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.
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.
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.