A new take on necking (in giraffes, that is)

Giraffes

Y’all menfolk will do some wacky stuff for sex, that is fo’ sure. (I get to say “y’all” with authority because I grew up in the South. Honest.) And so it goes in the animal kingdom too. New research published in the Journal of Zoology throws its weight behind a synthesis of the “necks for sex” and “necks for competition” hypotheses. Both have sought to explain why, exactly, giraffes evolved such funky, long, energy sucking necks. The “necks for sex” hypothesis asserts that sexual selection was the driving force influencing neck shape, whereas the latter asserts that competition for partitioned resources was the driving force. A pair of South African researchers asks, Why can’t it be both forces acting together?

Great question. Both hypotheses have their strong points and weak points. The authors write:

The main challenge for the competing browser hypothesis is to explain why giraffe have remained about 2 m taller than their tallest competitors for over 1 Myr, whereas the sexual selection hypothesis cannot provide an adaptive explanation for the long neck of female giraffe.

So, the necks for sex part… this video shows two male giraffes fighting rather violently by slinging their heads at one another’s neck, shoulder and body. Other sources report giraffes will push against each other, using their powerful necks as leverage.

I am no giraffe expert, but wow, those guys are clubbing the $#@! out of each other. 

If giraffes evolved long necks as an adaptation to survive droughts by being able to munch on leaves highest up the trees (thereby beating shorter competition, like rhinos that browse at lower levels), then one prediction is that both male and female giraffe should have equally long necks. But if giraffe evolved long, strong necks as a way to beat the $#@! out of each other (see above) in order to win fertile females, then one prediction is that male giraffe should have significantly longer and stronger necks than females. The authors point out several foraging studies that depict a certain fuzziness to the first idea, because apparently many giraffe browse at shoulder level, and browse more efficiently here, rarely using their elevational advantage to strip leaves from higher levels of acacia trees. (They also point out that during winter when leaves are scarce, rhinos simply knock the trees over to get at the leaves, suggesting there is more than one way to skin that cat.) They do point to one foraging ecology study published in 2007 that lends strong support to the necks for browsing hypothesis (check out this post about it).

They also outline a few lines of evidence supporting the necks-for-sex theory, including that females have been observed to prefer mating with males with longer necks, that males with longer/stronger necks often win clubbing contests and dominance displays, and that traits which are selected for sexually typically exhibit a positive relationship between size and shape (or to put it another way, changes in body size lead to changes in proportion of a feature relative to the body). And apparently, the literature supports all of this for the giraffe, so say the authors. They point to a previous study showing isometric scaling for the females but allometric scaling for the males. Hmmm!

The authors cite one study reporting direct observations of larger-necked males winning clubbing contests and gaining access to females in estrus, but they report they do not know of any studies providing direct observations to support the longer necks for food theory. They also cite two studies, one of Namibian giraffes and one of Zimbabwean giraffes, showing that  positive allometric relationships for neck size are more likely to occur with the males, again supporting the necks for sex hypothesis.

Figure-1

This graph shows male (closed circle) and female (open circle) Namibian giraffes plotted for the relationship of their neck size (y-axis) to their body mass (x-axis). As you can see, the distribution for the males is of a much higher ratio. Using the same data that produced this graph, they also tested the same sample and found that the males had significantly greater differences in head mass. They note that, “Males, but not females, in Namibia also continue to invest in heavier heads as they age,” further supporting the sexual selection hypothesis. Comparing skulls of males and females that were the same length, they cite studies that document the male skulls to be up to four times heavier.

The extra investment in longer necks comes with a cost though, in terms of both physiology (it takes a heck of a high blood pressure to bridge the gap between the giraffe’s heart and brain), and predation. One study cited in this paper says that males were 1.8 times more likely to be eaten by lions than females. The higher predation rate may be because a male’s health may plummet more quickly than a female’s in times of limited food, due to their higher caloric needs, rendering them more vulnerable. But to test this, the neck sizes of preyed-upon giraffes would need to be measured, and it doesn’t appear anyone has tested this yet. (Shout out if I’m wrong.)

Despite the majority of the evidence they review pointing toward sexual selection, it kind of begs the question, how did large necks become selected for in the first place? Because you need a long neck at the beginning… One idea may be that long necks were orginally spawned in response to accessing better forage, but became an exaptation maintained by sexual selection. Another idea looking at it from an evolutionary perspective may be that the “small sideways pointing horns of the ancestral giraffids..evolved to the short blunt ossicones we see on the extinct long-necked Samotherium and all subsequent giraffids” in an exchange from head wrestling (butting, locking horns) to clubbing and using the stout ossicones to try to and splinter an opponent’s bones or inflict damage. But for this latter idea to be right, the exchange in head-gear would need to occur at the same time that the giraffe’s neck became too long for head butting. And on this point, it’s not clear to me from the paper if that is known from the fossil record or not.

Even though the necks for sex hypothesis seems on the whole more supported within this paper than the necks for browsing hypothesis, the authors did not really discuss in depth why females would also have long necks (albeit smaller than the male’s) or why they even have ossicones at all. (I can’t help but think of birds of paradise where there is extreme morphological differences in the male birds which are sexually selected for… ) Of course, there’s always the possibility that it is not one or the other hyothesis, but a happy coincidence of both driving the equation at different points in time. Given the evidence supporting both, this may be where the answer lies.

This was my first foray into the literature on why giraffes have long necks. If you crave more on giraffe neck conundrums, check out these posts from Tetrapod Zoology:
Testing the flotation dynamics and swimming abilities of giraffes by way of computational analysis
Dammit, and I sooo loved the necks for sex hypothesis

Notes:
Simmons, R., & Altwegg, R. (2010). Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe Journal of Zoology DOI: 10.1111/j.1469-7998.2010.00711.x

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