Growing taller changes the journey blood must make inside a giraffe. The head becomes farther removed from the heart, while its bones, muscles and skin expand. It would be reasonable to expect the blood vessels themselves to undergo substantial reorganisation. But which parts of their arrangement are established early, and which change as new structures develop?
Haley D. O’Brien and colleagues investigated that question by reconstructing the arteries of three giraffe heads. Their 2016 PeerJ paper showed an unexpectedly consistent pattern across widely separated ages. The most informative finding was already present in the youngest specimen.
Making the hidden branches visible
The animals were three unrelated female Masai giraffes: a full-term stillborn calf, a six-month-old juvenile and a senescent adult. Their remains came from zoological collections; none was killed for this study.
The researchers introduced a contrast medium into the vessels and scanned the heads with computed tomography. That made the branching arteries distinguishable from surrounding tissues. Digital reconstruction then allowed the team to follow a vessel through the skull and soft tissues and compare its route with the corresponding vessel in another specimen.
This was particularly useful where a dissection might remove one structure to reveal another. In a digital model, the skull could be made transparent or viewed from different directions while the artery remained in place.
The specimens had different histories and preservation limits. Previous examination of the stillborn calf had removed or cut some tissues, and not every part of the neck was available in every animal. The comparison therefore concerns structures the team could actually reconstruct, rather than three equally complete circulatory systems.
The brain’s supply was organised early
Many mammals deliver blood towards the brain through an internal carotid artery. In giraffes and many other even-toed hoofed mammals, the adult arrangement includes a carotid rete: a network of small arteries near the brain, supplied through other branches. The extracranial portion of the internal carotid artery is closed rather than remaining the principal open route.
The developmental question is when that transition happens. In the full-term newborn giraffe, O’Brien’s team found an already developed rete and a closed extracranial internal carotid artery. The main arrangement supplying the brain was therefore present before the calf would have begun its postnatal growth.
The juvenile and adult largely shared that pattern. Becoming much taller did not require building a wholly new map of the principal cranial arteries during the stages represented here.
That is a statement about anatomical layout, not about identical blood pressures or flow at every age. Vessel dimensions and physiological regulation can change while the branching plan remains recognisable. The study mapped preserved anatomy; it did not measure circulation in awake growing animals or test how the rete behaves during drinking.
The distinction helps separate this developmental finding from the functional questions in our guide to giraffe blood pressure. A network’s presence tells researchers where blood can travel. Establishing how pressure is regulated requires a different investigation.
One unusual face prevented a misleading growth story
The juvenile had a comparatively reduced facial artery. An enlarged branch from another arterial route supplied areas of the face that the larger facial artery served in the newborn and adult.
Had the team compared only the juvenile with one older animal, this difference might have looked like a normal developmental transition. The newborn complicated that interpretation: its facial pattern more closely resembled the adult’s. The authors therefore considered the juvenile arrangement likely to reflect individual variation.
That reasoning is one of the study’s most useful lessons. A series of animals of different ages does not automatically document a single individual’s growth sequence. The third specimen provided a check on the temptation to arrange every difference into a simple timeline.
Growing ossicones did change their blood supply
Around the ossicones, the bony head appendages, the comparison did reveal a plausible developmental sequence. The newborn’s developing ossicone lay above a dense arterial mat. That particular underlying mat was no longer present in the juvenile, while the adult had vessels distributed around and within the ossicone.
These local changes accompanied the formation and growth of a structure that differs dramatically between a newborn and an adult. They show why describing the whole head as either “unchanged” or “remodelled” would miss the biology. Its principal arterial routes could remain stable while smaller networks changed with the tissues they supplied.
The scans also highlighted a methodological boundary: they could show a soft-tissue region without identifying its microscopic tissue type. Questions about how the ossicone ossifies required comparison with histological work, as discussed in our ossicone explainer.
Additional specimens, especially from prenatal stages and early life, could test when the brain-supply pattern becomes established and how often the unusual juvenile facial arrangement occurs. The three-head comparison has already narrowed the developmental question. Much of the arterial framework is in place at birth; understanding growth now means examining how that framework changes in scale, function and its local branches.
Sources
Haley D O’Brien, Paul M Gignac, Tobin L Hieronymus, Lawrence M Witmer (2016). A comparison of postnatal arterial patterns in a growth series of giraffe (Artiodactyla: Giraffa camelopardalis). PeerJ. DOI: 10.7717/peerj.1696. Read the full paper.
