A giraffe does not inhale through its neck directly into the blood. Air must first travel along a long windpipe before it reaches the lung surfaces where gases are exchanged. Some air remains in the conducting passages at the end of each breath, outside those exchange surfaces.
That creates a geometric problem. A longer tube holds more air unless it becomes narrower. But a narrower passage is harder to move air through quickly. As a giraffe grows, its breathing system must accommodate both its increasing body size and the changing proportions of its neck.
Graham Mitchell and John Dawson Skinner investigated that problem using measurements from 46 giraffes. Their 2011 paper found that lung mass grew more slowly than body mass and that windpipes became relatively more slender with increasing size. Calculations based on those measurements suggested that an ordinary breath occupied a growing fraction of the available lung volume. The distinction between the measured anatomy and the calculated breathing pattern is essential to understanding the result. Read the focal paper.
A sample spanning much of a giraffe’s growth
The researchers examined animals from legal culls in Zimbabwe between 2006 and 2009. Their sample included juveniles and adults of both sexes, ranging from 147 to 1,441 kilograms. It was a comparison of different animals at different sizes, rather than repeated measurements as particular giraffes grew.
Body mass was estimated using two approaches: adding the weights of dissected body parts and calculating mass from body length and chest girth. The average of those estimates was used in the analysis.
Lungs were removed and weighed. The windpipe was measured in place, before removing it could change its shape through elastic recoil. Length and diameters at three positions provided the information needed to estimate its internal volume as a cylinder.
Lung mass is a direct anatomical measurement; lung air volume is not the same quantity. To estimate volume, the researchers used a published relationship between lung tissue mass and lung volume across mammals. Applying that relationship to giraffes assumed that a given quantity of giraffe lung tissue enclosed air in approximately the same way as the mammalian comparison.
The resulting estimates are useful, but they cannot be described as 46 direct measurements of maximum lung inflation. The study’s later estimates of different breathing capacities depend on further assumptions built on that first conversion.
Bigger lungs, but less lung per kilogram
The giraffes’ lungs became heavier as the animals became larger. The interesting result was that lung mass increased less rapidly than body mass. An adult therefore had more lung tissue in absolute terms, yet less in relation to each kilogram of body.
The estimated lung volumes followed the same pattern. Across the sample, the authors calculated approximately 46 millilitres of lung volume per kilogram, compared with about 75 millilitres from their general mammalian prediction. The comparison is with an equation representing mammals at similar body masses, not with a second experimental group measured alongside the giraffes.
That difference also became more pronounced with size. Estimated lung volume per kilogram declined along the giraffe growth relationship, whereas it increased slightly in the general mammalian relationship used for comparison.
A relatively small lung does not by itself demonstrate inadequate oxygen supply. Gas exchange also depends on the surface and thickness of the exchange barrier, blood flow and the rate at which fresh air arrives. The study’s anatomy identifies a possible constraint to investigate, rather than measuring an animal’s exercise limit.
The windpipe’s two competing consequences
Windpipes ranged from roughly 0.8 to 2.5 metres long. They became wider in larger giraffes, but length increased faster than diameter. In proportional terms, the tube became more slender.
This helps restrain the amount of air occupying a long passage. Even so, the volume calculated for the windpipe was somewhat greater than the general mammalian estimate at comparable body size. The paper treated that volume as an approximation of respiratory dead space: air that does not exchange gases during a breath.
The approximation leaves things out. The branching airways inside the lungs also contain non-exchanging air, and physiological dead space can include regions that receive air but little effective blood flow. Windpipe geometry alone cannot measure all of it.
There is also a mechanical cost to keeping the windpipe narrow. For a given flow, a narrower passage requires greater air velocity and can increase resistance. The authors discussed this as a potential constraint during increased breathing demand. They did not directly measure airway resistance in these 46 animals.
Why the predicted larger breath needs careful interpretation
The researchers next estimated tidal volume, the amount of air moved in an ordinary breath. They started with the value predicted for a mammal of that mass and added the extra dead-space volume calculated for a giraffe.
This choice effectively asked what breath size would compensate for the additional airway space. It is not independent evidence that a living giraffe actually makes exactly that adjustment. Some conclusions about adequate resting ventilation follow partly from the way the calculation was constructed.
Combining that estimated breath size with a relatively small estimated lung gave a potentially important result: each ordinary breath used a larger share of total lung volume as body size increased. Other calculations suggested limited room for an additional deep inhalation. These are predictions about breathing reserve that would be strengthened by direct physiological measurements.
A separate study by Görel Nyman and colleagues illustrates the difference. They collected expired air from one young giraffe during clinically required anaesthesia and recorded its breathing directly. Its ordinary breath was 2.7 litres, and the physiological dead-space fraction after subtracting the measuring equipment was about 26%. Those measurements came from one particular anaesthetic condition, so they neither validate nor overturn a growth model for awake giraffes. They show the kind of functional evidence that anatomy alone cannot supply. Read the direct physiology study.
A growth problem that can now be tested
The strongest result of the 46-animal study is the combination of two anatomical trends: relatively less lung tissue in larger giraffes and a windpipe that grows longer faster than it grows wider. Both trends matter when predicting how much fresh air can reach the exchange surfaces and how much room remains to increase a breath.
The model offers specific questions for later measurements. Does the share of lung volume used in quiet breathing increase with maturity? How much can giraffes deepen their breaths during exercise, and when does a faster breathing rate become the main adjustment? The original calculations give a reason to ask those questions, while the measured anatomy shows why a giraffe’s respiratory system cannot be understood simply as a larger version of a smaller mammal’s lungs.
Sources
Graham Mitchell and John Dawson Skinner (2011). “Lung volumes in giraffes, Giraffa camelopardalis.” Comparative Biochemistry and Physiology Part A 158: 72–78. Full paper.
Görel Nyman, Bengt Röken, Eva-Maria Hedin and Göran Hedenstierna (2016). “Case Studies in Physiology: Ventilation and perfusion in a giraffe–does size matter?” Journal of Applied Physiology 121: 1374–1378. Full paper.
