When it comes to feats of extreme endurance, bar-headed geese leave most animals in their wake – even other birds. Migrating back and forth across the Himalayan Mountains from their northern nesting grounds to overwinter in South Asia, the large birds have to sustain flight – one of the most metabolically demanding activities – at altitudes where oxygen is scarce. Sabine Lague and colleagues from the University of British Columbia and Queen's University, Canada, explain that most birds are well equipped to deal with the metabolic demands of flight, and bar-headed geese have souped-up systems to ensure that they deliver sufficient oxygen to their toiling flight muscles in the thinnest of air. However, the team explains that most of our understanding of the physiology of these extreme aviators is based on observations of animals raised at sea level; yet, the true athletes are born and raised at altitudes of over 3000 m on the Tibetan and Mongolian plains.
Curious to find out how acclimatisation and growing up in a rarefied atmosphere affects the birds’ respiratory and cardiovascular systems, Lague, Beverly Chua, Tony Farrell, Yuxiang Wang and Bill Milsom set up a lab on the shores of the high-altitude Lake Qinghai, China, to find out how the birds’ ventilatory and cardiovascular systems compared with those of bar-headed geese raised back in Vancouver at sea level.
The team gently cradled the birds in a sling and placed their heads in a large Plexiglas chamber to measure oxygen consumption. They also recorded the pH, the concentrations of certain ions, and the oxygen content of the blood, in addition to measuring blood pressure and other cardiac variables to monitor how the hearts coped as the oxygen supply was gradually reduced to levels in excess of those experienced during the epic migration. Impressively, Lague and colleagues found that the high-altitude animals had lower metabolic rates than the sea-level birds and the Tibetan geese were able to breathe harder than the Vancouver birds as the oxygen levels plummeted. However, when they scrutinised the oxygen carrying capacity of both groups of birds, the high-altitude birds carried no more oxygen than the birds reared at low altitude. And when the team analysed the function of the birds’ cardiovascular system, they found that their hearts were able to pump blood just as effectively when the oxygen supply was high, but as the oxygen supply was reduced, the hearts of the birds from Lake Qinghai were able to pump blood at twice the rate of those from the geese raised in Canada.
‘High-altitude-reared bar-headed geese exhibit a reduced oxygen demand at rest and a modest but significant increase in oxygen uptake and delivery during progressive hypoxia compared to low-altitude-reared bar-headed geese’, say Lague and her colleagues, who are now keen to know more about the mechanisms that allow the Tibetan bar-headed geese to outperform their cousins in British Columbia.