The Whole Two Yards: Giraffe Neck Growth Patterns

18 January 2013 by Anne-Marie Hodge, posted in Uncategorized

Extreme adaptations seem to serve as canvases upon which people paint their various pet theories about evolution. The origin of the giraffe’s nearly two-meter long neck has long served as fodder for “just-so” stories, and has been featured in everything from Rudyard Kipling’s tales as a metaphor for self-improvement to countless biology textbooks as the default illustration for Lamarck’s theory of evolution via inheritance of acquired characteristics. Amongst the speculation as to why any reasonable species would develop such an extreme feature, two hypotheses predominate: 1) Giraffe necks elongated in order to enhance resource access, either through a co-evolutionary dynamic with Acacia tree height or through competition to obtain more browse material than shorter ungulate species; or 2) long necks are the result of sexual selection, akin to peacock tails, that result in more successful mate acquisition for longer-necked individuals—either because female giraffes think long necks are sexy or because males use them as weapons to compete for mates. It also could have been a combination of both, or long necks could have enhanced capacities for thermoregulation, or for vigilance . . . or something else. The debate over which mechanism applies in this case has defied resolution to this day.

By Luca Galuzzi, via Wikimedia Commons

The giraffe’s neck actually contains the same number of cervical (neck) vertebrae found in nearly all other mammals. (For a discussion of the very few mammals to deviate the magic number of seven cervicals, see this Endless Forms post). The rate at which these vertebrae develop relative to other parts of the giraffe’s body, including its head and overall body mass, however, has not been adequately explored.

The gangly, awkward teenage period is painful enough even for those of us that are not destined to end up as awkwardly gangly as giraffes for the rest of our lives.  This state of comical disproportion (in our eyes, of course) is not an entirely lifelong condition for giraffes, however: they are born with extended but not extremely long necks, and their cervicals don’t begin to extend rapidly until later in life (Van Sittert et al. 2010). But just how rapidly, relative to the rest of the body? Is there a difference between male and female growth patterns? What can the answers to these questions tell us about the evolutionary roots of the giraffe’s long neck?

These questions motivated a group of researchers from the University of Wyoming and the Centre for Veterinary Wildlife Studies at the University of Pretoria to investigate growth patterns of the necks and heads of giraffes, and their findings appear in a recent issue of the Journal of Zoology (Mitchell et al. 2013). They analyzed numerous morphological measurements in order to determine whether neck growth scaled allometrically with head and body growth—ie, whether the neck maintained a consistent size ratio with the rest of the giraffe’s body, or whether a giraffe goes through an “awkward teen phase” with proportions that are skewed relative to those of mature adults. The researchers also compared growth patterns between the sexes and examined sexual dimorphism to determine whether developmental patterns differ between males and females.

It turns out that there is no significant difference between male and female giraffes in neck growth rate (measured by both neck mass and neck length), but that in both sexes the neck length did indeed increase at a faster rate than body mass. The rates of neck and body growth rates are similar to one another in youngsters, but the necks outpace body mass once puberty hits, elongating out of proportion to the growth of the rest of the body. In other words, there is now scientific evidence that giraffes go through a gangly teenage stage.

Juvenile giraffe, via the "Wind and Sea" blog

 

Interestingly, a giraffe’s head actually grows at a slower rate than its overall body mass. Males did end up with slightly heavier heads and necks, due to thicker ossicones and skull bones. The researchers attribute this to basic differences in sex steroid levels, and it should also be noted that males have a much longer total growth period during their lives than do females. These factors underlie the slight male:female body size differential typically found even in mammals that lack significant sexual selection dynamics. Giraffe head and neck growth rates and the ratio of neck mass to total body mass both remained similar between the sexes,  meaning that they differ a bit in size but not in basic growth pattern.

This low degree of sexual dimorphism in skull and neck proportions has important implications for our understanding of giraffe evolution and biology. If giraffe necks had developed due to pressures from one form of sexual selection, dubbed "female choice," we would expect to find one sex (in the mammal world it would typically be the males) with profoundly more extreme features. In this form of sexual selection,  extreme features would provide the basis for the other sex (typically the females, sorry guys) to choose a mate from their array of suitors based on display characteristics of some sort, some more exaggerated than others. In the other type of sexual selection--"male-male competition"--giraffes would use their massive necks to battle for access to mates, which themselves would exhibit little aesthetic choosiness and would just go with the males that were the most successful at winning fights.

There are popular video clips of giraffes engaging in dramatic bouts of “necking”, in which they appear to fight by swinging their long necks and heavy heads at each other like weapons.  On the surface, it seems like this may represent sexual selection via male competition. Mitchell et al., however, point out that studies have shown that large males rarely participate in these activities, and it’s more often immature males with female-like head and neck sizes that do all of the “fighting” in order to establish a dominance hierarchy. Most importantly, the winners of the fights don’t go on to acquire more mates (Pratt & Anderson 1982).

Giraffes possess an extremely exaggerated feature, to be sure, but the lack of difference between males and females tells us that it’s unlikely this was due to sexual selection. This makes it much more likely that some form of enhanced resource utilization or physiological efficiency was the mechanism for neck elongation over evolutionary time.

The issue is far from resolved, however. Alternative mechanisms require further research and testing before they can claim to have “won” as evolutionary explanations for the length of giraffe necks. Moreover, there have been noteworthy back-and-forth on the validity of these theories. Some studies have suggested that giraffes rarely browse at the full height that their necks can reach (Young & Isbell 1991). Also, in order to “out-reach” other browsers, they would only need to reach two meters into the canopy, not five meters, and being that high off the ground actually makes it harder to see predators such as lions that slink along the ground (Cameron & Du Toit 2005).  Conversely, although Cameron & Du Toit (2007) found that 57% of the forage consumed by giraffes grew below two meters, i.e. in the height range of other herbivores, they suggest that the giraffe’s long neck allows it exclusive access leaves in the center of shrubs and bushes by reaching down from above. All of this will require broader and deeper investigation, of course, and in the end it is likely that the selective pressure that have produced such long necks is attributable to more than one precise factor.

Although it is most famous for its long neck, the giraffe exhibits an array of highly specialized features, and Mitchell et al. review a few fun facts about giraffe morphology. For example, relative to total cranial mass, the sinus cavities of giraffes are massive relative to those of other artiodactyls, allowing the development of such an enormous head without the skull becoming debilitatingly heavy. In addition, the cervical vertebrae close to the head are lighter than those lower in the neck, further decreasing the mechanical stress of holding a head that high on a neck that long. The occipital condyles, which allow the head to rotate on the neck, have such a wide range of motion that the giraffe can actually partially lay the top of its head along its neck while feeding, a sort of reverse flamingo strategy. Fancy, no?

By Marcus Obal, via Wikimedia Commons

 

ResearchBlogging.org

References:

Cameron, E.Z. & du Toit, J.T. (2005). Social influences on vigilance behaviour in giraffes, Giraffa camelopardalis. Animal Behaviour. 69:1337–1344.
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

Pratt, D.M. & Anderson, V.H. (1982). Population, distribution, and behaviour of giraffe in the Arusha National Park, Tanzania. Journal of Natural History 16:481–489.

Van Sittert, S. J.; Skinner, J. D.; Mitchell, G. (2010). From fetus to adult – An allometric analysis of the giraffe vertebral column. Journal of Experimental Zoology Part B Molecular and Developmental Evolution 314B(6):469–79.

Young, T.P. & Isbell, L.A. (1991). Sex differences in giraffe feeding ecology: energetic and social constraints. Ethology 87:79–89.


3 Responses to “The Whole Two Yards: Giraffe Neck Growth Patterns”

  1. Laurenann Reply | Permalink

    Poor Lamarck. He'll never live it down!
    Also, when sexual selection leads to dimorphism, does that mean that every sexually dimorphic trait is genetically sex-linked? It seems like there might be a few traits out there that, when selected for in the male, also get passed down to his sons AND daughters. That would mean the females could get caught up in the run-away selected trait and end up just as weird/sexy as the males, wouldn't it? Obviously there are other indicators that looking like Ryan Gosling isn't what's driving the giraffe neck, but it's strange that a lack of dimorphism would be a clear-cut indicator that sexual selection wasn't involved.
    It would also be interesting to see if giraffes' necks are still lengthening in recent evolutionary history or if they've reached the extent gravity defiance!

    • Anne-Marie Hodge Reply | Permalink

      Lauren: that's a really great point about sexual selection. The typical explanation for "runaway selection" (sexual selection that results in super exaggerated traits) is that males inherit the display trait, and female offspring inherit the *preference* for the trait, creating more and more demand which creates more and more selective pressure, resulting in a snowball effect that is essentially a form of intraspecific co-evolution between male traits and female preferences (see also: Fisher's "sexy son" hypothesis). This is a hypothesis, of course, and may or may not apply to any given case.

      But your question mainly addresses why females don't also inherit these weird traits if they are getting half of their genes from these bizarre males. For one thing, females tend to be more functionally constrained due to the demands of reproduction, and ostentatious displays would be detrimental if they attracted predators to a nest, took resources away from nurturing offspring, etc. So this would discourage the development of display traits that aren't linked to the male chromosome in species with XY sex determination. Studies have shown that sexual selection for extreme ornamentation occurs more "easily" in species in which females are the heterogametic sex, (ZW rather than XY sex chromosomes; ie, birds). This could be why we see more "female choice" type sexual selection in birds and more "male-male competition" type in mammals, and also indicates that the issue you bring up is a very real constraint and is important to consider.

      If you're interested in reading further about XY vs ZW systems in the context of sexual selection, these are two good papers on the topic:
      http://www.ncbi.nlm.nih.gov/pubmed/16210543
      http://rspb.royalsocietypublishing.org/content/258/1351/83.short

  2. Erasmus Reply | Permalink

    Sorry if I'm late to the party...

    Of course finding "the answer" can be complicated by the possibility that more than one selective pressure can be going on at the same time. For example, females could be selecting for, choosing, longer necks, AND all longer necked offspring could have a selective advantage by being better at reaching up, in, or wherever. If both things are going on we would expect to see a trend towards longer necks with minimal/no sexual dimorphism, which is what you describe as the current state of affairs for giraffes.
    On a another topic, evidence from bovids and cervids suggests that females that have "horns" (in the broad sense) will use them, either inter- or intra-specifically. I've only seen vids of putative males fighting. Anyone ever see a female bash her head against anything?

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