Giraffe Necks Not for Sex

Giraffes have an unmistakable, awkward charm about them. But these megaherbivores can also be quite brutal. In intense bouts, male giraffes compete for dominance by steadying their legs and swinging their necks to deliver sledgehammer blows to each other with the stout ossicones atop their heads.

For a time, it seemed that this violent behavior might be the secret to why giraffes are so oddly proportioned. Sexual selection, rather than the quest to reach ever-higher foliage, was picked out as a possible explanation for the strange anatomy.

In a 1996 study, zoologists Robert Simmons and Lue Scheepers proposed that the “necking” competitions between males favored the evolution of ever-longer necks. Imagine an archaic population of short-necked giraffes that competed like their modern counterparts. Better-endowed males would be more likely to win bouts – especially in early stages of fighting when males measure up against each other to see how they’re matched – and would obviously have a greater chance of passing on their winning trait to their offspring. Female giraffes would then be carried along by the evolutionary tide, developing long necks even though they don’t battle as the males do.

Simmons and Scheepers supported their idea with observations of giraffe biology and behavior. Most of the time, they argued, giraffes feed on relatively low patches of plant food that don’t require a long, energetically-expensive neck. Furthermore, male giraffes appeared to have more massive necks than females, and added neck mass throughout their lives. Males seemed to “invest” more in their necks than females, and they took this degree of sexual dimorphism as a sign that sexual selection had drastically shaped giraffe anatomy.

The “Necks for sex” hypothesis grated against the traditional explanation for the giraffe’s strange anatomy. From the 19th century on, the evolutionary impetus for giraffe elongation was thought to be competition for food. Amidst an ancestral population of relatively short-necked giraffes – animals akin to today’s okapi – individuals with necks ever so slightly longer than the rest were better able to reach food resources high and low, and therefore survived to produce more offspring. Repeat this step over and over and over again over millions of years, and natural selection produces the modern leggy, long-necked giraffe.

Both scenarios are plausible. That’s why the necks for sex hypothesis kicked up so much controversy (spilling over into paleontological circles regarding the even more impressive necks of sauropod dinosaurs). As zoologists investigated the idea and amassed additional observations, however, it became apparent that dominance battles and courtship rituals had little to do with the origin of long-necked giraffes.

Contrary to what Simmons and Scheepers suggested, giraffes truly do gain a feeding advantage by being able to browse both high and low. And a 2009 study of giraffe growth by Graham Mitchell and coauthors failed to find any significant difference between the sexes, contrary to the expectation that male competition drove the evolution of the towering giraffe neck. Simmons and Scheepers contested this conclusion, but still softened their position by acknowledging that the entire argument about giraffe evolution has been based on the behavior of modern animals. The current function of an organ, appendage, or other trait may not reflect the evolutionary pressures that molded that sfeature during prehistory – co-option is rife in evolution.

Now Mitchell and colleagues have issued a follow-up study about how the necks and heads of giraffes grow.  After examining 65 male and 71 female giraffes from two populations, the zoologists found only the barest difference between the two sexes – namely, that the skulls and ossicones of male giraffes are slightly heavier than those of females. Male giraffes also add neck mass for longer than female giraffes, yet, despite this, the neck mass of male and female giraffes are comparable. The very slight differences between male and female giraffes, Mitchell and coauthors argue, are common among large herbivorous mammals and are caused by differences in sex steroids.

The researchers did not find any sign that the giraffe’s neck evolved because of sexual selection. In fact, it was only in the heavier skulls of male giraffes – their primary weapon in necking contests – that Mitchell and coauthors could pick out sex differences. Additionally, citing previous behavioral studies, Mitchell and colleagues point out that the most necking is done by young males that are proportionally the same as female giraffes, and that these contests establish dominance rather than direct access to mates. Nor do males with the longest or most massive necks always win these contests, underscoring the point that “dominance and neck morphology are not linked, and, therefore, necking is very unlikely to have driven sexual selection for a trait such as neck mass.”

Living giraffes can’t tell us everything we wish to know about how these awesome mammals evolved. There’s still a great deal we don’t understand about giraffe prehistory, and what happened to the diversity of forms that once roamed the Old World. Still, we can still look for traces of evolutionary pressures and prehistory in living animals. Given their biology, ecology, and behavior, there’s no sign that male competition was responsible for the origin of the giraffe’s most characteristic feature. Plucking leaves from a range of heights may have been more important in giraffe history, but, to investigate that story, researchers must turn to the fossil record. The secret of the giraffe ultimately rests where clues from the present meet prehistory.

Reference:

Mitchell, G., Roberts, D., van Sittert, S., Skinner, J. 2013. Growth patterns and masses of the heads and necks of male and female giraffes. Journal of Zoology. doi:10.1111/jzo.12013