How geese fly so high
One of nature’s marvels is the annual migration of millions of birds. And among them are the bar-headed geese that routinely fly over the Himalayas. But how do they cope with an ascent from near sea level to extreme altitude and back down again in just a matter of hours? To find out, Jessica Mier, who’s both a physiologist and a NASA astronaut, developed a way to simulate their migration in a wind tunnel…
Jessica - These birds migrate annually over the Himalayas flying regularly between 5000 and 6000 metres through those passes. And of course the bar headed goose is quite famous for these early anecdotal reports where early explorers climbing the Himalayas say that they heard and saw a bar headed geese in the distance. So perhaps they're even flying as high as over the summits of the very highest mountains above 8000 metres. At these altitudes, the oxygen level is significantly lower than it is at sea level. So at these altitudes around 5000 6000 metres we're talking only half the amount of oxygen available when we start talking as high as the summits of the Himalayas. That is close to only about one third of the levels that we have here at sea level.
Chris - And of course humans do go to those sorts of altitudes, but they do it often with supplemental oxygen and they do it with enormous amounts of adaptation. First, they don't just go straight to the top of Everest, they go via base camp and so on, and spend weeks acclimatising. And these animals are effectively doing this straight off the bat...
Jessica - That's exactly right. And to me that is one of the most interesting points. These birds cross from sea level in India, over the Himalayas and in only about seven to eight hours, completely contrary to what you just mentioned, us humans taking weeks in order to acclimatise.
Chris - So how did you explore how they're not just flying at these tremendous altitudes but doing it without this acclimatisation period that, if we were to do that, we would be dead?
Jessica - We decided we needed a very controlled setting and we needed the birds to be very comfortable with the experimenters and comfortable with the equipment. So our idea was to fly these birds in a wind tunnel, measure things like heart rate, things like how much oxygen they use, how much carbon dioxide they produce, and even measure the level of oxygen and the temperature in their blood vessels while they were flying. We wanted to do this not only in normal oxygen levels but also the reduced oxygen levels when they were at altitude, flying over the Himalayas. We made a mask for the birds out of a thin sheet of plastic that we can form-fit over the beak to collect exhaled air so we can know how much oxygen the birds use, and how much carbon dioxide they produce. The other thing that the mask allowed us to do was to reduce the overall amount of oxygen that the birds were breathing. We also had a little backpack recorder system that had heart rate electrodes so we could get the ECG - electrocardiogram. And then also in indwelling oxygen electrodes that measured oxygen and also temperature in the arteries and the veins.
Chris - And when you do all this what actually emerged what was the pattern that you saw in these birds?
Jessica - Flying for birds is the most expensive form of locomotion for any vertebrate species. So we knew there was gonna be a big increase in metabolic rate and also there would be an increase in heart rate. So that of course makes sense: it's extreme exercise. What we didn't know was how it would change between these normal oxygen levels and the reduced oxygen levels. And, interestingly, as we thought, metabolic rate did increase during the flight: it went up about 16 times compared to rest. And what we found was that increase was associated with an increased amount of oxygen that was transported per heartbeat, with only a very modest increase in heart rate. So these geese appeared to have a lot of extra leeway in terms of their cardiac reserves - what we think of as how much the heart can pump. The heart rate in those reduced oxygen conditions was not any higher than in the normal oxygen levels. So although there was an increase of course from a resting condition to a flying condition, the heart rate was the same whether or not the oxygen level was normal or the reduced oxygen. The difference that we found between the two different conditions flight in the reduced oxygen level was achieved through a reduction in metabolic rate. So that means that the geese were actually using less oxygen in those flights and the lower oxygen conditions than they were in the flights with the normal oxygen conditions.
Chris - Have you any idea how they are doing this how are they registering this change and therefore knowing to keep their heart rate under control and how are they shifting their metabolism like that and doing it so quickly?
Jessica - We don't know for sure the answer to that question, but we have some hypotheses. First of all, the geese could be minimising oxygen required for other processes that are less essential during the flight. The gut of certain migratory species decreases in size a little bit before a long distance migration. This is also something that we see in diving animals. They will drastically change their blood flow, change what else is going on during a particularly long dive in order to preserve oxygen for critical organs like the heart and brain. Another thing that could be going on is that the birds are just simply adopting more efficient flight patterns and we did see some differences that basically made the geese fly a little bit more efficiently with the up stroke and the down stroke during flight.
Chris - Regardless though of behavioural changes and how the animals move, the tissues in the animals and specifically those really metabolically active tissues like the nervous system they're going to be seeing in these animals a sudden drop in oxygen, aren't they. Do we have any idea as to how the birds defend against that? Because if you did that to a person it would almost instantly result in a stroke!
Jessica - Right. So what we did see was that the arterial oxygen level was actually maintained throughout the flights. So, overall, there was a decrease when the animal had less oxygen on board but it was still maintained throughout the flight, so it didn't seem to be dropping to too critical of a level. The venous oxygen actually decreased during the initial portion of the flights. And that sort of shows us that the birds are continuing to extract more oxygen for those exercising tissues where they needed it. The other interesting finding that went along with this was that the temperature in the veins actually decreased during the flight. This is interesting because it can mean that there is significantly more oxygen in the blood hemoglobin, the protein that binds oxygen in the blood. When it's cooler the hemoglobin can actually hold on to more oxygen in the blood. So if you have a decrease in temperature there you can actually load and bind more oxygen at that site, meaning the bird could actually have more oxygen in their blood than they would otherwise if the temperatures were higher.
Chris - And it's not just the geese that are flying high you'll hopefully going to be doing this soon as well, aren't you?
Jessica - Yes that's right. In just less than three weeks I will be launching for a six month mission aboard the International Space Station. So the tables have turned. Now it is my turn to be the one poking and prodding for the advancement of science!