OpenLab Finalist: Giraffes – Necks for food, or necks for sex?

Author’s note: Today the finalists for the 2010 Open Laboratory anthology were announced, and I am proud to say that – for the fourth year in a row – one of my posts made it onto the list of winning entries. To celebrate, I have reposted it below.

The entire list of finalists is fantastic and displays the talent which permeates the science blogohedron. By the looks of it, the 2010 anthology is going to be the strongest edition of The Open Laboratory yet. (I should note that I was one of the judges for this year’s edition, though I hasten to add that I had no input about any of my own entries.) Congratulations to all of this year’s winners!

The neck of a giraffe (Giraffa camelopardalis) is a marvelous thing. Comprised of only seven vertebrae* – no more than in your neck – the towering feat of natural engineering is at once stunning and ridiculous. How could such a structure have evolved? This question is not just a throw-away. For the past century and a half, naturalists have been vexed by the long neck of the giraffe.

*[This would not be a good science essay if I didn’t have to throw in some kind of caveat before I got out of the first paragraph. There is an ongoing anatomical debate about whether giraffes have only seven neck vertebrae or eight, the extra one being a modified part of the back. If you want to read up on the technical details of this hypothesis, see Nikos Solounias’ 1999 paper on giraffe anatomy.]

As is traditional for big questions in evolution, the conundrum of the giraffe’s neck is often traced back to the work of Charles Darwin. Darwin, I was taught in my high school and college lectures, proposed that there were individuals among an ancestral population of giraffes which just happened to have slightly longer necks than their fellows. This allowed them to reach higher branches, and as a consequence these giraffes were more reproductively successful since they persisted on an untouched food source while giraffes of lesser stature perished due to competition. (This simple trajectory was often accompanied by a cute little picture of a modern giraffe placed next to an ungainly “ancestral” species which was identical in all respects to its descendant except neck length.) It was a neat and simple equation – short-necked giraffes + natural selection + time = long-necked giraffes – but, like so many standard classroom tropes, it is also historically and biologically wrong.

Even though the long neck of the giraffe would appear to be a stunning example of the power of natural selection, in the first edition of On the Origin of Species by Means of Natural Selection Darwin focused on a very different part of the mammal’s anatomy; it’s tail. The giraffe’s tail, although appearing to be “an organ of trifling importance”, was an excellent flyswatter which has probably played at least some role in helping individuals resist the attacks of biting insects. From the beautiful intricacy of the vertebrate eye to the relative crudeness of the giraffe’s tail, natural selection could be consistently applied to explain the origins of adaptations.

Darwin did not explicitly consider the neck of the giraffe until much later, and when he did it was as a response to one of his most serious critics. In 1871, the naturalist George Jackson Mivart published a book-length rebuttal to evolution by natural selection titled On the Genesis of Species. Like many other late 19th century naturalists, Mivart accepted evolution but rejected natural selection, and one of the major points of his book was that natural selection could not account for the intermediate stages between an ancestral and descendant form.

The giraffe, said Mivart, was a perfect example of natural selection’s inefficacy. Assume that a giraffe’s long-neck truly was an adaptation for reaching high foliage during droughts which denuded other sources of food. This would be consistent with natural selection, Mivart argued, but if droughts truly had favored the lengthening of the giraffe’s neck then it is surprising that no other herbivores were adapted in the same way. Likewise, the changes which must have occurred between the ancestral and modern giraffe must have extended beyond neck length alone. Various bits of anatomy and physiology would have had to have carried favorable variations in order for it to even be possible for giraffes to evolve long necks, and these changes – which would have increased the mass of the giraffe – would have required that it take in even more food under stressed, drought conditions.

Mivart had set up a straw man argument, but Darwin took this criticism seriously. In the sixth and final 1872 edition of On the Origin of Species Darwin responded at length to Mivart’s claims, and in the case of the giraffe he accepted Mivart’s scenario as a starting point. Given the competition which would have occurred among herbivores during times of drought, Darwin surmised, it would not be surprising that the ancestral giraffes would have been adapted to  reach a level of vegetation inaccessible to other herbivores. Competition would drive forms apart rather than keep multiple forms in close competition by adapting them in the same way. (A.R. Wallace considered this same point over a decade earlier.) Nor were Mivart’s objections about variation and nutrition especially important. Giraffes, like any other species, expressed variation, and the ability of individuals to reach food inaccessible to others would have led to the success of the taller forms. And, as far as drought conditions were concerned, Darwin countered that living giraffes were abundant in Africa. If the large, modern animals could survive in temporarily stressed conditions, then why not intermediate forms which have since been replaced?

Through accepting Mivart’s starting conditions, Darwin had presented a defense of his conception of evolution while also outlining a plausible pathway for giraffe evolution. What Darwin did not do was present a detailed, historical explanation for the form of the giraffe. Indeed, Darwin was typically careful in handling the history of life, and even when he was privately enthusiastic about fossil forms which exhibited transitional features – such as the feathered dinosaur Archaeopteryx – in his public works he avoided laying out precise step-by-step scenarios which would have taken place in the distant past. Darwin knew that paleontologists had only just begun to probe the fossil record, and so outlining phylogenies or tracing evolutionary lines of descent was a risky maneuver since those findings would almost certainly have to be revised.

The exchange between Mivart and Darwin did nothing to resolve the question of how giraffes had evolved. Proposing plausible adaptive scenarios was easy, but actually testing them has been another matter altogether. Even now, after nearly a century and a half since the exchange between Darwin and Mivart, the evolution of the giraffe’s peculiar neck remains contested, and the scope of the present debate has been shaped by two competing evolutionary hypotheses.

The standard hypothesis, which is an extension of the argument Darwin outlined in 1872, is that competition for food drove the evolution of elongated necks. Through the race to reach the most nutritious browse, natural selection would have resulted in the evolution of a giraffe able to reach otherwise unexploited resources above the heads of other herbivores. The idea that giraffes gain such a benefit was supported by one of the few experimental studies to look at this question. As illustrated by Elissa Cameron and Johan du Toit in a 2007 study of giraffe feeding ecology, it was found that lower-level herbivores deplete the abundance and quality of browse available to giraffes. By excluding browsers from feeding on certain Acaia trees, the scientists were able to measure just how much of a tree’s foliage competing herbivores scarf up, and it became apparent that giraffes would definitely get the most from each mouthful by browsing high when low- and mid-level browse had been cleared away. Giraffes can feed at a variety of levels, and this ability to reach high during times of tough competition certainly provides them with an advantage.

The competing hypothesis is that giraffe necks evolved as a result of sexual selection. Even though naturalists have long-recognized that male giraffes often swing their necks to batter each other with their stout ossicones – Darwin even mentioned it in passing in Descent of Man – this habit was not thought to be especially relevant to giraffe evolution until a 1996 paper by Robert Simmons and Lue Scheepers entitled “Winning by a Neck: Sexual Seletion in the Evolution of Giraffe.” Citing the observation that many giraffes they observed fed at a lower level than they expected, Simmons and Scheepers argued that a long neck did not confer that much of a feeding advantage. The competition between males, which appeared to have longer and stronger necks, had driven the evolution of the giraffe neck. Female giraffes were somehow carried along as sexual selection among males kept pushing the limits of neck length.

The hypothesis proposed by Simmons and Scheepers has been controversial from the start and has received a good deal of criticism. One paper, published by G. Mitchell, S. J. van Sittert, and J. D. Skinner in the Journal of Zoology last year collected data suggesting that male giraffes do not energetically invest more in the growth in their necks than females do. In fact, not only did the necks of female giraffes continue growing throughout their lives, but they also added neck mass faster than males, and whatever differences there were between the necks of female and male giraffes appeared to be attributable to differences in overall body mass rather than the true sign of sexual selection. If the differences between living giraffes were so minimal, it seemed unlikely that males had truly driven the evolutionary change through sexual selection.

Simmons and co-author R. Altwegg have just responded to this study in a new Journal of Zoology paper. In surveying the debate they state that neither the food competition nor the necks-for-sex hypothesis may be able to provide a comprehensive explanation for giraffe evolution. Although they dispute the findings of Mitchell, van Sittert, and Skinner – arguing that the data collected by the other team actually represents a significant disparity between males and females triggered by sexual selection – they rightly note that most of what has been said about giraffe necks has depended upon the anatomy and behavior of living animals. The arguments and experiments about the necks of living giraffes have more to do with the evolutionary pressures which are maintaining the form of the giraffe, but they may not be able to tell us very much about how long necks evolved in the first place.

As identified by Simmons and Altwegg, the long neck of the giraffe may have evolved in response to some ecological change but then was co-opted into other functions which caused further alterations under different evolutionary pressures. The advantages provided by the giraffe’s long neck today – be it in terms of feeding, sexual selection, or both – cannot definitively tell us why the trait evolved in the first place. The current function of a trait is not necessarily representative of why it evolved – an important caveat explicitly underlined by paleobiologists such as Stephen J. Gould and Elisabeth Vrba several decades ago.

If the necks of modern giraffes are at least partially attributable to a shift in function, then our ability to answer the question of giraffe evolution cannot be based upon living animals alone. In order to test their preferred sexual selection idea Simmons and Altwegg suggest going back to the fossil record to see when giraffes evolved the blunt ossicones males use in their competitions and how this corresponds to neck length. If the evolution of blunt ossicones tracks neck elongation, they hypothesize, then this might be an indicator that head-swinging contests had something to do with the neck elongation (while the browsing hypothesis would have more to do with detecting a relationship between neck and leg length which would have pushed giraffes over the heads of competing browsers). Correlation does not imply causation – even if the evolution of blunt, vertical ossicones and long necks coincided, that does not mean that the two are evolutionarily bound together – but by investigating these questions scientists would add a much-needed historical angle to research into giraffe necks.

Unfortunately, fossil giraffes are not exactly a hot area of research, but recent work and forthcoming papers have the potential to provide a little more context to explain the origin of the giraffe’s impressive neck. Over the past several years it has been hinted that fossil giraffe expert Nikos Solounias has been describing the nearly-complete neck of a fossil giraffe called Bohlinia, a genus closely related to – if not ancestral to – the first members of the genus Giraffa. The full publication has yet to appear, but, as figured in Donald Prothero’s book Evolution: What the Fossils Say and Why it Matters, this specimen is intermediate in form between that of earlier, shorter-necked giraffes and the living giraffe. When compared to the greater swath of giraffe diversity, it may be a crucial part of detecting the pattern of giraffe neck evolution.

An outline of that pattern came out of a review published last year by N. Ludo Badlangana, Justin Adams, and Paul Manger. According to the known swath of fossil giraffes, significant neck elongation began around 14 million years ago during the Late Miocene – after the lineage to which the relatively short-necked okapi split off – and by about 5 million years ago giraffes of modern proportions had evolved. As bracketed between known fossil types, the transition between the early, short-necked forms and the first long-necked giraffes probably occurred between 14 and 12 million years ago, and if further discoveries bear this out then it appears that the elongation of giraffe necks occurred during a global pattern of aridification in which grasslands replaced forests. Again, this correlation does not by itself imply causation, but by investigating the tooth wear of giraffes before, during, and after this shift it could be detected whether their feeding habits changed and could have been part of the reason for neck elongation. This, in turn, might provide some insight into the tempo of giraffe evolution. Did their neck vertebrae slowly become elongated at a near-constant rate, or did the altered activity of regulatory genes cause faster, large-scale shifts which turned out to be advantageous in the altered habitat?

Ultimately, a combination of natural history, embryology, and paleobiology will be needed to fully understand the unique anatomy of giraffes. This is not something which will be accomplished in a year or even ten, but will take the persistent investigations of many researchers working across a variety of scientific disciplines. For the moment, the question of “How did the giraffe get its long neck?” must be answered with “We do not yet know”, but that is as it should be. It is better to admit that we are still unraveling a mystery than to dogmatically assert that all is solved and that all the uncharted places on the evolutionary map have been filled in. Giraffes, just like every other organism alive today, have a long evolutionary history stretching back to the last common ancestor of all life on earth, but understanding how they were shaped over the span of Deep Time is an ongoing endeavor which is just as inspiring as it can be frustrating.

References:

BADLANGANA, N., ADAMS, J., & MANGER, P. (2009). The giraffe cervical vertebral column: a heuristic example in understanding evolutionary processes?Zoological Journal of the Linnean Society, 155 (3), 736-757 DOI: 10.1111/j.1096-3642.2008.00458.x

Cameron EZ, & du Toit JT (2007). Winning by a neck: tall giraffes avoid competing with shorter browsers. The American naturalist, 169 (1), 130-5 PMID: 17206591

Mitchell, G., van Sittert, S., & Skinner, J. (2009). Sexual selection is not the origin of long necks in giraffes Journal of Zoology, 278 (4), 281-286 DOI: 10.1111/j.1469-7998.2009.00573.x

Simmons, R., & Altwegg, R. (2010). Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe Journal of Zoology, 282 (1), 6-12 DOI: 10.1111/j.1469-7998.2010.00711.x

Simmons, R., & Scheepers, L. (1996). Winning by a Neck: Sexual Selection in the Evolution of Giraffe The American Naturalist, 148 (5) DOI: 10.1086/285955