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 lead 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