Logging of physiological variables during migration has shown that average heart rates during flight increase with rising elevation ( FIGURE 2B), and geese spend a greater proportion of time flying with near maximal heart rates when altitude exceeds 4,800 m ( 4). 16 with permission.Īs bar-headed geese fly to higher elevations, it becomes progressively more difficult to generate lift in the decreasing air density. Image was modified and reproduced from Ref. B: on their northward migration out of India, the ascent over the mountains is very steep (minimum climb rates of 0.8–2.2 km/h) and occurs very quickly (<1 day) ( 17). The colored background shading indicates elevation, and the white crosses are the world's highest mountains (all over 8,000-m elevation). The high-altitude migration of bar-headed geeseĪ: satellite tracking of migrating bar-headed geese (colored lines are individual geese) shows where individuals from a Mongolian population cross the Himalayas on their southward migration to India. Maintaining water balance during flight should also be a major challenge in the dry air at high altitudes, given that water loss can constrain flight duration at sea level in some species ( 11). At the same time, the temperatures at high altitudes can be very low, well below freezing year round in the high Himalayas ( 56), which could require additional metabolic energy for thermogenesis if the heat production from exercise is not sufficient to maintain body temperature. Therefore, bar-headed geese face the challenge of sustaining the high rates of O 2 consumption needed for flapping flight, which ranges from 10- to 15-fold above resting levels during steady flight in a wind tunnel at sea level ( 51), in air that can severely limit aerobic metabolism in many lowland animals ( 49). In fact, the atmosphere atop the highest peaks in the Himalayas is believed to have scarcely enough oxygen to support basal metabolism in humans ( 53). The level of hypoxia at these elevations, even the lowest at which bar-headed geese cross the mountains, is sufficient to reduce maximal O 2 uptake rates in humans substantially ( 53). Although the accuracy of auditory and visual observations are questionable ( 16), there are anecdotal reports of bar-headed geese flying even higher, above the highest peaks in the Himalayas (of which there are 14 above 8,000 m), where the P o 2 is only one-third of the sea-level value ( 46). Most birds reach altitudes of 5,000–6,000 m during the migration, where the P o 2 is roughly half of that at sea level, and they occasionally fly even higher (e.g., one recorded bird reached 7,290 m) ( 16, 24, 47). Geese migrating between India and Mongolia have been tracked by satellite telemetry crossing the Himalayan mountains across a broad front ( 47) ( FIGURE 1). What is the evidence that bar-headed geese can in fact accomplish this paradoxical feat? What physiological mechanisms underlie high-altitude flight? Recent efforts to address these questions, from characterizing the physiological ecology of the natural migration to elucidating the unique respiratory and metabolic physiology that underlies it, are shedding new insight into the paradox of high-altitude flight in this species.īar-headed geese fly at altitudes that are extremely challenging to lowland humans and animals. Therein lies the apparent paradox that has intrigued so many scientists: bar-headed geese must be capable of sustaining the high oxygen demands of flight in air that is exceedingly oxygen-thin. As the most metabolically intense form of vertebrate locomotion, flight demands an extremely high rate of oxygen consumption ( 51), yet the air at high altitudes in the Himalayas contains only one-third to one-half of the oxygen that is available in air at sea level. Bar-headed geese can be found anywhere from Mongolia to the Tibetan plateau in the summer, where they raise young before the majority take long flights south to the Indian subcontinent in the autumn, and return again the following spring ( 24, 47). Since early mountaineers and naturalists first sighted bar-headed geese migrating amidst the Himalayan mountains, the migration of this species has been a fascination to both scientists and the general public. Then, as if from the stars above me, I heard the honking of bar-headed geese.” –Lawrence Swan ( 46) Coming from the south, the distant hum became a call. “On one cold and still night in early April, I stood beside the Barun glacier.
0 Comments
Leave a Reply. |