May 19 2009
The story of the giraffe’s neck is a classic of high school biology textbooks. For this reason everyone “knows” that giraffes evolved longer necks in order to reach the leaves at the top of trees. However, this has never been clearly established and the real story is much more complex. There is, in fact, an enduring controversy over exactly what factors led to the elongation of the giraffe neck, highlighted by a recent study examining one hypothesis – sexual selection.
But before we get to that study, some background on giraffe necks.
The most obvious feature of the giraffe is its long neck. For some reason the evolution of the giraffe neck became the standard example in textbooks. Stephen J. Gould did a survey of biology textbooks and found that 100% used giraffe evolution as the example to distinguish Lamarckian evolution from Darwinian evolution (which itself is based upon a misconception of Lamarck’s career, but that’s another story). Meanwhile, Darwin did not use the giraffe’s neck as an example of natural selection, and regarded it as a speculative “just so” story.
Since Darwin there has been speculation and controversy over the evolution of the giraffe’s neck, but never any consensus. There is therefore a stark contrast between the scientific reality and the textbook fiction regarding giraffe evolution – but what’s new.
The standard (textbook, that is) story is that ancestral giraffes were selected for longer necks because that enabled them to reach leaves higher up in trees than other animals. Therefore in times of scarcity they would have access to food that other animals could not access, and this conferred a survival advantage. There is nothing wrong with this story, and it likely holds a kernel of truth. There is simply no evidence to support this particular selective pressure, and there are plausible competing hypotheses. Unfortunately the giraffe fossil record is sparse, so we cannot turn to fossil evidence to settle the question.
It is true that the giraffe’s reach does give them access to leaves in tall acacia trees. It also gives them access to leaves deep within trees. However, giraffes also feed off low bushes by bending their neck, and they do not show any preference for high or deep leaves during the dry season when food is scarce. So at least at present the long neck does not seem to be a significant hedge against starvation.
An alternate hypothesis is that the long neck evolved in response to the evolution of long legs, which themselves evolved for some other reason, such as speed in evading prey. Giraffes cannot support themselves with their knees bent. In order to drink water on the groun they must splay their forelegs (with knees straight) and then use their long necks to reach the ground. Therefore it is possible that necked elongate simply to keep up with the growing limbs.
Other hypotheses include the increase in skin surface areas for cooling, and increased head height to keep a better eye on predators.
Male giraffes also use their long necks in mating competition. They slam their necks against together to settle male rivalries. This could explains why males have slightly longer necks than females. This hypothesis was the focus of this latest study.
Giraffe researcher, Professor Graham Mitchell of the University of Wyoming, and colleagues addressed the sexual selection hypothesis by seeing if the giraffe’s neck meets the criteria for sexual selection: they should be exaggerated in the male, the neck should be out of proportion to other body parts and they should confer no other advantage, and may even be a detriment.
To be clear, it is possible for a feature to be selected for sexually but still have some other advantage. The criteria above are used only to demonstrate that sexual selective pressures are present. If other selective pressure are also present, that makes it more difficult to isolate sexual selection as a factor.
What Mitchell found was that the giraffe’s neck does not meet any of the above criteria. Male necks are longer than female necks, but only in the same proportion to overall body size. Males are bigger than females, but their necks are not propotionally longer.
Giraffes do invest more energy into developing their necks than other parts of their body, but there is no difference beetween the males and females. If males were being sexually selected for long necks, then males should invest more proportionally than females.
There also does not appear to be any detriment to the long necks for males – they are no more vulnerable to prey, for example. It is not possible to say that the long necks do not convey any other advantage – that is the very heart of the controversy.
It occurs to me, however, that all three criteria in the end derive from the same fact – male and female giraffe necks are proportionately similar, therefore males do not invest more energy in their neck nor do they suffer any more disadvantage from them. So one might argue that the first criteria being negative (no difference between males and females) automatically negates the other two. However, if the first criteria were positive (relatively longer necks in males) then the second and third would still be required, so they are not entirely redundant.
So after this study we are still left without any definitive answer to the question of what selective pressures lead to the giraffe’s long neck. This question may not be resolved unless and until we find some fossil evidence to examine. And even then, the fossils may not settle the question.
It is possible that the question cannot be answered because there is no single answer. The more we learn about evolution the more it is undestood that selective pressures are complex and inter-dependent, and there are forces at work in evolution other than natural selection – such as genetic drift and pre-adaptation.
Therefore perhaps all of the hypotheses regarding the evolution of the giraffe’s neck are true. Longer legged giraffes would have been able to run faster and see farther, but needed longer necks to then drink ground water. Meanwhile the longer necks extended access to food. Males later started banging their long necks together are part of their mating competition.
The whole giraffe system had to work together every step of the way, with each change opening up new possibilities and also creating its own selective pressures back on the whole system.
Why did giraffe ancestors head down the tall-necked path in the first place, while other African mammals did not? I chalk that up to the quirkiness and contingent nature of evolutionary history. The giraffe ancestor had a morphology which made the eventually evolution to its current long-necked form possible, perhaps even probable. Some set up circumstances, including the behavior of the ancestors themselves, then created a positive feedback loop of selective pressure sending them down the evolutionary path to tall necks.
Reverse engineering these circumstances may be as complex as watching pingpong balls bouncing around in one of those the air-chambers used to select numbers for the lotto, and then trying to explain why a particular number came up based upon all of the collisions that took place. In other words, there is a certain amount of chaos to the particular details of evolutionary history that therefore may forever defy detailed explanation.
But what we can define are the broad brushstrokes – the dominant factors at work. We can also expand our understanding of the many possible ways in which evolutionary changes occur, even though we cannot always determine which factors were present in any particular case.
Further, while Mitchell’s study is far from conclusive by itself, he demonstrates that we can apply scientific methods to questions of evolutionary history and can use data to infer which selective factors were more likely to be dominant. That may not lead to a simple and clean evolutionary tale (suitable for a high school textbook), but nature is notoriously insensitive to our desires and sensibilites.
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