Giraffe- A Physics Approach

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

Spring 2010 - Flat Stanley on the Sea

-Written for Anya Z for her 2nd grade Flat Stanley class project.

Today was another early morning for the sake of science! Flat Stanley and I donned our our suits and grabbed our snorkels, fins and towels. And we jumped in a boat! My invertebrate zoology class had a field trip where we were looking for invertebrate specimens. When we got to the boat place, they suggested we borrow their wetsuits, because the water was cold!

Flat Stanley on the boat with mangroves behind.

So we suited up (wetsuited up, that is) and hit the waves. We were in Biscayne Bay, so on one side was Key Biscayne and other Keys, and other was Miami. It was really fun. Soon we came to a sand bar, an area where the water was shallower, and we began to explore that. There were some corals on the surface along with some seagrass. Did you know that coral isn’t a rock or a plant, but actually a bunch of tiny creatures that are related to jelly fish?

Coral we found.

We found an eggsac that looked really cool, and once we shoveled up some sand, we found funny creatures called sipunculans which look like worms. Somebody even saw a sting ray! I really liked finding a sea urchin which covered itself with seashells for protection. Sea urchins move using their long spines to propel them forward, and a sticky substance at the end helped them hold the shells on. I thought it was a rock at first, that’s how many shells it had on it. I hoped we’d find a decorator crab. That’s a crab that uses sea anemones to help it eat, by holding them in its pinchers. Pretty smart for animals without a backbone, huh?

Our specimens, including sea urchins.

The water was super cold. I raced back to the boat to take some pictures of everyone coming back with our specimens. Then we looked at what we caught and threw back some things which would do better in the ocean. Our teacher fell into the water while trying to clean out a cooler!

Shivering, we carried back our specimens.

Then we got going to our second site of the day, a place near the mangroves. It was really cool. Mangroves are a type of tree that can survive in salt water for various reason. They have really big roots which prop them up out of the water. They are called “prop roots.” There are lots of snails growing on mangrove roots.

Mangroves.

They also found a few more urchins and stuff, but most of us were so cold we couldn’t look very well. After all, to snorkel, you should have your face underwater, right? Have you ever gotten brain freeze from eating something cold? Well, today I got brain freeze just from sticking my head underwater! How cold! We got out of the water pretty quickly, because it was so cold, but not before taking a picture of ourselves with the Miami skyline in the background. Very pretty.

Representing "the U" in muddy mangrove waters.

As we started back for the marina (it’s like a parking lot for boats), someone shouted “hey look, a dolphin!” and sure enough, there was a dolphin to our left, jumping through the waves. A perfect end to a boat ride. Of course, we all went out for lunch afterwards, being quite hungry. We got pizza. I have to say, Miami is awesome, mainly for the ocean, but nothing beats Chicago pizza. Make sure to give Stanley a slice when he gets home, he’ll be missing it!

Hello pretty dolphin!

Spring 2010 - BioBlitz 2010

It was far too early for college students when we filed neatly onto the boat. When your normal bedtime is in the first few hours after midnight, a 5 o’clock meeting time and 6 o’clock underway time is a stretch. We were groggy and felt gross and some of us couldn’t form complete sentences, but we were ready for an awesome day snorkeling around Soldier Key for BioBlitz.

We filled the cabin of the R/V Coral Reef II, Chicago’s Shedd Aquarium’s research vessel. We munched on muffins and bagels and brought out a plethora of invertebrate identification books and began to study while we chugged towards our destination. The ten of us students were joined by our TA, our professor, and the head of the Marine Science Department at the University of Miami. Between those three, we knew we’d have a good chance at finding and identifying a good number of things (guesses ranged mostly from 90-220). Upon arriving at Soldier Key, our exhaustion was mostly gone, replaced by a sense of excitement for what we were off to do- explore a beautiful and pristine part of nature for the benefit of science.

“Thalassia!” came the first cry from the water. Turtle grass was officially our first observed species, followed quickly by various species of snails and crabs, until we reached the wreck.

Soldier Key was used as a rum running station during the prohibition. The wreck we saw was the remains of a barge that had sunk there during that time period. Schools of grunts and mullet swam past, unconcerned with the new school of large strange “fish” that had descended upon them. Closer to the dock, large rocks harbored any number of marine gastropods. And when you noticed one or two moving a bit more quickly than any other snail you’d ever seen, a quick peek identified the creature as a tiny hermit crab inhabiting a snail shell. We nearly spent the majority of our time hovering near the remains of the dock, where little schools of sergeant majors darted between rocks and snails vied for space in the intertidal zone.

A bit further out, Queen Conch, sea cucumbers, brittle stars, and sea urchins rested on the substrate or hid beneath rocks and in crooks and crevices. One guy caught an octopus as it sped away from its disturbed home. The person picking up the rock made another brilliant discovery.

“You guys are going to want to see this! Get over here with that camera!” yelled Dr. Dan Diresta, standing with our professor, Dr. Peter Glynn, and the two boys who had found the octopus and… surprise! Her eggs!

Absolutely magnificent, they were translucent, allowing us to see the developing embryos within. I snapped a few (closer to eight) pictures of the octopus and the eggs before we let her go. We continued around the island, fighting a ridiculously strong current by using rocks to propel ourselves forward. But it was worth it to find the beautiful corals on the other side. Having circumnavigated the island, we swam for the boat- and lunch.

Our species count grew as we scarfed down sandwiches and leafed through the myriad of books we’d brought, double and triple checking each species against two or three books at some points. Of course, it wasn’t completely necessary to have the books for some of the species. Dr. Diresta and Dr. Glynn are encyclopedias in their own right. While it was clear that they were making us make the effort to identify what we had seen or collected, they were on top of most of the identifications, being very familiar with some of the groups we were looking at.

Then we were back in the water. The currents were faster this time around, and going the opposite direction from the boat, but I’m glad we got in. We found a few mantis shrimp and some new sponges we hadn’t seen on our first run. An awesome find was a bunch of lobster hanging out under a coral head, and a Florida native in our group saw and identified a Loggerhead turtle fighting the current, just like we were. Our TA got a few great shots of some fish beneath the wreck of the barge. Then we hopped onto the Coral Reef II’s zodiac and headed back to identify our last few species. All species identified, we lifted anchor and headed home.

On our way back, we came to our final count for Soldier Key- 206 different species! We were about as proud as we were exhausted as we said goodbye to the crew of the R/V Coral Reef II and transferred all our stuff onto the RSMAS dock. For the University of Miami Invertebrate Zoology class, BioBlitz 2010 was a definite success.

-Cara Ruffo, Junior in Marine Science and Biology at the University of Miami from Chicago, IL