Q. And what are you watching for?
A. We’re measuring oxygen consumption rates. If I were to stick an elite Olympic athlete, a cross-country skier, onto a cycle ergometer and ask them to wear a mask and say, ‘O.K., go as hard as you can go, and I want to measure your peak metabolic rate,’ one of the ways we can quantify that is in oxygen consumption. So I can say, ‘This human athlete is consuming four milliliters of oxygen per gram of body weight per hour.’
A hummingbird can easily hit 40 milliliters of oxygen per gram per hour. And if I ask the hummingbird to do extra, if I give it a little bit of extra weight to wear, that can go up to well above 60. So, their tissues are using oxygen at rates that are many, many times what we can possibly achieve.
Q. And they need the oxygen to help them metabolize the relatively enormous amounts of sugar they are taking in to get the energy to power their muscles?
A. I did a calculation back in graduate school. It turns out that when they’re hovering around and foraging during the day, they’re pretty much exclusively burning the sugar that they’ve been eating in the last 30 minutes to an hour. And so, they have this incredible ability to move sugar through their system. And I did the calculation and said, ‘O.K., if I scale one of my hummingbirds up to adult male human size, my size, how much sugar would I need to drink per minute if I were theoretically a hovering hummingbird as big as I am?’ It turned out to be right around the amount of sugar that’s in a can of Coca-Cola per minute. I haven’t actually tried to do this. My doctor advised against it.
Q. So they move sugar through their system an awful lot faster than we do?
A. Unlike us, hummingbirds can use the glucose that they’re ingesting in nectar and can move it through their guts, through their circulatory system, and to their muscle cells so fast that they can essentially keep that pipeline going in real time.
You and I can’t do that. We can support some small portion of exercise with newly ingested glucose, about 30 percent. But what’s just as remarkable is that the diet of hummingbirds is nectar and that’s half glucose and half fructose.
Fructose is getting a lot of bad press these days because of high fructose corn syrup in the Western diet and its association with metabolic disease and obesity. We’re not good at using fructose at all. Hummingbirds can use that fructose at very high rates.
Q. How do they do it?
A. Birds and nectar-feeding bats have evolved the ability to enhance the flux of nutrients like small sugars like fructose and glucose or amino acids to more effectively absorb their food. And getting them to their tissues is enhanced because hummingbird muscles and hummingbird hearts and hummingbird blood vessels are so good. The hummingbird heart rate is high and it’s pumping so much blood per unit of time. They have lots and lots of capillaries that allow the blood to get up close and personal to their muscle cells.
And hummingbirds can apparently take up fructose in their cells, and we’re trying to figure out what enables that. We do know that there is a different form of glucose transporter that is a specialist at taking up fructose. In our muscle cells that transporter is barely present. But hummingbird muscle fibers tend to have a lot of this transporter. So, we think we’re a little closer to understanding how they can take up fructose so fast. But the story’s not yet complete. We need to do some follow up work in order to really confirm that.
Q. What about your recent work on whether big or small hummingbirds are more efficient at energy use?
A. Hummingbirds really do vary in size. Our Ruby-throated hummingbirds weigh less than a penny, 2.5 to 3 grams. Down in South and Central America, you can find some much larger hummingbird species. Up in the range of 10-12 grams. And then there’s one species, the giant hummingbird, that sits at about 18-20 grams.
In essence, larger hummingbirds have higher efficiency. They’re converting a greater proportion of their sugar energy into mechanical power to hover than our smallest hummingbirds.
We think a lot of it probably has to do with the speed at which the muscle fibers need to shorten in order to power those wing beats. Small hummingbirds beat their wings at a higher frequency than larger hummingbirds. It’s something they need to do to generate sufficient mechanical power. But the faster you move or the faster you make a muscle shorten, there is evidence that suggests the less efficiently it does so.
And so, what we see is that the apparent efficiency of the smallest hummingbirds is down around 10 percent. If you go up to the larger hummingbirds they are — they’re up in the range of, you know, 30 percent.