I used to love Rudyard Kipling’s Just So Stories when I was a kid – my favourite was the tale of How the elephant got his nose (cue crocodiles, the great grey green greasy Limpopo River, & the insatiable curiosity of the elephant’s child). Charming stories & great fun, but of course no evidence that things actually happened that way 🙂 But what about the giraffe’s long neck?
For years the accepted story was that this had evolved as an adaptation which allowed giraffes to browse high in the trees, using a resource not available to other, shorter-necked competitors. The late Stephen Jay Gould (1996) characterised this as a fairy story that Darwin was far too smart to tell, but which entered our high-school texts as a classic case nonetheless. The idea was tested in a 1991 study that looked at giraffe feeding ecology – including the heights at which male & female girafffes actually feed (Young & Isbell, 1991). The researchers found that male & female giraffes foraged at different heights in the trees & that feeding patterns depended on social makeup of the groups. But they also found that rates of giraffe feeding peaked at intermediate heights equal to approximately 60% of adult giraffe height. The intersexual difference in feeding height can be partly explained by differences in the animals’ heights – but the researchers also suggested that it could be due to male-male competition for mates. Dominant males, feeding higher in the canopy, don’t lower their heads between feeding bouts & so may be more alert to the presence of competing males.
Subsequent studies seemed to confirm this, & in 1996 Simmons & Scheepers suggested that giraffes’ necks had been shaped by sexual selection, rather than selection pressures directly related to feeding. Male giraffes fight by clubbing each other with their heads; swung at the end of a long neck, the head can strike with a fair amount of force. There’s footage of male giraffes ‘neck-fighting’ on the net, here, for example – I winced when I viewed this & heard the impacts. And I’ve seen the young male giraffes in our local zoo doing the same thing:
Simmons & Scheepers found that dominant male giraffes are larger than other males; that male giraffes have larger, heavier skulls than the females; & that larger males exhibit something called ‘positive allometry’ (predicted by sexual selection), whereby they invest relatively more resources in growing their massive necks compared to smaller males. Their work gained wide acceptance & you’ll find it in evolutionary biology textbooks – I’ve talked about it in my own lectures (where we use Freeman & Herron’s Evolutionary Analysis).
But science is not static. And now another study has reopened the discussion around how the giraffe got its neck. Brian Switek has written about this on his blog, Laelaps, but briefly: Micthell et al. (2009) tested the sexual selection hypothesis by taking measurements from 17 male & 21 female giraffes (both juvenile & mature animals). They found no significant differences between males & females for any of the dimensions measured (head mass, neck mass, neck & leg length, & neck length: leg length). Their conclusion: that there are minimal morphological differences between the sexes & that any differences that do exist are simply due to the fact that males end up bigger than females. In other words, you can’t put the long necks down to the results of male:male competition. So are we back to the drawing board?
Brian points out that this highlights one of the difficulties with basing evolutionary hypotheses solely on morphological data from living representatives of a species – we don’t know how well these models actually reflect what’s happened in the past. We have no idea of how the neck was used in the past, regardless of what living giraffes do. As Brian says, We should not confuse what an organ is used for now with what led to its origin: they are not always the same.
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Update: you might also be interested in reading Steven Novella’s take on the whole story of giraffe necks (& thanks to Darcy for providing the link!)
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G.Mitchell, S.J.van Sittert, & J.D. Skinner (2009) Sexual selection is not the origin of long necks in giraffes. Journal of Zoology (on-line, 6 pp.) DOI: 10.1111/j.1469-7998.2009.00573.x
T.P.Young & L.A.Isbell (1991) Sex differences in giraffe feeding ecology: energetic & social constraints. Ethology 87: 79-89