In the early morning near Etosha in Namibia, two southern giraffes (Giraffa giraffa) silently approach the water. With the male standing close behind her, the female takes one last look around and a sniff of the air before deciding it is safe enough to drink.
That extraordinary neck gives giraffes access to browse high above the reach of most other herbivores. But there’s a catch. Despite having the longest neck of any living animal, a giraffe’s neck is still too short to reach the ground past those extraordinarily long legs.
So, to drink, this southern giraffe must spread her front legs wide and lower her head towards the water.
It is an awkward position – and a vulnerable one in a landscape where predators are looking for their next meal. An animal superbly adapted to browse several metres above the ground temporarily gives up some of the height, vision and mobility that help keep it safe, simply to drink.
Behind her, the much darker male appears to be keeping watch. Male giraffes often become darker as they mature, although colour isn't simply a measure of age; very dark males may also be signalling competitive or social status.
Do giraffes get struck by lightning?
"Again and again it crumpled to the ground like Bambi on ice…" Watch heartwarming footage of newborn gira"foe calf tries to stand
Bending down creates another, less visible problem. An upright giraffe needs a blood pressure of more than 200 mmHg to maintain blood flow all the way up to its brain. To drink, it then lowers its head by around 3-5 metres. How does it make that enormous change in height without either starving its brain of blood or overwhelming the delicate blood vessels in its head?
Giraffe cardiovascular physiology is remarkable. Changes in vascular resistance, heart rate and venous pressure and valves in blood vessels all help maintain blood flow to the brain. Even swallowing may play a part: the passage of water down the oesophagus can compress the jugular veins, helping to restore blood pressure while the giraffe drinks.
The image of a giraffe drinking looks peaceful. But physiologically, evolutionarily and in terms of predation risk, there is an extraordinary amount going on.
The price of reaching the highest leaves is having to solve the problem of getting back down to the water.
