The neck of a drinking giraffe carries two streams towards its body: swallowed water inside the oesophagus, and blood returning from the head through the veins. Christian Aalkjær and colleagues suggest those journeys may be connected. The muscular wave moving water could press on neighbouring veins and help drive blood uphill towards the heart.
The clue emerged in their 2025 paper, Hemodynamics and Drinking in the Giraffe. Implanted recorders showed an initial fall in arterial pressure near the heart as giraffes lowered their heads, followed by recovery while they drank. To understand why that recovery was surprising, it helps to follow what a change in head height does to the blood already in the neck.
Why lowering the head can leave the heart less well filled
With its head upright, a giraffe must push blood uphill to the brain. Pressure falls along that rising column of blood, so the heart needs to generate a high pressure for enough to remain at head level. Our giraffe heart and blood-pressure guide explains the standing animal's challenge.
At the water, the geometry reverses. Gravity adds to arterial pressure towards the head, while blood returning through the jugular veins must move upwards. Veins can expand and hold more blood. If some of the circulating supply accumulates there, less returns to fill the heart before its next beat, reducing its ability to generate pressure.
That possibility had already been demonstrated under anaesthesia. In a 2009 study led by Emil Brøndum, five giraffes were supported upright while their heads were lowered. Their jugular veins expanded, an estimated 1.2 litres of blood accumulated, and arterial pressure fell. Raising the head returned blood to the central circulation. Those animals were not drinking, so the experiment left open what swallowing might add.
An awake giraffe therefore has two linked problems to manage during a drink: keep blood returning to the heart, and regulate the pressure and flow reaching the head. Aalkjær's team combined recordings during voluntary drinks with separate studies of blood vessels and fluid pressure around the brain to investigate both.
Watching a drink from inside the circulation
The researchers fitted giraffes with implanted recorders under anaesthesia. After overnight recovery, the animals could move, feed and drink voluntarily while cameras recorded their behaviour. Matching the video to the internal measurements revealed when a pressure change occurred: during head lowering, during the drink or during head lifting. Measurements near the neck base and near the skull also separated two places where gravity should have different effects.
The main drinking-pressure series came from seven giraffes. The work took place in enclosures in South Africa and Denmark, with different subsets of animals supplying different measurements. These recovered, instrumented giraffes allowed the team to observe real drinking; the sample still leaves open how much the response varies in wild animals of different sizes and ages.
In the seven-animal pressure series, arterial pressure near the base of the neck initially fell by an average of about 24 millimetres of mercury as the head descended. During drinking, it moved back towards the previous upright level in most animals. Meanwhile, arterial pressure near the skull rose in the four giraffes with those measurements. Pressure could rise at the lowered head while falling near the heart because gravity and changes in heart filling were acting on different parts of the system.
Heart rate also slowed during drinking and briefly accelerated after the head rose. Within roughly 10–20 seconds of lifting, arterial pressures returned to their upright steady state. The drink was a sequence of adjustments, not one fixed head-down condition.
The clue in a swallow
The jugular veins carry blood from the head towards the heart. During drinking, pressure in the jugular near the neck base rose, and six of the seven animals showed repeated pressure oscillations. Most recorded oscillations were faster than the heartbeat, suggesting that something besides each heartbeat was driving them.
At the same time, muscular waves could be seen travelling down the side of the neck. These were associated with peristalsis, the coordinated contractions that move swallowed material along the oesophagus. The oesophagus runs close to the jugular veins. A moving bulge of water and contracting muscle could press on a neighbouring vein; valves and surrounding tissues could help direct the displaced blood towards the heart.
That is the proposed swallowing-assisted pump. Returning more blood to the heart could improve its filling and help explain why arterial pressure recovered while drinking continued, even though heart rate had slowed.
An unplanned observation sharpened the question. One giraffe lowered its head towards an empty reservoir. Arterial pressure near the neck base dropped and remained low through the apparent attempt to drink. Actual drinking often showed recovery at the same head position. This single episode suggests that head position alone may not explain the whole response, but it was not a replicated experiment isolating swallowing.
Together, the pressure rhythms, visible neck movements and recovery during real drinks support the swallowing explanation. Its decisive prediction remains to be measured: a pulse of jugular blood moving towards the heart with each swallow. That would connect the observed pressure pattern directly to the proposed pump.
Blood speed stayed steady as pressure changed
Getting blood back to the heart solves only half the problem. The lowered head also receives blood under increased arterial pressure. If the resistance along that route stayed unchanged, a greater pressure difference would tend to drive more flow. Small vessels that narrow can oppose that increase, much as narrowing an opening restricts how much liquid passes through.
In two giraffes, the team recorded blood speed in a carotid artery, which carries blood towards the head. Speed stayed steady during drinking, then briefly fell by about 20% after the head lifted. This was speed rather than total volume per minute; the researchers treated changes in speed as a guide to changes in flow because the earlier experiment found that carotid diameter stayed unchanged with head position.
Combining pressure and speed let them calculate an index of resistance. It increased by the end of a drink and dropped rapidly after head lifting. The sequence suggests regulation suited to the changing problem: greater resistance while the head is down, then a quick easing as blood again has to climb. Repeated drinks revealed the pattern within those animals, although the flow evidence comes from just two giraffes.
The tissue experiments offer a mechanism for changing resistance. Small arteries dissected from the brain, tongue and neck muscle constricted when internal pressure increased. This is a myogenic response: muscle in the vessel wall tightens in response to stretch. It can regulate how much pressure and flow reach smaller vessels downstream.
The brain arteries responded most strongly at lower pressures than arteries from the tongue or neck muscle. Their surroundings offer a possible reason. Blood-vessel walls respond to the difference between pressure inside and outside them. In three anaesthetised giraffes, separate measurements showed that fluid pressure around the brain was higher with the head lowered. Greater pressure outside a brain vessel can partly counter the pressure inside, changing the load its wall experiences.
These experiments help explain how stable blood delivery could coexist with changing pressure. The arteries can actively restrict flow, while fluid around the brain may reduce the pressure difference across vessel walls. Bringing those findings together with the awake recordings supports the authors’ account of coordinated regulation, though pressure inside the brain’s smallest vessels was not directly recorded during a drink.
The next observations that would bring the picture together
Several observations could help explain how these mechanisms work together.
Does a swallow send blood towards the heart? Aligning voluntary swallowing with suitable jugular-flow measurements would test the central prediction: a pulse of returning blood linked to the swallow rather than the heartbeat.
What differs between drinking and simply lowering the head? Repeated observations of naturally occurring head lowering and drinking at comparable heights could establish whether pressure recovery consistently accompanies swallowing across animals. This would extend the clue provided by the single empty-reservoir episode.
How does the response change with body size? Comparing appropriately obtained recordings from giraffes of different heights could show how the timing and magnitude of adjustments vary with the distance between heart and head. The existing two-animal blood-speed sample cannot answer that question.
The familiar sight of a giraffe spreading its legs to drink is therefore also a window into rapid coordination inside the body. Brøndum’s earlier work showed how the neck veins could temporarily hold blood away from the heart. Aalkjær and colleagues added the active drink, revealing recovery while the head was still down. That is the new puzzle a swallowing-assisted pump may solve.
Sources
- Aalkjær, C., Damkjær, M., Baandrup, U. T., Bertelsen, M. F., Brøgger, T., Brøndum, E., Danielsen, C. C., Funder, J. A., Grøndahl, C., Hasenkam, J. M., Henriksen, P. G., Secher, N. H., Skovgaard, N., Smerup, M. H., Telinius, N., Østergaard, K. H., Bie, P. & Wang, T. (2025). Hemodynamics and Drinking in the Giraffe. Acta Physiologica, 241(5), e70046. DOI: 10.1111/apha.70046. Published open-access article in Aalborg University's repository.
- Brøndum, E., Hasenkam, J. M., Secher, N. H., Bertelsen, M. F., Grøndahl, C., Petersen, K. K., Buhl, R., Aalkjaer, C., Baandrup, U., Nygaard, H., Smerup, M., Stegmann, F., Sloth, E., Ostergaard, K. H., Nissen, P., Runge, M., Pitsillides, K. & Wang, T. (2009). Jugular venous pooling during lowering of the head affects blood pressure of the anesthetized giraffe. American Journal of Physiology—Regulatory, Integrative and Comparative Physiology, 297(4), R1058–R1065. DOI: 10.1152/ajpregu.90804.2008. PubMed abstract and bibliographic record.
