In 2007, I was presented with the opportunity to travel to Namibia for a few weeks to work with local children. While there I was also lucky enough to take the time to go trekking in the Namib Desert and go on my first safari. This meant that for the last few days on the trip I camped in Etosha National Park, watched over by the Milky Way with the sounds of nearby animals to keep me company. This was the first time I ever saw a giraffe in its natural habitat and definitely the first time I ever saw a giraffe drink. Have you ever seen a giraffe drink? It’s quite the palaver, as you can probably imagine. To give you an idea, imagine you’re a giraffe and although you have that elegant long neck, you are also blessed with tall, thin legs. You see the problem? Well, giraffe actually get around this by doing their version of the splits so that their heads can actually get to the ground and they can drink easily but this poses another question – why does all the blood not go to the head like it would us if we were to do the same as this? We have all done a handstand and felt that uncomfortable sensation as the blood rushes to the head. Conversely we have also all felt the sensation of light-headedness when we stand up too quickly, so why do giraffe not regularly faint as they raise their heads? So many questions and so little time!
The best place to start seems like the beginning. You see, giraffes necks don’t start as long as they become. This elongation largely takes place after birth, as giraffe mothers would have a difficult time giving birth to young with the same neck proportions as adults. Basically, if a giraffe’s neck started proportionately the same length as the mothers then they would struggle to drink the mother’s milk. As they grow, the neck grows proportionately longer than the rest of the body and the other problems develop, or would if it weren’t for some very clever solutions! In order to push blood up to its head in the first place the giraffe must be able to generate blood pressures twice that required by a human. To do this it has a huge heart that can weigh more than 11kg and measure 60cm long! (For those of you who prefer imperial, that’s 25lb and 2ft!) The thickest wall of the heart can even measure 7.5cm (3inches) thick. To put this into proportion, a human heart weighs an average of 0.25-0.35kg compared the giraffes 11kg!!This all means that the blood pressure is enough to get blood up to the brain but can pose a problem to the legs. High blood pressure in those legs could cause some serious damage! The giraffe tackles this problem by having thick tight skin over the legs that act a little like a pressure bandage preventing fluid build-up.
Unfortunately the same technique can’t be used for when the giraffe lowers it’s head! So how does it cope? Well, when the giraffe lowers its head it utilises the valves in its jugular vein which prevents the backflow of blood into the head. In addition to this the giraffe has something called a ‘rete mirabile’ (latin for ‘wonderful net’), a complex of arteries and veins lying very close to each other, which works to equalize the pressure. A lot of mammals have this structure but once again, giraffe’s are special. The vessels of the giraffe rete have elastic walls which can accommodate excess blood when the head is lowered so that the brain is not flooded. On top of this they also have a connection between the carotid artery and vertebral artery which drains off a portion of the blood before it ever even reaches this net. The rete mirabile also plays a part in stopping the giraffe fainting when it puts its head back up. Imagine the damage the giraffe could do to itself if it hit its head on the floor from that height (something wildlife vets have to bear in mind when they dart them)! When the giraffe raises its head again the rete mirabile vessels are elastic enough that they constrict and retain sufficient blood so that the brain is never deprived.
So there you go, a giraffe’s neck may have many benefits but it definitely poses some unique challenges that the giraffe has evolved to cope this because, let’s be honest, we can’t have a load of giraffe stumbling around the place!
Sidenote: The plural of ‘giraffe’ = giraffe or giraffes? Discuss!