, and the cupboard proved surprisingly bare. Contrary to Darwin’s theories, research from East Africa showed that tall giraffes frequently fed from low-growing grewia bushes in winter (females especially so) and ate faster and more regularly at low heights. This was a surprise for such a well-oiled idea, and it set me on a journey searching for alternative data after I was struck by an idea while watching three young males sparring (“necking”) in the Sabi Sands Game Reserve in South Africa.
Unlike other species, they were clubbing each other with their heads, using their long necks to gain momentum and speed. Could giraffes have evolved a long neck as a club-like tool? When my friend and colleague Lue Scheepers told me that he had measured giraffe head and neck mass from culled giraffes in Etosha National Park, it allowed me to analyse their growth unprecedentedly, and I uncovered an unexpected relationship. Males continued to grow their necks and heads in terms of mass well past the time females stopped – around the age of eight. This continued into old age (about 28 years for the massive, black-coated males).
They were continuing to invest in the very section of their bodies one would expect them to if the neck was a sexually selected “club” used by males to beat their rivals. Indeed, this continued investment (about 4kg per year for necks and an astonishing 1kg per year for their heads) is predicted by sexual selection and is called allometric growth. This describes a process when a trait grows disproportionately in relation to other body proportions. And so the “necks-for-sex” idea was born. And it’s had growing pains ever since!
Science proceeds apace when new ideas emerge and we are bombarded with new angles and new data. It is both exciting and challenging as each new angle is re-examined. That is what several groups of researchers have done. Data from Zimbabwean giraffes that had also been culled were weighed and measured by researchers from the Mammal Research Institute in Pretoria. They, too, looked for allometric growth patterns in the necks and heads of male and female giraffes and found, contrary to the large Etosha sample, that there was only isometric growth (i.e. necks in proportion to other body parts). Once their paper was published with the provocative title “Sexual selection is not the origin of long necks in giraffes”, it became clear that there was a contradiction in our relative observations of the males.
Statistical ecologist Res Altwegg’s independent analysis indicated that the findings in this paper showed that the growth patterns for males were allometric and not isometric, and the findings supported the necks-for-sex idea, contrary to what was concluded in the paper. We published our analysis in a 2010 paper and took the debate forward by suggesting that palaeontologists could help the debate by looking for a simultaneous change in neck length with a change in the “headgear” of giraffids in the fossil record.
Why is this important? Giraffids, of all hoofed animals, have a wider diversity of antlers, horns and headgear (14 to be precise) than any other group of ruminants, alive or dead. These range from singles ‘bosses’ (one reinforced keratinous mass placed on top of the head – similar to the base of a buffalo’s horns), to short forward-facing horns, sideways-pointing horns, backwards-facing racks and antlers, and even animals with four horns. We suggested that the change to the small blunt horns (or ossicones) we see today would have coincided with a longer neck (to aid the transition from wrestling or head butting to the clubbing of modern giraffes). If this is indeed the case, then a sexual selection origin would be supported.
In 2022, unexpected support for this theory came from the Junggar Basin of northern China, where a group of palaeontologists led by Shi-Qui Wang uncovered one of the smallest known giraffids ever found. This little animal (Discokeryx xiezhi), standing about 1.2 m at the shoulder, was a giraffid based on its dentition, cervical vertebrae and a curious semicircular canal unique to giraffids. Importantly, Discokeryx showed all the hallmarks of an elaborately strengthened neck, with massive vertebrae, a reinforced skull and a disc-like helmet, all beautifully designed for hitting opponents with huge forces. Their detailed analysis suggested that ancient giraffes were already on a sexually-selected pathway of promoting a “necks for sex” lineage some 16.9 million years ago!
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The most recent assessment of the size and shape of extant giraffes comes from Doug Cavener in Pennsylvania, USA. Cavener and fellow researchers combined data from live zoo animals with photographic measurements of wild males and females in Tanzania. By measuring neck, leg and torso lengths from zoo animals of known age, they found that adult females surprisingly have longer necks for their body size than males (the female’s bodies were still smaller than males’, but their necks were proportionately longer). Cavener et al. suggested this offered no support for the necks-for-sex idea and instead promoted the original feeding competition idea because females could use longer necks to feed deeper inside bushes.
However, I argue that this is not a test of the necks-for-sex idea because that hypothesis does not make any predictions about the length of female necks. Their necks can be shorter or longer as far as sexual selection is concerned. The hypothesis concentrates on males having long, strong necks for beating rivals to gain access to females on oestrus. This theory posits that female necks are long because they were “dragged along” by the well-known genetic correlation between the sexes, experimentally shown in many lab experiments with fruit flies and other species. This genetic correlation also explains other non-adaptive traits like nipples on human males and tusks on female elephants. Thus, Cavener et al.’s results provide interesting insights but do not constitute a test of the necks-for-sex idea.
Ever since Darwin proposed sexual selection as an explanation for gaudy tails, elaborate songs and bizarre appendages, a sceptical audience has doubted the simple explanation that females prefer males for traits offering no survival advantages. But these traits keep popping up, and at least in animals that can be tested, there is strong experimental evidence that mates prefer long tails and elaborate songs. These are passed onto sons and daughters, who will outcompete rivals directly or indirectly for mates.
It is important to note, as did the genius Charles Darwin, that traits or behaviours can rarely be assigned to one selection pressure or another but are more often due to the sum of numerous drivers, including predators, mates, food and climate. In proposing the necks-for-sex idea, we have provided an alternative to natural selection. Longer necks may have started with a slightly longer neck in the giraffe’s short-necked ancestors, allowing individuals to survive lean times by reaching into tall trees. If that precluded the incipient giraffe from head-butting or wrestling its rivals, then a head-clubbing (necking) method of fighting would have arisen. Thus, both natural and sexual selection can be said to have shaped the bizarre animals we see today.
Science moves forward in leaps and bounds and does so by questioning our perceptions and theories. As we researchers continue to question, giraffes will continue elegantly browsing and gracefully fighting, oblivious to the brouhaha they cause for curious naturalists and evolutionary biologists.
About the author
Dr Robert Simmons, based at the University of Cape Town’s Fitzpatrick Institute, is a behavioural ecologist, conservation biologist and ornithologist specialising in the ecology of raptorial birds, cats and giraffe.
References
Cavener, D. R., Bond, M. L., Wu-Cavener, L., et al. (2024). Sexual dimorphisms in body proportions of Masai giraffes and the evolution of the giraffe’s neck. Mammalian Biology.
Mitchell, G., van Sittert, S., & Skinner, J. D. (2009). Sexual selection is not the origin of long necks in giraffes. Journal of Zoology, London, 278(4), 281–289.
Pellew, R.A. (1984), The feeding ecology of a selective browser, the giraffe (Giraffa camelopardalis tippelskirchi). Journal of Zoology, 202: 57-81.
Simmons, R. E., & Scheepers, L. (1996). Winning by a neck: Sexual selection in the evolution of giraffe. The American Naturalist, 148(5), 771–786.
Simmons, R. E., & Altwegg, R. (2010). Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe. Journal of Zoology, London, 282(1), 6–12.
Wang, S.-Q., Ye, J., Meng, J., et al. (2022). Sexual selection promotes giraffoid head-neck evolution and ecological adaptation. Science, 376(6598), 201–205.
Young, T. P., & Isbell, L. A. (1991). Sex differences in giraffe feeding ecology: Energetic and social constraints. Ethology, 87(1-2), 79–89.
Further reading
- The giraffe is a wonder of evolution, and a vital part of Africa’s ecosystems. Read all there is to know about the planet’s tallest creature here.
- Why do giraffes have such long necks? A new study questions whether feeding or mating played the bigger role in giraffe neck evolution. Read about Doug Cavener’s exploration into the driving force behind giraffe-neck evolution here.
- Under pressure – genetic research on giraffes reveals evolutionary secrets of how they cope with high blood pressure and maintain bone density. Read about the pieces of the giraffe evolution puzzle here.
- Do giraffes choose their social groups based on appearance? A recent study investigates if giraffes form bonds based on spot shape
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