In 1953, a mountain climber reported seeing a bar-headed goose (Anser indicus) soar over the peak of Mount Everest. The nearly 9-kilometer feat—2 kilometers higher than any other animal has been known to fly—was thought physiologically impossible. Now, researchers who raised 19 of the geese—named for the black stripes on the backs of their heads—have shown the birds really do have what it takes to fly so high.
The team trained the youngsters to fly in a large wind tunnel wearing backpacks and face masks full of sensors that recorded their heart rate, blood oxygen levels, temperature, and metabolic rate—how many calories they burned per hour. The researchers simulated low-, medium-, and high-altitude conditions by altering the concentration of oxygen supplied to face masks worn by each goose as it flew in the tunnel.
Birds already have a better heart and lungs than mammals for sustained physical activity. And researchers knew that bar-headed geese have even larger, thinner lungs that let them breathe more deeply and an even bigger heart to pump more oxygen to muscles than other birds.
The wind tunnel experiments showed that when the concentration of oxygen was at its lowest—like the 7% found on top of Mount Everest versus 21% at sea level—the geese’s heart rate and frequency of wing beats remained the same even as their metabolic rate dropped. Somehow, the birds managed to cool down their blood—the measured blood temperature dropped—so it could take up more oxygen, the researchers report today in eLife. This cooling likely helps compensate for the very thin air, the team says.
Although well trained, the birds were only willing to stay airborne a few minutes—or less—when wearing their backpacks and flying at “high” altitudes. So it’s not clear whether these adaptations alone are what make it possible to fly the 8 hours it takes to climb over Mount Everest, or allow the geese to complete 4000-kilometer migrations across Central and South Asia, which they can do with no training. But those few minutes showed these geese really could fly by climbers on top of Mount Everest.