Lucy Hawkes / Lecturer in Physiological Ecology / University of Exeter UK
We first heard the extraordinary tale of Dr Lucy Hawkes‘ research at World Extreme Medicine Expo in London this year. A comparative physiologist, Lucy has spent the last few years chasing the Bar-Headed Goose, one of the world’s highest flying birds, across the Tibetan Plateau. High technology meets old mountain tales meets the Christmas goose in our final article before the holidays.
The legend
In 1960, some seven years after the first successful summit of the 8848m Mt Everest, Sir Edmund Hillary led an expedition back to the Himalayas to investigate, among other things, whether Yeti really existed. He took with him an accomplished team, including mountaineers, physiologists and two animal biologists. One of the biologists, Dr Lawrence Swan, published a detailed account of Himalayan flora and fauna. Dr Swan documented Himalayan frogs, leeches, lice and fish but nothing astounded him more than a sighting of a most unseemly high altitude mountaineer, a goose!
“On one such cold and still night in early April, I stood beside the Barun Glacier. At 16,000 feet [4,880 metres], above nearly half of the atmosphere, the stars were brighter; and old familiar constellations acquired new, small bits of light unseen at lower elevations…and then a sound came, a quiet hum muffled by distance… Then as if from the stars above me, I heard the honking of bar-headed geese”
Dr Lawrence Swan, Tales of the Himalaya (1961)
Lawrence Swan later spoke with George Lowe, one of Edmund Hillary’s original team on Everest in 1953. George Lowe was, incidentally, the recipient of the famous quote: “Well George, we knocked the bastard off”, Hillary’s first words upon greeting his team on his descent from the summit. George Lowe allegedly reported to Lawrence Swan that he had seen bar-headed geese flying above him from the high slopes of Everest and the legend of the extreme high altitude flight of the bar-headed goose was born. The Guinness Book of Records listed the goose as the world’s highest flying animal and it became the stuff of biology textbooks.
My own research, as a post-doctoral researcher for Dr Charles Bishop (Bangor University) and Prof Pat Butler (University of Birmingham), funded by the BBSRC, aimed to find out how this migration might actually take place using latest state-of-the-art animal tracking devices, which we attached to the geese using tiny ‘backpack’ straps and which would collect the bird’s location and altitude on an hourly basis.
The Reality of Bar-Headed Geese
Interestingly, out of almost 150,000 GPS locations from 91 birds, we never recorded a goose flying higher than 7,290 metres, and 98% of the locations we received were from lower than 5,500 metres. We showed that geese tended to minimise altitude wherever possible and typically travel through the valleys of the Himalayas and not over the summits.
At the same time though, the geese showed a world-record rate of climb as they made their journey from their sea-level wintering grounds in India onto the Tibetan Plateau to breed in the summer. The maximum climb rates of 2.2 km/hr (120ft/min for all you pilots) would have been sufficient, for example, to get the geese from sea-level to the summit of Everest in just over four hours (if indeed they had travelled to the summit, which they didn’t). Even average climb rates would have taken them to the summit in just over eight hours. These impressive climbing flights onto the Tibetan Plateau are achieved without acclimatisation and without rest stops, and further, our data suggest they are achieved in the dead of night when conditions are calm, but perhaps surprisingly when there is no wind assistance.
How do they do it?
To anyone who has travelled to altitude, or indeed to anyone who has read some of the thrilling accounts of expeditions to Everest, K2 and beyond, it is clear that exercise at altitude is extremely challenging. This is because as altitude increases, air pressure decreases (there is less atmosphere weighing down on you), which means that the total amount of oxygen in the air around you decreases as well.
At Everest base camp, for example, there is a little over half the oxygen per unit volume of air than there is at sea level. The Xtreme Everest expeditions studied the effects of ascent up Everest on climbers. They found that the partial pressure of oxygen dissolved in the blood (PaO2) was at the very limit for survival and was accompanied by sustained hyperventilation of up to 90 breaths a minute, leaving no real additional capacity for exercise. Essentially, summiting Everest without supplemental oxygen could only be achieved via extremely short anaerobic walking bouts, paid back over regular rest periods. There’s more: hiking in humans consumes about six times more oxygen relative to resting metabolic rate, varying with walking speed and incline. Flight, however, is much more costly than walking or even running at maximum sprint speed. In birds, flight consumes about 12-15 times as much oxygen as resting. So how can birds fly where we struggle to walk without supplemental oxygen?
A Lesson in Efficiency
Birds have a highly evolved respiratory system that makes them extremely efficient at extracting oxygen. In fact, even your average London pigeon is far better at it than the finest human athletes ever recorded. We humans, like all other mammals, use our muscular diaphragm to draw air into the lung, extracting oxygen across the alveolar surface. We then reverse this process to expel the air back out of the lung, releasing carbon dioxide.
Birds have pulled this system apart and separated the two roles of the mammalian lung: ventilation and gas exchange. While the bird lung still extracts oxygen from the air as ours does, it uses a separate system of eight to nine bellows or ‘air sacs’ to ventilate the lung. The air sacs, located in the birds abdomen, thorax and clavicular area actually set up a uni-directional system of air flow through the lung. Yes, you read that right, air travels one way through a bird’s lung, meaning that they extract at least twice as much oxygen as we do per unit of time because they are moving fresh air across the lung surface during both inhalation and exhalation. Thanks also to this unidirectional ventilation, the bird lung remains fully inflated at all times because it is constantly receiving a fresh supply of air, either from the trachea during inhalation, or from the abdominal air sacs during exhalation. (Ever vigilant, Adventure Medic then asked – where does the air come out from? Well, the answer is here, and it is not as rude as you might first imagine…)
Not only do birds have this incredible adaptation, but the surface area of a bird’s lung is about two and a half times greater than it is in mammals and the surface itself is about four times thinner, meaning that the oxygen has a smaller distance to diffuse into the blood stream. As you can imagine, although this varies by species, it makes for one heck of a capacity to extract oxygen, and therefore to allow birds to cope with altitude and the oxygen demands of flight.
Goose vs. The World
Among the birds, bar-headed geese are thought to be champions. Using a variety of laboratory in-vivo and in-vitro studies, comparative physiology researchers have showed that bar-headed geese have an enhanced capacity to hyperventilate relative to other birds and that cerebral blood flow is not affected by hyperventilation (i.e. by hypocapnia) as it is in humans. As you might imagine, this dramatically reduces their susceptibility to High Altitude Cerebral Edema (HACE), although this reduced sensitivity to HACE is actually common among all birds.
Bar-headed geese also have a higher density of capillaries in the flight muscles than other birds and, importantly, also have a higher capillary density in the cardiac muscle, about 30% more than other species that have been investigated to date, allowing them to sustain high heart rates even in extreme hypoxia. Bar-headed geese also have a higher proportion of oxidative muscle fibres in the flight muscle and have positioned their mitochondria closer to local blood capillaries to reduce the distance over which oxygen needs to diffuse.
Their haemoglobin has a lower P50 than many other bird species so, like fetal haemoglobin, it can pick up more oxygen even when ambient conditions are hypoxic. Their haemoglobin also has greater temperature sensitivity than other birds. This last adaptation is the subject of novel research by Professor Bill Milsom and collaborators at the University of British Columbia and seeks to examine whether breathing cold air, as the geese likely do at altitude, might further enhance oxygen uptake (watch this space!).
In our own research, we showed that bar-headed geese were capable of maximum treadmill running exercise for at least 15 minutes in extremely hypoxic conditions simulating the summit of Mount Everest. By exposing the geese to an atmosphere of only 7% oxygen (compared to the 21% at sea level) and by measuring their heart rates, metabolic rates and blood gases as they ran at their very fastest speeds achieved in training, we demonstrated that they can sustain arterial oxygen saturation at values that are similar to when the birds were in sea-level (normoxic) conditions. In other words, they could still obtain and supply to their body a sufficient amount of oxygen for exercise, even at the (simulated) summit of Everest.
A Mere 455 Beats Per Minute
Finally, using latest state of the art loggers, we have also recorded the heart rates of wild bar-headed geese flying across the Tibetan Plateau. In our treadmill studies, we recorded heart rates as high as 500 beats per minute while the geese ran at top speeds in hypoxia. We found that their heart rates during flight in the wild were comparatively, surprisingly low, at 328 beats per minute, and geese spent only 2.3% of their time with heart rates over 455 beats per minute, suggesting that cardiac capacity for work does not appear to limit them at altitude.
We also brought together this work to model what the maximum flight altitude might be for this species. Using the Fick Equation for the convection of blood, we can estimate what we think the maximal supply of oxygen might be to the tissues and using flight biomechanical theory (aircraft physics!) we can estimate how the demands of flight should increase at altitude as the birds fly into ever thinner air, which would require them to flap harder to stay aloft. We predict that the maximum altitude to which these geese should be able to fly should be around 8,000 metres, and therefore that the paradigm of flight over Everest is at the very least, not the “normal” way for these geese to travel.
And Geese Might Fly…
So how can we explain what George Lowe saw back in 1953? Well, it remains entirely possible that some geese were really flying that high, perhaps aided by a favourable updraft to increase their altitude above Everest. It is also possible that George Lowe did not see what he thought he did as there have been a range of studies documenting decreases in cerebral function at altitude and suggesting that as many as a third of climbers above 7,500 metres may hallucinate to some extent. We may never solve the mystery of what George Lowe really saw as he sadly passed away in 2013, but we would love to hear from anyone in the Adventure Medic community with fresh sightings. In the meantime, we continue our research into the adaptations of other birds that travel to high altitudes, putting the feats of man to shame!
Photos: Lucy Hawkes, Nyambayar Batbayar and Coke Smith
