Dammit, and I sooo loved the 'necks for sex' hypothesis

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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).

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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.

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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.


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Jack Cohen insists the giraffe's neck is neither "for" browsing nor "for" sexual competition, though it might easily do those jobs in addition to its primary one. Cohen points out that the giraffe is a long-legged animal, whose gait is a pace (left legs, right legs, in that order) so that the longer the legs, the better it is at getting around.

You see the problem when this animal gets thirsty. How the hell would a long-legged short-necked giraffe ever reach the water? So Cohen is saying that the giraffe's neck is an analog of the elephant's trunk: not a device for getting up high to eat, but for getting down low to drink.

It seems in this case that both hypotheosises may well be mutually inclusive.

"Vampire bats coming next I promise, unless I get distracted (which I will)."

why you.....:)

This is truly one of the best blogs I've ever seen.You may be a tease,but you do eventually deliver the goods;with a joyous detour all along the way!

How could there be any controversy over how many neck vertebrae a giraffe has? When I visited the Harvard Museum of Natural History a couple of years ago, I walked right up to the giraffe skeleton on display and counted them myself. Sure enough, seven. Did they reassemble the skeleton wrong?

Actually, my five-year-old daughter and I went around to many of the mammalian skeletons in the room, big and small, and counted out each. Seven, every time, except for the whales. We also looked for the patellae on each skeleton -- with the same exception, of course. It seemed a good way to help her discover her kinship with them all.

By Nathan Myers (not verified) on 02 Feb 2007 #permalink

Even if Simmons and Scheepers was the only data available, it's far from obvious to me (from the details you give anyway) that it indicated that the primary function of the neck wasn't to increase browse height.

My point is that, arguably, in most places most of the time (including where and when S and S did their survey) there's plenty of browse down low for everyone, both giraffes and others. And why bother reaching for the sky when you don't have to?

It's only when there's a bad season -- maybe once a decade or even once in a century -- that all that low-level browse gets eaten, and longer necks come into their own in the feeding competition stakes.

But these bad seasons are the critical times, the times when the less fit get clobbered, the times when nearly all the selection takes place.

By John Atkinson (not verified) on 02 Feb 2007 #permalink

hoofed mammal researchers

Sounds like something from the Far Side.


Thanks to all for their comments. It's clear that there is not one magic answer to the mystery of the giraffe's neck, and I agree that we should not imagine it to have evolved for one function only.

Nathan: counting giraffe neck vertebrae is not so simple in my experience. The problem is that the eighth vertebra looks more like a cervical than a thoracic, and has a junction with the ninth that looks more like the junction you normally see between the seventh and the eighth. The soft tissues are also significant in this debate: the brachial plexus (the big nerve cluster associated with the base of the neck) is adjacent to the eighth vertebra in giraffes, whereas in other mammals it is adjacent to the seventh. It is these factors which suggest to some that the eighth vertebra should be regarded as part of the neck, in which case giraffes have 8 neck vertebrae. Like I said though, not everyone agrees with this.

"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."

It's not helped by the fact that whenever such research confirms what folk had long believed and it makes the media, some bloviator will be going on about it being a waste of money and a sign of how dumb scientists are 'Next thing, they'll be wanting money to study whether the sun is up during the day!' That has to make folk handing out limited research money have second thoughts.

hoofed mammal researchers

Sounds like something from the Far Side.


Ah, the wonders a hyphen can work...

By David Marjanović (not verified) on 03 Feb 2007 #permalink

Derek, giraffes run using a rotary or (less often) a transverse gallop, the footfall being: left front - right front - right - left rear. They do not pace or "rack" like a camel with both the front and hind foot on each side moving together. Actually for the most part they move like an over-legged horse and not a camel.

I think trying to find a single reason for differential survival in giraffe legs and necks is very human but very wrong.


I spent a not-very-long time observing giraffes at Longleat a year or two ago, and I saw giraffes using both gaits in a short space of time.

Regarding necks as a sexual signal (or, let's call a spade a spade, necks as penis extensions): I've never found this compelling because I come at the problem from the sauropod end. While I could just about buy it for a giraffe lineage that's lasted a couple of poxy million years and spanned maybe half a dozen genera, I find it very hard to swallow that clade as long-lived and diverse as Sauropoda (150 million years, 120 genera currently named with plenty more in the works) could have pursued this so diligently with only one significant reversal (in Dicraeosauridae). OK, this is the dreaded Argument From Personal Incredulity ... but I am really incredulous :-)

By Mike Taylor (not verified) on 05 Feb 2007 #permalink

Further to Mike's comments. I spent several years (part-time) working with giraffes in London Zoo and have seen them in many other zoos and in the wild. I can assure everyone that they do pace and this is their primary mode of locomotion. In fact, I have never seen them use another form locomotion at slow and medium speeds, and i am unaware of any record of them doing so.
Pacing is an inherently interesting form of locomotion, observed in camelids, cheetah and maned wolves naturally and it can also be seen in some breeds of dog and horse. However, apart from the giraffe, for all of thee it is an ininnerant for of locomotion and even if used more often than not, it is not obligitory.
I have often wondered if many sauropods were obligae 'pacers' based on their respective limb lenghts. I'm sure Mike will deluge me at some point when he spots this!

I find the long necks for drinking explanation to be implausible. I would expect that there is no reason why giraffes couldn't kneel down to drink.

"I find the long necks for drinking explanation to be implausible. I would expect that there is no reason why giraffes couldn't kneel down to drink."

Well, they don't "kneel" down, even though their necks are quite short to reach the water from the top of their legs. Instead, they splay their front legs, resulting in a very awkward stance.

Maybe this detail can be used both ways; for some reason they can't "kneel" down, or doing so and moving from such position is potentially dangeorous, so longer necks became an advantage for drinking and running away fast. OR could be argued, perhaps, that if necks had really been selected for such function, they'd be even larger so to really make the drinking process less uncomfortable. I think that the latter counter-argument is weak, anyway, it could well be that the long neck was selected for such function, and simply there were not enough variation or even structural possibility to the neck be of the perfect length for such function. It stumbles in collateral maladaptation or even developmental constraints.

An interesting text on the giraffe's neck and the search for explanations for adaptations in general: "The Giraffe's Short Neck", by Craig Holdrege.