Blue throated hummingbirds are tiny, energetic birds with fast beating hearts to support their high metabolism. Their scientific name is Lampornis clemenciae and they are native to western North America. Standing just 3-4 inches tall and weighing 2-6 grams, blue throated hummingbirds have incredibly rapid heart rates to supply oxygen and nutrients to their tissues during flight. Understanding the physiology of their cardiovascular system provides insight into their adaptation for hovering and efficient energy use.
Anatomy of the Hummingbird Heart
The hummingbird heart is proportionately larger than the hearts of other birds in order to meet the high metabolic demands of flight. The heart makes up around 2.5% of a hummingbird’s body weight, compared to just 0.8% in most other birds and 0.5% in humans. Structurally, it is similar to the hearts of other birds and mammals with four chambers: right and left atria, and right and left ventricles. The muscular walls of the ventricles are especially thick to provide the force needed to rapidly circulate blood. The stroke volume, or amount of blood pumped per heartbeat, is the highest of any animal relative to its size. Oxygenated blood from the lungs flows from the left atrium to the left ventricle, then out to the body through the aorta. Deoxygenated blood returns from the body to the right atrium, then fills the right ventricle before being pumped through the pulmonary arteries to the lungs. This pulmonary circuit provides gas exchange to replenish oxygen levels.
Heart Rate at Rest
When sitting at rest, a hummingbird’s heart rate slows down dramatically compared to the rate during flight. However, it is still very rapid with a resting heart rate of around 250 beats per minute. This is over 10 times faster than the average resting human heart rate of 60-100 bpm. The rapid rate at rest allows their heart to quickly increase output when transitioning to the high energy demands of flight. For comparison, the average resting heart rate for other small birds is around 500 bpm. The lower rate in hummingbirds at rest may be an adaptation to conserve energy.
Heart Rate During Flight
A hummingbird’s heart rate accelerates dramatically during the sustained exertion of hovering and flight. Flight muscle makes up around 25% of their body weight, requiring huge amounts of metabolic energy input. While feeding, their heart rate can reach as high as 1260 beats per minute. This allows cardiac output to increase to around 10 times the resting rate. Remarkably, this means their heart is beating around 20 times per second while they feed.
The full range of heart rate acceleration allows hummingbirds to modulate heart function depending on their activity level. Just a brief 10 second feeding bout may result in a breathing rate of 250 breaths per minute and a heart rate spike of 400 to 500 bpm. More sustained hovering flight during feeding or display dives elicits peak rates. This gives the cardiovascular system impressive flexibility to match oxygen and nutrient delivery to the needs of the muscles and organs.
Blood Circulation Time
Hummingbirds have the highest mass-specific metabolic rate of any animal, requiring large amounts of oxygen and nutrients. To support this, they also have the fastest blood circulation time of any bird. Their small blood volume of just 1-2ml means it can quickly complete each circulation cycle. At rest, it takes just 2 seconds for blood to fully circulate through their vascular system. Oxygenated blood moves from the lungs to the muscles and organs within this short timespan. For comparison, it takes 20 seconds for human blood to fully circulate.
During flight, their circulation time further accelerates to just 1 second. This means their entire blood supply circulates through the lungs and back out to the body once every second. Such fast circulation facilitates the exchange of oxygen, carbon dioxide, glucose, and other metabolites required to power sustained hovering and flight.
How Heart Rate is Regulated
Hummingbirds have evolved an efficient physiological mechanism to rapidly modulate heart rate in relation to their activity level and oxygen needs. This regulation centers around coordination between the sympathetic nervous system and oxygen sensing centers in the aortic arch.
The sympathetic nerves cause the release of norepinephrine which increases heart contraction strength and rate. When a hummingbird flies, enhanced sympathetic outflow directly accelerates heart rate to increase cardiac output.
Specialized oxygen sensing glomus cells in the aortic arch also detect drops in blood oxygen levels during exertion. This triggers enhanced sympathetic outflow to speed heart rate. By coordinating circulatory adjustments with metabolic needs in this way, hummingbirds can achieve remarkable cardiovascular flexibility.
Cardiovascular Adaptations for Hovering
Many unique cardiovascular adaptations allow hummingbirds to support the extreme metabolic requirements of sustained hovering flight. Hovering is the most energetically expensive form of flight due to the constantly flexing and supporting the weight of the body. Hummingbirds are the only bird group that can truly hover due to the following adaptations:
- Very high wingbeat frequency – Around 50 beats per second
- Inverted aerodynamic lift during the upstroke
- Rotating wings to generate lift throughout the stroke cycle
- Rapid heart rate – Up to 1250 bpm while feeding
- Short circulation time – One second or less
- High capillary density in the muscles
- Near instant oxygen diffusion into the tissues
This allows them to beat their wings sufficiently quickly while maintaining blood circulation and oxygen delivery to the flight muscles. No other bird can simultaneously achieve this heavy aerobic exertion and mechanical lift requirement. The incredibly rapid heartbeat is a key factor enabling sustained hovering by powering their unique flight biomechanics.
Impacts of Prolonged Fast Heart Rate
The near continual state of exercise means most hummingbirds have a short lifespan of just 3-5 years. The prolonged tachycardia and associated high blood pressure places severe oxidative stress on their cardiovascular system. They are prone to heart enlargement and weakening of the heart muscles over time.
Hummingbirds may experience ischemia and erratic heart rhythms leading to sudden death. Their heart accounts for over 10% of their oxygen consumption at rest due to the energy demands of the cardiac muscle contraction. The consistent need for rapid transport of oxygenated blood leaves little capacity for endurance at the upper limits. However, their physiology remains highly effective for meeting the cardiovascular demands of their energy-intensive lifestyle in the short term.
Measuring Heart Rate in Hummingbirds
Hummingbird heart rate has been measured using a variety of methods in field and laboratory studies:
Electrocardiogram (ECG) Sensors
Fine electrodes contact the skin to detect electrical activity during each heart contraction. This produces an ECG trace from which heart rate can be derived. Lightweight telemetry allows hummingbirds to be monitored during free flight.
Doppler Effect
A directional high frequency sound beam is aimed at the heart. The variations in reflected ultrasound waves allows measurement of the speed of valve and blood flow. This data provides an accurate non-invasive measurement of heart rate.
High Speed Video
Recording hummingbirds at 500-1000 frames per second allows the rhythmic pulsations and movements associated with each heartbeat to be visualized. Frame counts permit heart rate calculations.
Audible Detections
In a quiet environment, the rapid lub-dub sounds of the heart valves closing can be detected by careful listening. Average beats per minute can be estimated by timing the intervals between sounds. This low-tech option provides reasonable accuracy.
Heartbeat and Temperature Regulation
In addition to delivering oxygen, the hummingbird’s rapid heart rate and circulatory system help maintain optimal body temperature during cooling conditions. Fluctuations in heart rate directly impact their ability to regulate temperature and prevent hypothermia.
Respiratory Evaporation
Panting increases evaporative heat loss from the respiratory tract which dramatically impacts body temperature balance. Hummingbirds have been shown to pant at up to ~450 breaths/minute during heat stress, coinciding with a reduced heart rate.
Peripheral Circulation Adjustments
More blood flow to the skin increases both radiative and convective heat loss to the environment. research shows hummingbirds can increase heart rate by up to 130 bpm during excessive heat, improving loss of internal heat from the body core to the peripheral tissues for cooling.
Shivering Thermogenesis
When cold, hummingbirds increase muscle contractions to rapidly generate heat. This depends on elevated heart rate to provide oxygen and energy, and may require rates over 1000 bpm during sustained shivering.
Comparisons with Other Bird Species
Hummingbirds have the highest known heart rate and most rapid metabolism of all birds. However, other avian groups also display physiological cardiovascular adaptations related to their ecology and flight style:
Small Passerines
Small songbirds have average resting heart rates of ~500 bpm. During flight their hearts can reach up to 1000 bpm, although this is not sustained. Their circulation time is about 20 seconds at rest. This moderate cardiovascular capacity supports varied bursts of activity.
Diving Birds
Penguins and other diving bird species have resting heart rates as low as 70 bpm to conserve oxygen. During a dive their heart rate further drops, even stopping entirely for some penguin species. This allows them to utilize oxygen slowly while submerged, extending dive duration.
Bustards
These large terrestrial birds have a very slow resting heart rate of just 130 bpm. However, their hearts can double in rate up to 260 bpm during flight. Their slow overall circulation matches their favored walking over sustained flight.
Galliformes
The order containing chickens, turkeys, and pheasants maintains higher sustained heart rates around 250-350 bpm. This elevates oxygen circulation to match the demands of wing-loading required for their short burst flight style.
Hummingbirds in Slow Motion
The following table compares speculated slow motion perception of hummingbird heart rate if humans experienced similar proportional increases:
| Animal | Heart Rate | Proportional Increase | Human Equivalent |
|---|---|---|---|
| Resting Hummingbird | 250 bpm | 10x increase over human | 1000 bpm |
| Hovering Hummingbird | 1250 bpm | 50x increase over human | 5000 bpm |
To match the proportional heart rate increases hummingbirds can achieve, an average resting person would experience a slow heart rate of 1000 bpm. Peak exertion would feel like 5000 bpm, over 80 beats per second! This highlights the truly remarkable cardiovascular capabilities of hummingbirds.
Conclusion
The rapid heart rate and circulation of hummingbirds provides an incredible example of evolution synchronizing physiology with ecology and locomotive style. Sustained hovering flight necessitates unmatched oxygen delivery which is facilitated by cardiovascular adaptations allowing heart rate spikes over 1200 beats per minute. The locomotive muscle demand is satisfied through precise neural and metabolic regulation of heart function linked with respiration. Further study of hummingbirds can provide more profound insights into maximizing exercise performance in other animals, as well as guidance for technological innovation to approach the extreme efficiency of hummingbird flight biomechanics. Truly understanding the speed and capabilities of the hummingbird’s heart requires perceiving time at their scale.
