Why do giraffes have such long necks?

The mechanisms of natural selection can be mysterious. The reasons for the long necks of giraffes are debated, as several explanations seem possible. An archaeological study of a distant ancestor rekindles questions about the evolutionary history of giraffes.

In the animal kingdom, giraffes stand out with a very long neck that allows them to taste the highest branches effortlessly. These kinds of peculiarities come from the evolutionary process of natural selection: an advantage that helps to survive in the face of one or more “pressures”. Over the decades, centuries, millennia, the individuals of a species who have an advantage against these pressures become the majority, since they transmit more of their genes to the following generations, where the disadvantaged individuals disappear.

The mechanisms of Evolution, first discovered by Charles Darwin at the end of the 20th century, are beginning to be better and better understood. But there are still mysteries at the level of certain species: why was such and such an advantage imposed on such and such an animal?

A fight of male giraffes. // Source: Wang Yu & Guo Xiaocong

In the case of the giraffe, we tend to consider that the advantage is food. Individuals with longer necks can reach a food source inaccessible to other herbivorous species, in highly competitive environments. But this explanation does not satisfy all scientists.

Some believe it is more of a reproductive benefit. Observation of the behavior of giraffes has shown that, to court females, males engage in violent combat during which the neck is a weapon. It serves them to powerfully project their solid head against the adversary, gaining momentum. In fact, the longer the neck, the more devastating the strike will be; and therefore males with the longest necks, being advantaged, are more likely to pass on their genes. This is how the long neck would have been selected by Evolution, generation after generation.

The long neck proof against blows

The debate only intensifies with an archaeological study reported in Science, this Friday, June 3, 2022. This work focuses on a distant cousin of the giraffe, whose bones were discovered twenty years ago in northern China. The species, named Discokeryx xiezhi, belonged to the superfamily Giraffoidea. It roamed the savannah during the Miocene, around 17 million years ago.

The in-depth study of the skull and vertebrae of this ancient herbivore shows that its bone structure enabled it to withstand strikes of extraordinary violence. In short, its head is particularly adapted to frontal combat, which suggests that, even among the distant ancestors of the giraffe, the neck was already a major asset.

The Discokeryx xiezhi species is depicted in the middle of this illustration — the two individuals fighting each other. // Source: GUO Xiaocong

The strength of evidence for this discovery, to explain the selection of the long neck of giraffes, remains limited because Discokeryx xiezhi is a different species. However, they belong to the same superfamily of the animal kingdom and adopted very similar behaviors. An environmental element comes to add its grain of salt: Discokeryx xiezh evolved in open grasslands, like giraffes, which increases the interest of the comparison.

Natural selection does not necessarily have a precise answer

Neck fighting was probably the main driving force of giraffes that evolved into long necks and high browsing was probably a consistent advantage of this evolution. “, say the scientists behind this research, in their conclusions.

The fact is that natural selection is a complex process, which takes place over dizzying timescales. It responds to several pressures that allow the survival of some specimens rather than others. This study does not lead to a definite or complete conclusion.

It is likely that the long necks of giraffes stem from a combination of advantages for the species: an advantage for males in reproductive fights; a food advantage for reaching high branches; advantage against predators. These different reasons may, together, make individuals with longer necks survive more easily and therefore pass on their genes more easily.

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