It doesn’t come to a surprise that drinking is when a giraffe is at its most vulnerable state. That is why giraffe have an ability to gain most of their water content from the leaves they eat, though they do still need to drink now and then. As you can imagine their long necks can make this simple task quite challenging. The giraffe splays their front legs apart and lowers its head towards the water to drink. You would think that all that hypertension (high blood pressure) would send all the blood in their 250 kg neck rushing towards their head, as it lowers it 6 metres to the ground, causing a huge surge to the brain and cause the giraffe to collapse from a stroke. Not to mention the loss of pressure the giraffe could experience when it lifts its head causing it to faint. Amazingly the giraffe has evolved a series of impressing mechanisms that prevent such horrific events from happening.
When the giraffe lowers its head, special shunts in the arteries carrying blood to the head restrict blood flow to the brain. The blood diverts into a web of small blood vessels known as the rete mirabile or ‘marvellous net’ and these vessels expand to the increased local blood pressure. In the giraffe’s jugular veins, there are valves that prevent blood from flowing backwards when the head is lowered to the ground. The giraffe is highly adapted to deal with this change in blood pressure and the series of mechanism in the neck work together constantly to monitor the pressure in the blood vessels and adjust accordingly. If danger arises, a giraffe can lift their head mid-drink and still maintain a good bloody supply to the brain.
Lifting their head from the ground comes easier to a giraffe than lowering it. This is due to the stiff ligament in their neck. It takes a lot of force applied to it, to be able to stretch it in order to lower the head. The muscles running along the underside of the neck applies the force on to the ligament as they contract. This pulls the neck down against the stretch of the nuchal ligament. To lift the head, the muscles must relax and like an elastic band, the ligament pings back and the head lifts again back into the resting upright position.
Now we understand how a giraffe can lower its head safely to water to drink. Let’s draw our attention to how they drink.
Think of a giraffe’s mouth as a pump, pumping water all the way up the neck, yet it does not enter the stomach when the head is down. The water drains down into the stomach with the help of gravity and wave-like muscle contractions (peristalsis) once the head is lifted. This means the neck holds the water whilst it is pumped in through the mouth. The jaw acts like the pump, as it pulls back and water rushes into the mouth. The epiglottis valve at the back of the throat remains closed to prevent water entering the throat. Only when the giraffe clenches its lips and relaxes the epiglottis, it pumps its jaw, so water captured in the mouth is pumped into the oesophagus.
This cycle repeats, moving more water up the neck into the oesophagus and researches estimate that a giraffe pumps the water at a speed of 6.7 mph which is fast enough to prevent it from rushing back out. A giraffe’s neck can approximately hold 1.3 gallons of water and researches have also revealed that each pump sends 10 ounces of water up into the throat. So, a giraffe that drinks for 25 seconds, can do on average 17 jaw pumps, which means it could nearly fill its whole oesophagus of water in the neck. Better so, as the giraffe does not want to have its head down for long when there are predators about.
Giraffe are at their most vulnerable to the threat of predators when they are drinking.
If the above information hasn’t wowed you enough, you may be surprised to find out that a giraffe has the same number of vertebrae in its neck as we have – which is seven. A flamingo has 19 bones that support their long necks, so why is it that a giraffe only has 7? Each vertebra in an adult giraffe can be up to 10 inches long, therefore, they do not require more bones to support their heavy necks but have larger and stronger bones.
Giraffe are fascinating and the evolution of their neck still puzzles scientists. It was originally thought that they evolved long necks for an evolutionary advantage over other herbivores, so they could feed off leaves on the top of tall trees. Scientist now have a new theory that the evolution of the giraffes long, powerful neck is to help males compete for females during mating seasons. Nevertheless, we cannot deny that giraffe are an extremely impressive species that has us all gazing up at them with amazement.
See you next Friday for more facts!
Resources
Solounias, N. (1999). The remarkable anatomy of the giraffe’s neck. Journal of Zoology, 247(2), 257-268. doi:10.1111/j.1469-7998.1999.tb00989.x
https://sciprogress.com/giraffe-vs-human-neck-bones/
https://www.insidescience.org/news/how-do-giraffes-drink-water
