Why do giraffes have long necks? We know that modern giraffes must have evolved gradually, but figuring out what selection pressures influenced giraffe evolution is another story altogether. One of the most popular recent explanations is that giraffes have long necks as a result of sexual selection.
The "necks for sex" hypothesis is primarily inspired by the contests between male giraffes. In these duels the males stand side by side and whack each other with their necks and ossicones ("horns"). This can be seen in the video below;
What does this behavior have to do with the evolution of long necks? According to the "necks for sex" hypothesis males with longer necks in an ancestral population of giraffes would have won more of these contests and thus been more successful at mating. It seems simple enough, but there is a problem: the selective pressure would just be on the males. Indeed, if this kind of competition between males was driving the selection for long necks we would expect to see more sexual dimorphism. Females, who don't engage in these contests, would probably have shorter necks than males. If little to no sexual dimorphism can be seen in living giraffes it is unlikely that sexual selection was the main cause for the evolution of the modern forms. To test this hypothesis G. Mitchell, S.J. van Sittert, and J.D. Skinner examined 17 male and 21 female giraffes that were killed during culling in Zimbabwe and reported their results in the Journal of Zoology.
What Mitchell and colleagues found was there were virtually no differences between males and females in terms of body size, neck length, and leg length that could be attributed to sexual selection. There was one interesting minor difference, though. The zoologists found that female giraffes had proportionally longer necks compared to foreleg length than males when the sexual selection hypothesis would predict that males would have longer necks. Both sexes also had necks that grew faster than the rest of their body, cutting down the idea that males "invested" more in their necks than females.
Simply put, the "necks for sex" hypothesis fails because there is no evidence of sexual dimorphism in giraffes that would result from male-male competition. The competitions between male giraffes are a consequence, rather than a cause, of neck elongation. Giraffe necks became elongated for some other reason.
The debate over giraffe necks illustrates the pitfalls of trying to figure out past evolutionary pressures based almost solely upon living animals (and just one living species, at that). Studies involving the benefits of having a long neck to feeding or intrasexual competition might be informative, but what giraffes do now might not tell us much about what caused their ancestors to evolve long necks. We should not confuse what an organ is used for now with what led to its origin: they are not always the same.
Strangely, a discussion of fossil evidence is almost always missing from hypotheses about the evolution of giraffe necks. Perhaps this is because we cannot observe the behavior or feeding patterns of extinct creatures, but studies of fossil giraffes could do much to inform discussions of giraffe evolution. In fact I recall seeing a fossil giraffe with a neck intermediate in length between ancestral and living forms in Don Prothero's Evolution: What the Fossils Say and Why it Matters. I do not know if a study of this animal has been formally published yet, but it could certainly be important to figuring out how giraffes evolved.
Mitchell, G., van Sittert, S., & Skinner, J. (2009). Sexual selection is not the origin of long necks in giraffes Journal of Zoology DOI: 10.1111/j.1469-7998.2009.00573.x
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Ah, interesting! I hadn't even considered the sexual dimorphism angle of the "necks for sex" theory. This study certainly puts the kaibosh on it, though.
Definitely not sexual selection... I've always thought it was to do with competition for resources, hence the only long-necked mammals in Africa being the giraffe and gerenuk, both of which browse on higher foliage than other ungulates (except elephants) can reach.
Mo, the sexual selection hypothesis was proposed by Simmons and Scheepers (âWinning by a neck: Sexual selection in the evolution of the giraffeâ American Naturalist 148: 771â786) based on the observation that giraffes actually do not tend to browse on all that high foliage.
Kai; See Darren's post (the second link). Giraffes do browse more widely than Simmons and Scheepers cited. This may not be important when food is plentiful but is more important when resources are slim. This is also addressed in the new paper I discussed here.
I don't have access to any of the cited works, but it would seem to me that people are a little quick to dismiss sexual selection.
It seems to me that if selection for long necks is sufficiently strong in males, all it has to be in females is relatively neutral. Considering the fact that it might still improve survival when resources are slim, it might outweigh any additional biological cost to having a longer neck in females.
I haven't actually studied sexual selection in years, so some one who knows better can tell me I don't know what I'm talking about it, but off the top of my head it seems to me that for sexual dimorphism to be a likely consequence, you need a situation where either A) the genes under selection are sex-linked (which I suspect is not the case, but I don't really know), B) where the benefit in one sex is balanced by a negative in the other, or C) where the benefit is derived by the dimorphism itself (such as situations where it might allow females to identify males and avoid wasting energy trying to mate with other females...)
Dr. Octopoid; Thank you for your comment, but I don't think we are being too hasty in ruling out sexual selection as the driving factor for the evolution of giraffe form. Hypothetically male-male competition could have provided some selection pressure that would have carried females along with it but we would still expect a stronger degree of sexual dimorphism than is present. Why would males and females be so similar, and females have proportionally longer necks, if male-male competition was the driving selective pressure?
As I stated towards the end of the post I think this is a case where we are trying to figure out the evolution of a kind of animal based almost solely upon the living animal. This can easily lead us astray, especially if "necking" among male giraffes is a consequence of having long necks and not the behavior that selected for that trait.
What if we're looking at the wrong end of the giraffe? Instead of looking at the neck, maybe we should be looking at the legs.
What if there's selection pressure for longer legs? The neck would then be "required" to elongate not to reach higher branches on trees, but to reach lower down to the ground to facilitate drinking water or for some other purpose.
I suppose proportionally shorter necks would be stronger, so could the sexual selection (for males) be for proportionally shorter necks (which are of course still long for eating)?
I enjoyed reading this post. I just received a gift of a stone-carved giraffe and had wondered how the long neck came about. Interesting thoughts in your article. Hopefully one of these days we'll know.
I guess what I'm questioning is why do we necessarily expect sexual dimorphism? While I'm sure there are a slew of genes affecting neck length, the vast majority are not going to be sex-linked in any way. If the long-neck alleles of these genes improve fitness in males and are of at least minimal overall impact on fitness in females, you're still going to see an increase of the long-neck alleles in both males and females. If there's no signficant negative, there seems to me no reason to select for differential development in females. This basically just hinges on whether the cost of growing and sustaining a long neck equal the benefits of having a long neck in times of famine (or another unnamed benefit).
Of course this is all just speculation, and I think it's probably at least as likely that the neck-bashing behavior developed after they had long necks to do it with, and not the other way around. In fact, it would seem like a strange behavior for an animal without a long neck to evolve, doesn't it? Ian's mention of the legs makes me wonder if this doesn't support the idea that there is a significant benefit just in being tall...they do have awfully long legs, and you wouldn't necessarily need to have long legs if you just needed a long neck to whack competing males with.
Anyway, I'm just playing with this... It sounds like even the people who actually know what they're talking about still know little enough about giraffe evolution that the best we can do for the moment is just speculate anyhow.
Doc Ock: Only time for a short answer, but sexual dimorphism would be expected because the behavior being posited as providing the selective pressure is male-male competition. When traits are the product of male-male competition they are often dimorphic (baboons are a good example). It also would seem strange to me that necks could be a product of sexual selection by male-male competition and be almost identical between sexes; even if females had long necks by being "carried along" by male competition I would still expect to see some significant degree of dimorphism.
Mo:
There are also climbing browsers in Africa (e.g., colobine monkeys and tree hyraxes) that theoretically at least may compete - or might have competed in the past â for foliage with giraffids. Also, it's not certain that the long-necked giraffids originally evolved in Africa; during the Miocene and the Pliocene, many different giraffid species were found all over Eurasia. And here they would have faced a slightly different set of potential competitors than in Africa.
Brian:
Mauricio Antón has illustrated some early giraffe species in the books Mammoths, Sabertooths, and Hominids (AgustÃ, J. & Antón, M., Columbia University Press, 2002) and Evolving Eden (Turner, A. & Antón, M., Columbia University Press, 2004). And these are 'giraffe-like' giraffids, not sivatherines (although he has illustrated some of those, too).
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JSB
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I can understand that one could reasonably infer sexual selection if you see sexual dimorphism, but I'm not sure that it logically follows that the reverse is true: that sexual selection necessarily results in dimorphism. If there is a net positive to having a long neck, or even no harm, then it should persist in both sexes.
It's quite possible (theoretically, anyway...I don't know what works been done on this) that long necks are the result of only a very few mutations (major single gene mutations affecting elongation of various body parts are well documented in other species). If that's the case in giraffe, one could easily get the exact same effects in both sexes, with only sexual selection.
Although you are technically correct that females had longer necks than similarly sized males, the differences were not significant, and given the small sample size and the need to find similarly sized individuals within that small group, I think these differences are pretty meaningless. If females were in fact larger it might suggest that sexual selection is not going on, but the fact they are essentially the same does not weaken the argument.
I'm not really arguing that sexual selection is the cause, what I'm arguing is that it remains plausible...
(And there IS sexual dimorphism among giraffes...size.)
This article might help:
http://news.softpedia.com/news/Why-Do-Giraffes-Have-Such-Long-Necks-414…
The highly homosexual nature of giraffes would exaggerate the long neck trait in females too - both males AND females are selecting for the long neck. It could also be true if the long legs are being selected for (requiring a long neck to eat from the ground - but not, of course, if the major food source is leaves from trees) but their behaviour would suggest it is the long necks that turn them on!
Male nipples.
I do wonder how these papers get published. Sexual dimorphism is not that easy to come by. If neck size is regulated by a set of hox genes and allied components, there's no simple way to gain neck size in males but not in females. And that's fine as long necked females who produce sons with the same genes ultimately produce more in the F2 generation.
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I can understand that one could reasonably infer sexual selection if you see sexual dimorphism, but I'm not sure that it logically follows that the reverse is true: that sexual selection necessarily results in dimorphism. If there is a net positive to having a long neck, or even no harm, then it should persist in both sexes.
It's quite possible (theoretically, anyway...I don't know what works been done on this) that long necks are the result of only a very few mutations (major single gene mutations affecting elongation of various body parts are well documented in other species). If that's the case in giraffe, one could easily get the exact same effects in both sexes, with only sexual selection.
Although you are technically correct that females had longer necks than similarly sized males, the differences were not significant, and given the small sample size and the need to find similarly sized individuals within that small group, I think these differences are pretty meaningless. If females were in fact larger it might suggest that sexual selection is not going on, but the fact they are essentially the same does not weaken the argument.
I'm not really arguing that sexual selection is the cause, what I'm arguing is that it remains plausible...
(And there IS sexual dimorphism among giraffes...size.)
hello, Nice article but I have some doubts hope you can clear out
How can you prove that there should be dimorphism if it was not proven that the trait is sex linked ?
Its a random equal shot for a female or a male giraffe to be produced due to the XY chromosomes, and if the length of necks were indeed not sex linked then it is easy to conclude that random chromosome paring would eventually lead to the equilibrium between male and female neck length. It seems to me that Mitchel in part proved a whole different thing that was is proposed. If the trait is actually proven not to be sex linked, then the hypothesis of sexual selection is acceptable. If however the trait was proven to be sex linked then yes sexual selection can be out weeded out.
Thanks