Take the giraffe's long neck. This one seems like a no-brainer: giraffes evolved long necks in response to food scarcity and competition with other browsing mammals. Having a slightly longer neck enabled the giraffe's ancestors to reach slightly higher browse than other mammals, and over millions of years, the trait was inherited and enhanced by natural selection, producing the majestic, iconic animal we see today.
Or so you were probably taught. Just like me. It makes perfect, logical sense; intuitively, it feels right. Darwin himself made the same speculation in the sixth edition of On The Origin Of Species:
So under nature with the nascent giraffe, the individuals which were the highest browsers and were able during dearths to reach even an inch or two above the others, will often have been preserved; for they will have roamed over the whole country in search of food. That the individuals of the same species often differ slightly in the relative lengths of all their parts may be seen in many works of natural history, in which careful measurements are given. These slight proportional differences, due to the laws of growth and variation, are not of the slightest use or importance to most species. But it will have been otherwise with the nascent giraffe, considering its probable habits of life; for those individuals which had some one part or several parts of their bodies rather more elongated than usual, would generally have survived. These will have intercrossed and left offspring, either inheriting the same bodily peculiarities, or with a tendency to vary again in the same manner; whilst the individuals, less favoured in the same respects, will have been the most liable to perish.Darwin's endorsement of this speculation has given it the force of orthodoxy, and this interpretation remains, 150 years later, the textbook example of a straightforward case of natural selection in action.
But it might be completely wrong.
I started thinking about giraffes after my interview at Let Them Eat Meat, in which I pointed out that evolution is more complex than most people realize, and therefore we should be wary of seemingly straightforward connections between food and (assumed) adaptations. What seems like a no-brainer might, on closer inspection, turn out to be a just-so story. As Stephen Jay Gould warned (in his classic essay The Tallest Tale, among many other places) "consequences of substantial import often arise from trivial triggers of entirely different intent. In other words, current utility bears no necessary relationship with historical origin."
In short, at the time of Darwin's writing, and even up to the time of Gould's, there was no body of evidence straightforwardly indicating that giraffes evolved long necks in response to dietary selection pressures. And despite the fact that there's been some excellent giraffe research done since then, there still isn't. The origin of the giraffe's long neck is still an intriguing mystery.
What makes a giraffe?
If you're like most people (and why wouldn't you be?), you're accustomed to defining giraffes by their long necks. It's pretty much their distinctive characteristic in the common imagination. But it turns out that in this case (as in so many others), the common imagination is wrong. Most giraffes throughout Earth history have not had long necks. For that matter, giraffes today don't have long necks, either.
|A giraffe struggles against its short neck to get a drink.|
Just look at the stance they have to assume in order to drink water: legs splayed, forming a triangle with the plane of the ground, stretching their necks to the water's surface... and then still barely able to reach.
Contrast this stance with that of any other terrestrial quadraped; say, the antelope, or the rhino. None of them have to stretch their legs in an awkward angle to take a sip. They just stroll up to the water's edge, bend their necks down, and drink. Their legs are never involved.
Not so with the poor giraffes, for whom every drink involves a yoga stretch. They may be the tallest land animals in the world, and their necks may be the longest among all others in absolute terms, but relative to their ability to get water, giraffes have shorter necks than any of their relatives among the ungulates, and arguably among any other group of terrestrial animals.
So upon closer inspection, it doesn't make much sense to define giraffes as a species based on their supposedly long necks. To figure out what makes a giraffe a giraffe, we have to look more precisely at their morphology and fossil record. What do modern giraffes, Giraffa camelopardalis, have in common with their closest living relatives, Okapia johnstoni? And what trait(s) do these two extant clades of giraffids share with their common ancestors and fossil relatives? What distinct anatomical feature(s) unites the living and extinct giraffids into a coherent taxon?
|Okapi: the other giraffe! No long neck here, either...|
This distinct trait first appears in the fossil record between 10 to 15 million years ago, with the rise of the Climacocerids, of whom the most famous member is the early giraffoid Canthumeryx syrtensis, who looked more like an antelope with horns askew than like what most people think of when they think "giraffe." C. syrtensis was a grazer/browser who lived in the lush flood plain/savannah/river basin habitat of what is now Libya, and had relatives all over North Africa and southern Eurasia, as far east as India.
In addition to being a relative of modern giraffes, C. syrtensis was likely also an ancestor of the sivatheres, a clade of bulky, moose-like giraffids who didn't have "long" anything, as far as we can tell.
Giraffokeryx is considered transitional between the canthumerycids and later giraffe relatives, and after it, things start looking a bit more familiar; its ancestors, the palaeotragines (who looked even more like the okapi than Giraffokeryx did), gave rise to the samotheres, who ultimately became the modern giraffe. The resemblance between okapi and basal palaeotragines is so strong that some have argued that the okapi is a "living fossil" representing the morphology of ancestral giraffids, unchanged over millions of years because it occupies a forest niche unchallenged by any other species.
The samotheres replaced most of the other palaeotragines by the Miocene/Pliocene transition (9 to 5 Ma), and were generally larger and longer-faced than their ancestors, looking more and more like modern giraffes as time progressed. It was with this group that the elongation of the neck and legs really began to take off, and the genus Giraffa (which once contained six species, and now only one) was born between 7.1 to 7.3 Ma.
So, Why The Long Neck, Mr. Giraffe?
Fascinating as this evolutionary history is, though, it still begs the question, "why did giraffes evolve long necks?"
But in a very real sense, that's the wrong question to be asking, let alone begging, since it's clear that most giraffes through time didn't have long necks, and relative to their own bodies, modern giraffes arguably don't actually have long necks, either. The real question, then, is, "what made Giraffa species get so much taller overall than their ancestors?" To make anything like an educated guess on that question, we have to look at the world in which giraffes emerged.
The emergence of "true" giraffes across the Old World and their subsequent retreat into Africa happened against the backdrop of the Miocene/Pliocene transition, a time of enormous climatic, geographic and tectonic change. Starting about 10 Ma, the Himalayas and the Tibetan plateau entered a second phase of uplift and the Paratethys Sea began to slowly dry up and retreat. At the same time, other geologic processes like sea-floor spreading, silicate weathering and sediment burial began to lower atmospheric CO2 concentrations.
All of these factors working in concert fundamentally altered the ecology of central Asia and north Africa, (heck, the rest of the world, too), where Giraffa's ancestors had thrived. The region stretching from western India to northern and eastern Africa went from a tropical rainforest environment dominated by C3 plants to an arid savannah populated by C4 grasses. Deciduous forest biomes gave way to conifers and other small-leafed, arid-adapted plants. Rainfall became more seasonal north of the Himalayas, the Tibetan plateau became drier, and summer and winter monsoons got stronger. South of the Himalayas, summer temperatures and rainfall increased dramatically. Collision of the African and Eurasian tectonic plates caused coastal upwelling of the Arabian Sea and created land bridges between Europe, Asia and Africa, allowing for the migration of giraffids (and other species) into Africa. By 6 Ma, C4 plants dominated central Asia, and the decline of giraffids there began in earnest. Then, between 6 Ma and 2.6 Ma (the Plio-Pleistocene transition), there was further uplift of the Himalayas (throwing monsoonal patterns further out of whack), an expansion of continental ice-sheets, and a prolonged period of global cooling. These factors led to the extinction of Giraffa species in Asia, and the retreat of remaining giraffids into Africa.
It seems likely that these upheavals served as the crucible in which Giraffa developed its incredible suite of adaptations, including its height, but nailing down precisely which of these factors caused which of the giraffe's traits is tricky, if not impossible. In particular, the intuitively-appealing idea that height provided giraffes with a selective advantage over other herbivores becomes murkier upon close inspection.
The first giraffe researcher to raise an objection to this idea was Chapman Pincher, who made the following points in a 1949 Nature article titled "Evolution of the giraffe":
- any dearth of resources that was long enough, severe enough and/or frequent enough to produce a selective pressure for long necks would cause a recurring wastage of young giraffes, and thus the extinction of the species; i.e., if young giraffes can't reach the high browse for which they're supposedly adapted until they reach adulthood, then they'll never reach adulthood!;
- the same dearths would have produced selection for long necks in other herbivores, yet only giraffes achieved this adaptation; and,
- adult male giraffes are, on average, one meter taller than adult females, who are in turn taller than all their young. Thus, dearths would place shorter members of the species at a permanent disadvantage, yet again ensuring extinction of the species.
Against these points, Pincher argued that the neck of the giraffe was only "long" because as their leg length increased, giraffes would need to be able to reach water sources from greater heights. Thus, their neck is just long enough to do the job, and no longer (remember their short necks?).
About 15 years later, A. Brownlee, also in a Nature article entitled "Evolution of the giraffe," agreed with Pincher's objections, but argued that height provided giraffes with improved thermoregulation and a better ability to avoid, detect and fend off predators.
The question of whether height provides a feeding advantage to giraffes is further complicated by the variant feeding patterns of giraffes in different habitats. In South Africa's Kruger National Park, for instance, male giraffes tend to feed at a greater height than females, who in turn feed at a higher level than young. Average feeding height is between 1.7 and 3.7 m, again with males tending to feed higher. This places female and young Kruger Park giraffes in direct competition with kudus, whose feeding height range is between 1 and 2 m.
But in Kenya's Tsavo National Park, giraffes spend about 50 percent of their feeding time browsing below a height of 2 m. And in the Serengeti, they mostly feed on low-growing Grewia bushes.
On balance, then, it appears that, at least in Kruger Park, height provides giraffes with only a marginal feeding advantage. A marginal advantage is certainly good enough for natural selection to work with, but it doesn't explain why giraffes grow to an average height of 5 m when an average feeding height of 3 m would be all that's needed.
Yet another team of giraffe researchers argued that long necks are driven by sexual advantage rather than feeding advantage. Male giraffes frequently use their necks as weapons in defense of females in estrus, as well as in dominance contests with other males. Thus, there would be sexual selection for long necks in males, and female offspring would inherit the trait without really "needing" it.
But then, here we are again (potentially) indulging the adaptationist fallacy that current use is necessarily related to evolutionary origin. It might be that giraffe height has nothing to do with any of these clear adaptive advantages. Or, it might be the product of all of them at the same time. As Darwin noted, "(t)he preservation of each species can rarely be determined by any one advantage, but by the union of all, great and small." Or, it could be a side-effect of pleiotropy.
The bottom line is, we just don't know for sure why giraffes evolved to be so tall. And it'll probably stay that way for a long time. Until it's figured out, we should avoid the temptation to take at their word anyone speaking authoritatively about the importance of specific traits... even Charles Darwin. Truth is, in such situations, they're claiming to know more than they possibly could.
- Brownlee, A. 1963. Evolution of the giraffe. Nature 200: 1022.
- Du Toit, J.T. 1990. Feeding-height stratification among African browsing ruminants. African Journal of Ecology 28: 55–61.
- Leuthold, B.M. & Leuthold, W. Food habits of giraffe in Tsavo National Park, Kenya. East African Wildlife Journal 10: 129–141.
- Mitchell G, Skinner JD: On the origin, evolution and phylogeny of giraffes Giraffa camelopardalis. Trans Roy Soc S Afr 2003, 58:51-73.
- Pellew, R.A. 1983a. The giraffe and its food resource in the Serengeti. I. Composition, biomass and production of available browse. African Journal of Ecology 21:241-267. 2. Response of the giraffe population to changes in the food supply. African Journal of Ecology 21:269–283.
- Pincher, C. 1949. Evolution of the giraffe. Nature 164 (4157): 29–30.
- Simmons, R.E. & Altwegg, R. 2010. Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe. Journal of Zoology 282: 6-12.
- Simmons, R.E. & Scheepers, L. 1996. Winning by a neck: sexual selection in the evolution of giraffe. The American Naturalist 148(5): 771–786.