Name:_____________________________ Date:_________________
What the Heck is up with that Neck?
Part 1: Competing Browsers
Introduction
Giraffes are ruminants, grazing animals that live exclusively on plants ­ grasses, leaves, bark, etc. They work just like
cows. Somewhat unusual cows. As you know, giraffes have tall necks, some reaching up to 3 meters (9 feet), often longer
than the rest of their body. The question many people including biologists have been asking is, why? Why are giraffe necks
so oddly long? More technically, what is a long neck an adaptation to? Does having a long neck improve a giraffes’ ability
to survive and make babies? What do you think?
Are giraffes’ long necks an adaptation? If so, to what?
One prevalent theory is the competing browsers theory. In the Evolution of Species, Charles Darwin hypothesized that
giraffes evolved to have long necks in order to feed on leaves that were high in the trees and out of reach of other animals.
The giraffe, by its lofty stature, much elongated neck, fore­legs, head and tongue, has its whole
frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food
beyond the reach of the other Ungulata or hoofed animals inhabiting the same country; and this must
be a great advantage to it during dearths.... So under nature with the nascent giraffe the individuals
which were the highest browsers, and were able during dearth to reach even an inch or two above
the others, will often have been preserved; for they will have roamed over the whole country in
search of food.... Those individuals which had some one part or several parts of their bodies rather
more elongated than usual, would generally have survived. These will have intercrossed and left
offspring, either inheriting the same bodily peculiarities, or with a tendency to vary again in the same
manner; whilst the individuals, less favoured in the same respects will have been the most liable to
perish.... By this process long­continued, which exactly corresponds with what I have called
unconscious selection by man, combined no doubt in a most important manner with the inherited
effects of the increased use of parts, it seems to me almost certain that an ordinary hoofed quadruped
might be converted into a giraffe.
­Darwin 1872, pp. 177ff.
Imagine you received a $300,000 grant to assemble a small team, go to Africa, hang out in the Savannah for 2 years, and
try to better understand whether giraffes necks are important adaptations for feeding.
1. What would you measure? Explain.
2. What you would expect to see if this idea is true?
Data Analysis:
Instead of going to Africa, a colleague of yours provides you with feeding height data from his recent trip. The researchers
followed the herd during the dry season. Each time they noticed an adult giraffe feeding for more than 10 bites, they
recorded the height of feeding. The data they gave to you include the heights of 100 feeding episodes (i.e., feeding at the
same height for more than 10 bites) for female giraffes.1 They also tell you that, on average, female giraffes are 4.2
meters tall and have a shoulder height of 2.8 meters.
Follow this link to open the data in SeeIt2 and open the “Female Feeding Height” data.
3. Does the data support the necks for browsing hypothesis?
Things to think about:
●
What is the range? What does this tell us?
●
What is the median? For these data, what does the median tell you?
●
Are the data skewed or symmetric?
●
How often did the giraffes eat from branches taller than the average female giraffe (4.2m)? How is it possible that
female giraffes fed from branches well above 4.2 meters?
●
How often do giraffes eat from a height lower than their average shoulder height (2.8m)?
●
Do these data support the hypothesis that giraffes have evolved to have long necks in order to feed on leaves that
were high in the trees and out of reach of other animals? What additional data would you like to see?
1
DATA SOURCE
2
Link to Giraffe Data in SeeIT:
http://centerforbiophotonics.github.com/SeeIt3/distributionsv3.html?!key=0AuGf3AP4DbKAdDNCMFhJTnZpSWtMR1dfZU0zSUtW
NXc
Part 2: Necks for Sex
Introduction
In addition to the data you examined in part 1, researchers have found other data refuting the hypothesis that giraffes have
evolved to have long necks in order to compete with other animals for food. These findings include:
●
●
●
In the Serengeti, giraffes spend most of the dry­season, when food is scarce, feeding from low bushes.
About 50% feeding occurs at less than half the height of giraffes and within reach of other competitors
Giraffes feed at faster rates when feeding below shoulder level
Imagine that you visit Africa to investigate this hypothesis further and while in the Savanna, you observe the following....
Giraffe Necking Video
Its called Necking. When females give the signal that they are ready to mate, Male giraffes stand side­to­side to battle for
dominance over the herd and exclusive mating rights.
Do you think necking may have played a role in the evolution of the giraffe’s long neck? What would you
measure to examine this theory?
This hypothesis, sometimes referred to as the “Necks for Sex” theory is fairly recent. In 1996, it was described by two
researchers, Robert Simmons and Lue Scheepers:
The novel hypothesis developed here rests on the premise that the extraordinary length of the giraffe’s
neck arises from its use as a weapon during intrasexual combat...During combat two males stand side
by side and exchange blows by swinging their necks first away from then at their opponents. The top or
back of the well­armored skull (Spinage 1968) is used as a club to strike the neck, chest, ribs, or legs of
the opponent with a force capable of knocking a competitor off balance or unconscious (Innis 1958;
Coe 1967; Spinage 1968). Since the energy delivered by a club (the head) increases in proportion to
the mass of the head and the length of the neck, we expect to selection to have favored longer and more
massive necks.
­ Robert E. Simmons & Lue Scheepers, “Winning by a Neck: Sexual Selection in the
Evolution of Giraffe” The American Naturalist, November 1996
Data Analysis:
You talk to your colleague and he is able to provide you with more data from his recent trip to Africa. This time, he gives
you data on the neck mass of 32 male and 32 female giraffes.3
If the data support the necks for sex hypothesis, what would you expect to see?
Follow this link to open the data in SeeIt and open the “Giraffe Neck Mass” data.
3
Data simulated from Simmons, R. E., & Altwegg, R. (2010). Necks­for­sex or competing browsers? A critique of ideas on the evolution
of giraffe. Journal of Zoology, 282(1), 6­12.
Do the data support the necks for sex hypothesis?
Things to think about:
1.
2.
3.
4.
How are the two distributions similar? How are they different?
What is the mean for males? For females?
Do these data suggest that there is a difference in the neck mass of male and female giraffes?
Do these data support the hypothesis that sexual selection contributed to giraffes’ long necks? What additional
data would you like to see?