It’s World Giraffe Day!

HungryKitty's baby giraffe

21st June is the longest day of the year, a fitting occasion to celebrate and raise awareness of the tallest living animal.

What better way to celebrate than with some neck anatomy!  The neck is just marvellous. 250kg of bone, muscle and gristly bits, with a head perched on top. You might think that it must take a huge amount of effort for giraffes to keep their heads up. Actually it’s the opposite..

Mammals have a ligament that runs behind their head and runs along the upper back – the nuchal ligament. In large grazing mammals this ligament supports the weight of the head. In giraffe the same ligament has to support the head AND the massive bulk of the neck. Giraffe have heavily invested in this useful structure.

Nuchal ligament
Photo by me, drawing taken from ‘Die Skelete der Wiederkäuer’, Alton 1823. I love this skeleton picture, although the posture is not really correct! Live giraffes in 1823 weren’t so easy to come by, I think they’ve used a camel for some inspiration. CC-BY-NC-SA

You can see how the spines of the vertebrae suddenly flare upwards as they transition from the lower cervical to upper thoracic region. These tall vertebral spines serve as the anchor point for this stout nuchal ligament. They give giraffe that characteristic humped back.

Think of the nuchal ligament as a giant and stiff elastic band, connecting the base of the skull to the upper back. This really nicely suspends the head and neck in the air, with minimal effort.

This means giraffes are quite happy with their heads in the air. But – you can take a giraffe to water, but you can’t make it drink without significant muscle work. They need to put in effort to get their heads down.

This ligament is stiff, but will stretch if you apply enough force to it. And it can stretch all the way to allow the head to reach the ground. The force comes from the muscles running along the underside of the neck. To lower the head, these muscles contract, and pull the neck down against the stretch of the nuchal ligament. Once the muscles relax, the ligament pings back and lifts the neck into its resting upright position.

Giraffe drinking from a ditch. The front legs are splayed, and one of the back legs is just lifting off the ground.
Photo by me (CC-BY-NC-SA). Giraffe drinking from a ditch. The front legs are splayed, and one of the back legs is just lifting off the ground.

The really amazing thing is that somehow, giraffes don’t pass out from fluctuations in blood pressure. When I was a kid I got stuck upside down on a ride in Dreamland (a now derelict 1920’s amusement park in Margate) and remember feeling blood rush uncomfortably to my head. That was probably only for 30 seconds, and I weighed about 45kg. Imagine the blood inside 250kg of neck pooling downwards as the head is lowered 6 metres to the ground. Giraffes have clever strategies to avert exploding head disaster syndrome, but they still avoid lowering their heads for more than a minute or so. Maybe because of these issues of blood pressure, maybe because their neck muscles will fatigue, or maybe so that they’re not attacked and eaten.

I spend a lot of time studying giraffe legs. But I still can’t help be distracted by the neck. It’s interesting in so many ways, from the bony anatomy, the mechanisms that regulate blood pressure, the evolutionary pressure to produce the bizarre size, its use during sparring…

Anyway – happy World Giraffe Day!

Chris

@thebadlizard

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