The main route supplying blood to a giraffe’s brain passes through an intricate arterial network at the base of the skull. That network, called the rostral epidural rete mirabile, presents an appealing possibility: perhaps its branching, winding vessels help manage blood flow as the animal raises and lowers its head.
Marna S. van der Walt and colleagues explored that possibility in a 2023 paper in Anatomia. They combined dissection and microscopic examination of giraffe tissue with an experiment using water and coiled plastic tubing. The study documented real anatomical features and proposed how they might contribute to circulation. Its experiment showed a physical effect in a simplified model; it did not measure how much protection the network provides in a living giraffe.
That distinction is central to understanding the paper. An anatomical structure can suggest a mechanism, while the mechanism still needs testing under the conditions in which the animal uses it.
Finding the network beneath the brain
The team obtained material from five giraffes that had been culled during reserve management in South Africa. The researchers were not involved in organizing those culls. Different specimens contributed to dissection, tissue microscopy and casts of the cavity holding the brain.
For four heads, the team introduced colored latex into the blood vessels: red for arteries and blue for veins. Once set, the latex helped them follow the vascular pathways through the neck and skull. They located the rete in a space called the cavernous venous sinus, immediately beneath the brain.
The dissection showed a conspicuous bend as the maxillary artery approached the network, followed by the much finer interconnected vessels of the rete itself. The paper’s photographs make the change in organization visible: a comparatively direct supply vessel leads toward a compact meshwork.
The latex did not fully enter that fine network. This limited what the authors could reconstruct and measure. In particular, they could not dissect out the complete meshwork to establish its total vessel length. The omission matters because length later became the variable in their physical experiment.
The study therefore supplied an anatomical description of where the rete sits and how vessels approach it, without a complete three-dimensional map of every route within it.
Muscle in the vessel walls
Microscopic sections from one head provided a closer view of arteries before, within and after the rete. The researchers used stains that distinguish tissues including muscle, collagen and elastic fibers, then compared the layers of the vessel walls.
The muscular middle layer was thinner in the small rete arteries than in the larger vessels sampled before and after the network. Nevertheless, the rete arteries contained substantial smooth muscle. That is relevant because vascular smooth muscle can alter a vessel’s diameter, influencing resistance to blood flow.
Seeing muscle is evidence of anatomical capacity, however, rather than a recording of what that muscle did during head movement. The study did not watch these vessels contract, measure their diameter in an awake giraffe, or establish the timing of any response.
The authors proposed that contraction might help direct a small volume of blood toward the brain when the head rises. It is a functional hypothesis built from the position and structure of the network. The tissue sections alone cannot determine the amount of blood moved or whether the proposed action occurs.
What the coiled tubing tested
To investigate the influence of a winding route, the researchers made a model from plastic tubing with a four-millimetre diameter. They wound lengths ranging from half a metre to six metres around a pipe, passed water through them, and measured the flow over one minute. Each length was tested repeatedly.
The broad flow result was clear. Water flow was about 648 millilitres per minute through the shortest tubing and 275 millilitres per minute through the longest. In this apparatus, increasing the length of the coiled route reduced the amount passing through each minute.
Those numbers describe the apparatus. They are not measurements of blood passing through a giraffe’s rete. The tube lengths also were not measured lengths from the giraffe network, which the incomplete latex filling had prevented the team from obtaining.
The model varied total tubing length along with the extent of the coiled route. It consequently does not isolate a special contribution from bends alone. Nor does a single continuous tube reproduce a network in which blood can divide among interconnected branches. The authors explicitly acknowledged the limitation of their serial arrangement.
Water in plastic tubing also lacks the living vessel wall’s ability to change diameter. The experiment can illustrate resistance in a winding pathway, but translating that result into the pressure reaching the brain requires a closer representation of the actual network and its operating conditions.
From an appealing mechanism to a measured one
Van der Walt and colleagues interpreted the rete as one component of the giraffe’s response to its large heart-to-head distance. They proposed roles in slowing flow toward the lowered head and maintaining supply when it rises. These ideas connect the observed anatomy to an important physiological problem.
The study did not directly record pressure or flow on either side of the rete in a living animal during those movements. It therefore cannot establish the size of the proposed buffering effect, or show that this structure alone explains the avoidance of fainting or injury. A model based on a measured branching geometry, together with physiological observations, would address that missing link more directly.
Our giraffe science library contains other approaches to circulation, from heart measurements to vascular anatomy. This paper’s contribution lies in bringing the rete’s physical structure into that discussion. The meshwork is real, its muscle is visible, and its winding routes raise testable questions. The precise share it contributes to controlling cerebral blood flow remains a question for evidence beyond a coiled tube.
Sources
Marna S. van der Walt, Willem Daffue, Jacqueline Goedhals, Sean van der Merwe and Francois Deacon (2023). “The Rostral Epidural Rete Mirabile: Functional Significance in Blood Flow Regulatory Mechanisms in Giraffe (Giraffa camelopardalis).” Anatomia 2, 138–155. Full paper.
