The Evolution of the Neck of the Giraffe
The giraffe holds the record as the
tallest land mammal. Native to African
savannah where there are stands of trees, giraffes are best known for their
distinctive patterning, which gave them their scientific name of Giraffa camelopardis and their height.
Male giraffes (bulls) can reach nineteen feet in height and females
(cows) can reach seventeen feet. Calves
are about six feet tall at birth.
The long neck of the giraffe, which
helps it achieve the status of tallest land animal, is a curiosity in the world
of animal physiology, and has led to many questions about evolution. The neck of the giraffe was the basis for
Lamarck’s theory of the evolution of acquired traits, which has since been
disproved. The evolution of the neck of
the giraffe is still contended today.
The giraffe, like almost all
mammals, has only seven neck vertebrae.
Instead of adding vertebrae to increase the size of the neck, giraffes
actually only elongated what was already there.
Some researchers actually believe that the giraffe did evolve an eighth
vertebra somewhere between the second and sixth vertebrae, and that what would
formerly have been the seventh vertebrae is utilized as a rib vertebra, rather
than a neck vertebra. This accounts for
the position of the shoulders farther forward on the giraffe than they are for
other ruminant mammals.
There are some advantages for having
the giraffe’s long neck. The situation
of their eyes at such a height gives them a good view of the surrounding savannah,
making them more aware of the approach of predators such as lions. The eyes of the giraffe are very large and
their vision appears to be very good.
Natural selection would favor longer necks because early giraffes would
have an advantage over their neighbors in spotting and fleeing from predators.
However, giraffes also evolved
defenses against predators, such as their powerful kick, which is capable of
crushing the skull of a large predatory cat.
Their distinctive coat makes them less visible to predators when they
are in wooded areas where their height does not give them an advantage. Therefore, the advantage of height for
spotting prey may not have influenced the evolution of the neck. Yet, once the long neck was established,
natural selection may have pushed the population towards longer necks than
would otherwise be expressed.
Another hypothesis for the evolution
of the giraffe’s neck is that the long neck gave them an advantage in reaching
a food source and thereby securing a meal during the dry season. The height of the giraffe due to its long
legs and neck gives it access to an average of two meters worth of foliage
beyond the reach of the other large browsers of the African savannah except the
elephant. Their height gives them an
advantage in reaching the leaves of their favorite tree, the Giraffe Thorn, or Acacia erioloba, which can grow up to
seventeen meters high. This isn’t the
only adaptation the giraffe has that makes it better able to eat the leaves of
the Acacia tree, which may suggest that the giraffe evolved to better equip
themselves to their diet. The long,
prehensile tongue of the giraffe allows it to reach past the thorns, and its
antiseptic saliva helps to quickly heal cuts made by the thorns.
But studies actually show that the
giraffe does not utilize its superior height during dry periods when
competition is highest, instead grazing on bushes at shoulder length and below
during these times. Females often graze
at shoulder height or below during both the wet season and the dry season. Usually it is only the males that browse on
foliage between five and six meters above the ground. This suggests that the
neck did not evolve because early giraffes with longer necks had a better
fitness because they could eat where other ruminants couldn’t during the dry
season when there was little food, but rather that the already long necks of
modern giraffes merely help them to reach higher leaves.
Recent studies have shown that
necking, a behavioral aspect of courtship where two bulls fight for dominance
by using their heads to club the other male, could factor into the extreme length
of the giraffe’s neck. The bull with a
longer and therefore heavier club has the advantage in a fight of this sort,
and the winner gets the female. This
usually ensures that the longer-necked dominant male passes on his genes to the
next generation. Because longer necks
have an advantage in this scenario, the genetic makeup of the population would
shift to favoring a gene that expressed greater elongation of the neck
vertebrae in offspring.
The long front legs of both the
giraffe and its nearest relative, the okapi, require the giraffe to splay the
front legs in order to bend down to drink water from the ground. This poses a disadvantage to the giraffe when
a predator is in range. The long neck
allows the giraffe to reach the ground within the range of merely splaying or slightly
bending their legs.
But this position puts the giraffe
at a disadvantage with blood pressure.
The heart is situated above the head and pumps blood too quickly to the
brain. Blood pressure would be too high
if the giraffe didn’t evolve a way in which to slow the flow of blood to the
brain.
The jugular vein of the giraffe
consists of a series of one-way valves which prevent the blood flowing from the
brain to the heart from flowing back to the brain when the head is lowered
below the level of the heart. Pressure
to the brain from blood flowing from the heart to the brain also threatens to
raise the blood pressure in the brain, but a sponge-like net of blood vessels
in the arteries leading to the brain, called the rete mirabile, buffers the pressure before it reaches the
brain. The way the giraffe drinks helps reduce the effect of the high blood pressure by lowering the
heart to reduce the pressure difference.
The length of the giraffe’s neck
raises the brain five feet above the heart.
Blood pressure at the heart is high as a result of needing to keep the
pressure in the brain constant. The
heart weighs around 24 lbs and is nearly 2 feet long to be able to pump blood
up that distance. At the heart, a resting giraffe has a blood pressure of
approximately 200 to 300 mm Hg. This
helps keep the pressure at the brain at 100 mm Hg. Expansion and contraction of arterioles
around the capillaries outside the head helps keep the circulation in the brain
constant when the head changes position with respect to the heart.
The giraffe also has to contend with
more dead air in its windpipe than other large mammals. Because of the length of its neck, it has to
breathe out five pints of air before it can take in more air. When the giraffe inhales, it must fill its
lungs and its windpipe. This reduces air
flow and forces the giraffe to breathe faster in order to put enough oxygen
into its windpipe. This reduces the
giraffe’s endurance.
The length of the neck is also
determined by how much weight the giraffe can hold up without overbalancing
forward, as this would greatly reduce fitness.
A striking characteristic of the giraffe is its relatively less stable
center of gravity compared to other quadrupeds.
The costs of their long necks increase with height, as taller giraffes
have to eat more to keep from tipping forward from the weight of their necks.
It is not certain why the giraffe
evolved its long neck. But by studying
the limitations imposed upon the giraffe and the benefits afforded the giraffe
by its neck can help scientists make more sense of a problem that has mystified
scientists since Darwin’s first proposal of evolution.
Bibliography
Apfelbach, R.
“Long-necked or steppe giraffes”.
1990. Grzimeck’s Encyclopedia
or Mammals Vol. 5. McGraw-Hill Publishing Company: New York, NY.Vrba, E.S., G.B. Scheller. Antelopes, Deer, and Relatives. 2000. Yale University Press: New Haven, CT.
Owen-Smith, R.N. Megaherbivores. 1988. Cambridge University Press: Cambridge, MA.
Prothero, D.R., R.M. Schoch. Horns, Tusks, and Flippers: the evolution of hoofed mammals. 2002. John Hopkins University Press: Baltimore, MD.
Solounias, N. “The remarkable anatomy of the giraffe’s neck.” J. Zool., Lond. (1999) 247, 257-268.
Simmons, R.E., and Scheepers, L. “Winning by a Neck: Sexual Selection in the Evolution of Giraffe.” The American Naturalist, Vol. 148, No. 5 (Nov., 1996), pp. 771-786
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