Engineered for Efficiency: How Hummingbirds Hover
By Margaux Winter
A common motif in energy consumption is that less is more. We assume that having less mass to move will improve the ability with which we can move that mass. Energy efficient cars are compact, and smaller animals seem to scurry along more quickly than their larger counterparts. This idea appears straightforward—moving each additional pound should require an additional input of energy. In quadrupeds, at least, this idea holds true. This is not the case for hummingbirds.1,2
Among vertebrates, hummingbirds have unusually high metabolisms due to the need to hover during flight.1 While most vertebrates must break down ingested sugars into glucose prior to undergoing cellular respiration, hummingbirds are able to use fructose and glucose as energy sources equally well. This adaptation has significant consequences for the hummingbird’s ability to stay airborne; unlike most vertebrates, which rapidly convert unmetabolized fructose into fat, hummingbirds are able to use these sugars to power their flight almost immediately after consumption.3
The extra energy that hummingbirds glean from fructose comes in handy--hovering during flight requires a remarkable 50 to 80 wingbeats per second.1 From an evolutionary perspective, the hummingbird’s lifestyle and anatomy have adapted to compensate these unique metabolic needs.
For one, hummingbirds maximize fructose intake by drinking floral nectar, a liquid that is high in sugar. Hummingbirds also have highly aerodynamic bodies.3 These light and nimble creatures have hollow bones, large chest muscles and hearts, and longer, stronger bones on the ends on their wings. These features endow stability, control, and agility during flight.3,4 Even more striking is the ability of the hummingbird’s morphology to adapt to its surroundings; during the winter, hummingbirds selectively molt their feathers to produce a more aerodynamic wing shape to reduce energy loss from wind friction.2
Due to the large proportion of energy spent on flight, hummingbirds’ hovering metabolic rate (HMR) and wingbeat kinematics make the hummingbird an important model organism to study the effects of wing morphology on metabolism.1 In a study conducted by the Welsh Lab at the University of Toronto, Dr. Derrick Groom found that larger body mass in hummingbirds correlates with increased metabolic efficiency.1 That is, larger hummingbirds use less energy to move each gram of body weight compared to their smaller counterparts.
Groom collected a wide variety of morphological, kinematic, and metabolic measures of hummingbirds mid-hover. Much like a driver might be interested in engine size to determine the horsepower of a gas-guzzling car, Groom was particularly interested in the effects of wing size on metabolic efficiency in hummingbirds. Put together, Groom used this data to uncover the influence of wing morphology—the wings’ shape, size, and mass—on HMR.1
Groom hypothesized that HMR would be negatively correlated with wing size and that metabolic efficiency would increase with increasing mass.1 While Groom and his colleagues did not find a direct relationship between HMR and wing size, they found significant effects among individual wingbeats. Specifically, the amount of energy required to beat a wing increases with the size of the wing, and accordingly, wingbeat frequency decreases with increasing wing mass.1 This combination of factors is referred to as a positive scaling of metabolic efficiency with wing size; to accomodate for a higher energetic cost of flapping, larger wings do not have to flap as many times per second to maintain hovering.1
This result has important implications for how we think about size and energy efficiency. Groom’s findings illustrate that the distribution of mass is more important than the overall mass of an animal when determining that animal’s metabolic payload. Indeed, Groom found that larger hummingbirds also tend to have larger wing size.1 By contrast, smaller birds were found to have a higher wingbeat frequency and an increased energy output, a pattern that is also found in bees.1
Research on the effect of size and wing morphology on metabolic efficiency isn’t just useful for studying animals. Groom’s observations may also have a profound impact on the way engineers design airplanes and spacecraft. When pilots look to the skies, they often think of ways to reduce the weight of cargo in their aircrafts, but perhaps this strategy is just a limitation of the way current aircraft are designed. Taking notes from hummingbirds, we may be able to design planes with flapping wings, capable of supporting a higher mass without a spike in energy consumption. Indeed, before humans take to the skies, we should emulate the creatures who have mastered the craft of flight over millions of years.
References
A common motif in energy consumption is that less is more. We assume that having less mass to move will improve the ability with which we can move that mass. Energy efficient cars are compact, and smaller animals seem to scurry along more quickly than their larger counterparts. This idea appears straightforward—moving each additional pound should require an additional input of energy. In quadrupeds, at least, this idea holds true. This is not the case for hummingbirds.1,2
Among vertebrates, hummingbirds have unusually high metabolisms due to the need to hover during flight.1 While most vertebrates must break down ingested sugars into glucose prior to undergoing cellular respiration, hummingbirds are able to use fructose and glucose as energy sources equally well. This adaptation has significant consequences for the hummingbird’s ability to stay airborne; unlike most vertebrates, which rapidly convert unmetabolized fructose into fat, hummingbirds are able to use these sugars to power their flight almost immediately after consumption.3
The extra energy that hummingbirds glean from fructose comes in handy--hovering during flight requires a remarkable 50 to 80 wingbeats per second.1 From an evolutionary perspective, the hummingbird’s lifestyle and anatomy have adapted to compensate these unique metabolic needs.
For one, hummingbirds maximize fructose intake by drinking floral nectar, a liquid that is high in sugar. Hummingbirds also have highly aerodynamic bodies.3 These light and nimble creatures have hollow bones, large chest muscles and hearts, and longer, stronger bones on the ends on their wings. These features endow stability, control, and agility during flight.3,4 Even more striking is the ability of the hummingbird’s morphology to adapt to its surroundings; during the winter, hummingbirds selectively molt their feathers to produce a more aerodynamic wing shape to reduce energy loss from wind friction.2
Due to the large proportion of energy spent on flight, hummingbirds’ hovering metabolic rate (HMR) and wingbeat kinematics make the hummingbird an important model organism to study the effects of wing morphology on metabolism.1 In a study conducted by the Welsh Lab at the University of Toronto, Dr. Derrick Groom found that larger body mass in hummingbirds correlates with increased metabolic efficiency.1 That is, larger hummingbirds use less energy to move each gram of body weight compared to their smaller counterparts.
Groom collected a wide variety of morphological, kinematic, and metabolic measures of hummingbirds mid-hover. Much like a driver might be interested in engine size to determine the horsepower of a gas-guzzling car, Groom was particularly interested in the effects of wing size on metabolic efficiency in hummingbirds. Put together, Groom used this data to uncover the influence of wing morphology—the wings’ shape, size, and mass—on HMR.1
Groom hypothesized that HMR would be negatively correlated with wing size and that metabolic efficiency would increase with increasing mass.1 While Groom and his colleagues did not find a direct relationship between HMR and wing size, they found significant effects among individual wingbeats. Specifically, the amount of energy required to beat a wing increases with the size of the wing, and accordingly, wingbeat frequency decreases with increasing wing mass.1 This combination of factors is referred to as a positive scaling of metabolic efficiency with wing size; to accomodate for a higher energetic cost of flapping, larger wings do not have to flap as many times per second to maintain hovering.1
This result has important implications for how we think about size and energy efficiency. Groom’s findings illustrate that the distribution of mass is more important than the overall mass of an animal when determining that animal’s metabolic payload. Indeed, Groom found that larger hummingbirds also tend to have larger wing size.1 By contrast, smaller birds were found to have a higher wingbeat frequency and an increased energy output, a pattern that is also found in bees.1
Research on the effect of size and wing morphology on metabolic efficiency isn’t just useful for studying animals. Groom’s observations may also have a profound impact on the way engineers design airplanes and spacecraft. When pilots look to the skies, they often think of ways to reduce the weight of cargo in their aircrafts, but perhaps this strategy is just a limitation of the way current aircraft are designed. Taking notes from hummingbirds, we may be able to design planes with flapping wings, capable of supporting a higher mass without a spike in energy consumption. Indeed, before humans take to the skies, we should emulate the creatures who have mastered the craft of flight over millions of years.
References
- Groom, Derrick J E, M Cecilia B Toledo, Donald R Powers, Bret W Tobalske, and Kenneth C Welch. "Integrating Morphology and Kinematics in the Scaling of Hummingbird Hovering Metabolic Rate and Efficiency." Proceedings. Biological Sciences 285, no. 1873 (2018): 20172011-20172011.
- Achache, Yonathan, Nir Sapir, and Yossef Elimelech. "Hovering Hummingbird Wing Aerodynamics during the Annual Cycle. II. Implications of Wing Feather Moult." Royal Society Open Science 5, no. 2 (2018): 171766.
- "University of Toronto, Scarborough." Hummingbird Metabolism Unique in Burning Glucose and Fructose Equally. Accessed November 02, 2018. http://ose.utsc.utoronto.ca/ose/story.php?id=5581
- Hedrick, Tyson L., Bret W. Tobalske, Ivo G. Ros, Douglas R. Warrick, and Andrew A. Biewener. "Morphological and Kinematic Basis of the Hummingbird Flight Stroke: Scaling of Flight Muscle Transmission Ratio." Proceedings of the Royal Society B 279, no. 1735 (2012): 1986-992.
