Hummingbirds are well known for their incredibly rapid wing beats and remarkable ability to hover in midair as they feed on flower nectar. Their tiny size and high metabolism require a very efficient circulatory system to meet their energy needs. This has led to the common belief that hummingbirds have the smallest heart of all birds and mammals. But is this really true? Let’s take a closer look at the anatomy and physiology of hummingbird hearts to find out.
Heart size relative to body size
Hummingbirds do have extremely small hearts compared to their overall body size. The average adult hummingbird has a total body length of 3-6 inches and weighs 2-20 grams. In contrast, the average adult human is 5-6 feet tall and weighs 62-204 pounds. Since heart size generally correlates with body size, it makes sense that hummingbird hearts are much tinier than human hearts.
However, when comparing creatures of vastly different sizes, it is more informative to look at proportional heart size instead of absolute heart size. Proportional heart size is the ratio of heart mass to total body mass. This metric provides a size comparison that accounts for differences in body size between species. By this measure, hummingbird hearts are not the smallest.
The bee hummingbird heart
The bee hummingbird (Mellisuga helenae) is the smallest bird species in the world, with an average adult body length of just 2-2.5 inches and weight of 1.6-2 grams. As such, they also have the smallest absolute heart size of any bird. Their tiny hearts are about 0.1 grams in mass, similar in size to a coffee bean. But proportional to their body size, bee hummingbird hearts make up about 5-8% of their total body mass. This is similar to the proportional heart size of other small birds and mammals.
The shrew heart is proportionally smaller
Shrews are tiny mammals that have proportionally smaller hearts than hummingbirds. The Etruscan shrew (Suncus etruscus) has a body mass of only 2-3 grams as adults. Their diminutive hearts weigh just 0.03 grams and account for only 1-2% of total body mass. Across vertebrate species, shrews have the smallest proportional heart mass. The combination of a very rapid metabolism and small body size appears to have driven the evolution of incredibly tiny hearts in these small mammals.
Why do hummingbirds have rapid heart rates?
Hummingbirds have amongst the highest metabolic rates of any animal. To meet the demand of their high energy lifestyle, hummingbirds have evolved hearts that beat extremely fast. The average resting heart rate of a hummingbird is around 500 beats per minute. During flight, their hearts can reach speeds of over 1200 beats per minute. This allows a hummingbird heart to circulate a large volume of oxygen- and nutrient-rich blood throughout its body to power energetically expensive hovering flight.
Rapid energy consumption requires a hyperactive circulatory system
Hummingbirds consume a tremendous amount of energy relative to their small size. To put it into perspective, a hummingbird may burn through energy at up to 10 times the rate of an equivalently-sized mouse at rest. This requires an incredibly efficient respiratory and circulatory system to deliver oxygen and energetic molecules to tissues at rapid rates. While flying or hovering, around 75% of their total energy expenditure goes towards powering flight muscles alone.
To meet these demands, hummingbirds have evolved hearts with the highest known mass-specific metabolic rates of any animal. Oxygen consumption in hummingbird hearts has been measured at 50 times the rate of human hearts per gram of tissue. This allows their small hearts to rapidly deliver nutrients and remove waste to match their extreme energy turnover.
High density of mitochondria supports energy needs
The cells that make up hummingbird hearts are packed with mitochondria, the cell’s “powerhouses” that produce energy molecules called ATP. The volume density of mitochondria in hummingbird heart muscle cells has been measured at 35%. Comparatively, human heart muscle cells have a mitochondrial density of only 22%. The very high density of mitochondria in hummingbird hearts allows them to generate the enormous amounts of ATP needed to sustain their unprecedented metabolic rates.
Differences in heart structure and function
In addition to their small size and rapid rate, hummingbird hearts have other specializations that enable their unique physiology and high performance:
- Higher stroke volume enables more blood pumped per beat
- Shorter systolic phase enables longer diastolic filling
- Thick ventricular wall allows more forceful contractions
- Specialized proteins involved in cellular calcium cycling
These adaptations allow hummingbird hearts to circulate blood extremely rapidly while maintaining sufficient cardiac output to perfuse tissues. This provides critical oxygen and energy to power flight muscles continuously for hours without fatiguing.
Oxygen transport is vital for meeting energy demands
A hummingbird’s metabolic machinery is impressive, but would be useless without an efficient respiratory and circulatory system to provide oxygen. Oxygen makes up a large fraction of a hummingbird’s body weight because their muscles require so much of it. Hummingbird flight muscles are made up of 35-45% mitochondria by volume. This allows them to extract oxygen from blood extremely quickly.
How do hummingbirds get enough oxygen?
Hummingbirds have the highest mass-specific oxygen consumption rates of any vertebrate. To meet this demand, they have evolved specialized mechanisms for maximizing oxygen uptake and transport:
- Large lungs relative to body size
- Unidirectional airflow through lungs enables maximum gas exchange
- High concentration of red blood cells transports more oxygen per volume of blood
- More capillaries around flight muscles facilitate oxygen delivery
These adaptations enable hummingbirds to rapidly take up oxygen from air and efficiently deliver it to working muscles. This supports the metabolic demands of hover-feeding and endurance flight.
Summary: How does heart size compare across species?
In summary, hummingbirds do not have the smallest hearts when accounting for differences in body size. The bee hummingbird has the smallest absolute heart size of any bird, but proportionally, shrews have smaller hearts than hummingbirds. However, hummingbird hearts are highly specialized to deliver oxygen and nutrients at extremely rapid rates to meet their high metabolism. Their small overall size belies their powerful performance.
Conclusion
While hummingbirds do not have the smallest proportional heart mass of all animals, their rapid heart rate and high cardiac output is truly exceptional. Hummingbird cardiovascular systems are specifically adapted to provide copious amounts of oxygen and energy to enable sustained hovering flight and rapid metabolism. When considering their energy needs relative to body size, hummingbirds have some of the highest performing hearts in the animal kingdom.