Hemodynamics and Drinking in the Giraffe

Swallowing water may help pump blood through a giraffe's neck. A giraffe bending towards an empty pond provided an unexpected clue: its blood pressure fell and stayed low. In giraffes actually drinking, pressure initially fell too, but then climbed again despite their heads remaining down.

Christian Aalkjær and colleagues recorded pressures in neck arteries and veins as instrumented giraffes moved and drank voluntarily in managed enclosures. Drinking-pressure measurements came from seven animals, with blood-flow measurements in two. Separate experiments on anaesthetised giraffes and isolated blood vessels examined how blood volume, fluid around the brain and small arteries respond to the enormous height changes between standing and drinking.

During drinking, pressure in neck veins oscillated at a rhythm that generally differed from the heartbeat. The authors propose that water moving down the food pipe squeezed neighbouring veins, helping return blood to the heart and restore arterial pressure. That pumping mechanism remains an interpretation of the measurements, not a directly isolated effect. Meanwhile, increased resistance in vessels supplying the head helped keep measured carotid flow steady despite rising pressure there. Drinking thus appears to involve two opposing processes at once: blood collecting in neck veins as the head drops, and swallowing helping move it back.