Posts Tagged 'giraffe'

Neck multifunctionality

Following on from my post on sauropod neck lengths (and indeed that of the SV-POW! boys and Tet Zoo too), at the end I made the inevitable comment that necks (and indeed other structures) can be multifunctional. A long neck can be an indicator of sexual selection at the same time as providing increased reach for food. As noted before, analogy plays an important role in working out (or at least hypothesising) palaeo behaviour, so are there any animals out there that seem to do this. Indeed there are, so step forward, the Galapagos giant tortoise.

Tortoise neck use - above, reaching for food, below, challenging rivals. By Darren Naish, from Taylor et al., 2011.

Research shows that those animals with longer necks do gain a distinct advantage in reaching higher placed food. The neck provides a genuine basis for natural selection based on neck length. However, it has also been shown that when tortoises stand off in dominance battles, the individual with the longer neck tends to win. So, necks would also appear to be under some measure of sexual selection / dominance as well.

So what about the giraffes? They have long necks and the males fight with their heads, so what’s going on there. Well males do have longer necks than females, but there’s not really anything to say that longer necked males do better. Bigger males do better (no surprise) but not through a longer neck per se. Plus of course the males are actively fighting with their heads. (And tortoises can be quite vicious if you’ve ever seen them fight, then bite and butt with their shells).

Posturing only gets you so far in nature. Sure, there are cheats out there (false cleaner fish, milk snakes, female mimic salmon etc.), but they can only prosper as long as they are in the minority. This is because sooner or later someone is going to square up to you in one way or another and find out if you really can back the bark with bite. If you can’t, you’re going to lose. And if say most of the population were lying, once a dominant animal (or predator etc.) finds out, then that is going to take over damned fast. So lying only works when there are few liars, and most things are honest. In other words, if they are advertising that they can win a fight, it’s because they can and will.

What does this mean for sauropods? Well is has been suggested in the past that sauropods might fight one another, with their necks. Now if this was going to happen you’d expect to see some evidence of this in sauropods. Like the especially tough and thick skulls of male giraffe, or the prow-shaped rams of some tortoises, or robust necks and heads in male sauropods and you’d see injuries from some serious sauropod neck-on-neck action. Only of course there aren’t any.

Instead sauropod skulls are incredibly weak and fall apart if you look at them funny, let alone ram them into something else at speed. And while the neck as a single unit might be quite tough, it has those lovely wafer-like lamina and those oh-so-thin cervical ribs. If they were fighting we’d see breaks, pathologies, healed bones and the rest. And you can’t cheat by just having a big neck and expecting the others to back down, you have to back that up or someone will realise it’s all talk.

I’m sure sauropods did fight on occasion, sooner or later animals of pretty much any species will come into competition and of course it is members of the same species that tend towards the fiercest competition. There will come times when accessing that water hole, or harem, or territory is critical and combat becomes inevitable. But was it with the neck? No. The neck might have been a *symbol* of the power of the individual even if it wasn’t used (pheasants and cockerels show off their colours to demonstrate their fitness, but they fight with their spurs).

Sauropod necks – what were they for?

Long time readers might well recall that for a few years I was a volunteer at London Zoo where I worked on what was the Cotton Terraces (now sadly rebranded as the “Africa Zone”) with various hoofstock and ungulates. It was, generally, an absolute joy and I learned a great deal about animal management and animals in general. Among my occasional charges were the giraffe and I have a long and abiding affection for them from my time there. This has in part manifested itself by my keeping up with giraffe research in the literature and when an opportunity arose to write about giraffes and dinosaurs, well, I could hardly resist.

So what do giraffes and dinosaurs have in common? Well not much to be honest, but perhaps inevitably the long necks of giraffe have been brought up a number of times in the past as analogues for the long necks of sauropods. It’s not unreasonable really as good analogues are important for inferring the functions and behaviours of extinct taxa and there are very few terrestrial long-necked herbivores out there, and even less that look even vaguely like sauropods. So the high reach of giraffes has been used to suggest sauropods reached high into the trees to feed.

Male giraffe sparring at Beijing zoo.

Set against this background has more recently been the controversy over sauropod neck postures and quite how much (or even if) they could raise their heads and necks. But it gets more complicated still (yay!). Back in the mid 1990s it was suggested that actually giraffe necks hadn’t evolved for feeding high in trees, but instead were sexually selected structures. Perhaps not surprisingly, the same hypothesis was then extended onto sauropods too! If they couldn’t raise their necks up, and the only obvious living example (in giraffes) was only an opportunistically high browser, then maybe sauropod necks were the result of sexual selection too? (Mike Taylor has a preview of these ideas here).

In a new paper out today myself and the SV-POW! boys (Messers [or rather, Doctors] Taylor, Wedel and Naish) take on this idea. The detail is of course in the paper but there are several interlinked threads to this paper that affect different lines of sauropod research and ecology. Perhaps most interestingly for me is the work on giraffes though, since actually the idea of sexual selection in these beautiful artiodactyls is far weaker than originally proposed and actually high browsing does seem to be the primary function of the neck.

Sauropod neck lengths. From Taylor et al. in press

Thus the original analogy can be restored and the analogy for sauropods having sexually selected necks rather falls by the wayside. Couple that with the existing work by my colleagues on the potential vertical reach of sauropods and another barrier falls by the wayside. Even if sauropods couldn’t reach high into trees though, that’s not necessarily a barrier to their long necks being adapted for feeding efficiency. Anyone who as seen a grazing goose will recognise their feeding pattern with the body of the bird only plodding along slowly but the long neck sweeping from side to side so that for every step forward (a big deal of effort for a 50 ton sauropod say) a fair area of new grass can be covered by the head. In short, a long neck can be efficient whether you are reaching up or not.

All together (and there is obviously more to this than I’m covering briefly here) we find no convincing evidence of sexual selection going on in sauropod necks and are satisfied that the long necks would have provided a significant horizontal and vertical reach and thus did afford a significant part of their feeding ecology. There’s no good evidence (at the moment, at least) that any sauropod necks were under sexual selection or indeed, that those of giraffes were. That doesn’t mean it wasn’t happening, but there is nothing to suggest it was.

This must, of course be hedged with a couple of most important and basic caveats to these kinds of papers. Critically is that of multifunctionality – that structures can have more than one function, so there could be some cryptic sexual selection ongoing alongside feeding advantages. Though as it happens in this case, it’s this very issue that helps us deal with some of the evidence for sexual selection. Secondly of course this paper is a review – we talk about general patterns and trends and evidence. We do not look at all sauropods in great detail and while we have great confidence in the findings of our paper we are talking about generalities – there is of course a reasonable chance that something was an exception.

Taylor, M.T., Hone, D.W.E., Wedel, M.J. & Naish, D. The long necks of sauropods did not evolve primarily through sexual selection. Journal of Zoology, in press.


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