Hummingbirds are amazing creatures known for their ability to hover in midair and fly backwards. They are also the only bird that can fly backwards. Another incredible feat of the hummingbird is its ability to enter a hibernation-like state called torpor.
What is torpor?
Torpor is a state of decreased physiological activity characterized by reduced metabolism, heart rate, and body temperature. It allows hummingbirds to conserve huge amounts of energy. Torpor is not the same as hibernation because the body temperature reduction is not as extreme and the state is much easier to come out of.
Hummingbirds enter torpor to survive periods when they cannot feed, such as overnight or during bad weather. Going into torpor slows down their metabolism and lowers their body temperature, allowing them to conserve energy when food sources are not available.
How low does a hummingbird’s body temperature get in torpor?
During torpor, a hummingbird’s body temperature can drop dramatically, from around 105°F (40.5°C) to as low as 50°F (10°C). Their heart rate also slows from around 500 beats per minute down to just 50-180 beats per minute.
This allows them to conserve an incredible amount of energy. Their metabolic rate drops to around 5% of their normal active rate. By lowering their body temperature and heart rate, hummingbirds can survive on the limited energy reserves in their fat stores and liver.
How long can hummingbirds stay in torpor?
Hummingbirds can stay in torpor for surprisingly long periods. During cold nights, they may enter torpor for up to 14 hours. In one study, hummingbirds were found to maintain torpor for up to 18 hours overnight.
During migration, when food sources are limited, they may stay in torpor for multiple days or weeks. Records show Calliope hummingbirds remaining in torpor for up to 11 days during migration. Meanwhile, Rufous hummingbirds have been documented surviving for up to 15 days while relying solely on torpor.
Typical torpor durations
- Overnight torpor: Up to 14 hours
- During migration: Up to 2-3 weeks
So in short periods, hummingbirds can stay in torpor for hours, while during migration they can prolong torpor for many days without food.
How do hummingbirds survive so long in torpor?
Hummingbirds have several remarkable adaptations that allow them to survive extended periods in torpor.
Fat stores
Hummingbirds build up fat reserves ahead of time and during migration. The fat provides an energy source for them to metabolize while in torpor. They can double their body fat content before migrating or entering long bouts of torpor.
Efficient temperature regulation
Hummingbirds can precisely control the decrease in their body temperature when entering torpor. Allowing their temperature to gradually decrease over several hours minimizes energy expenditure.
Slowed metabolism
By slowing their metabolism to around 5% of normal, hummingbirds don’t burn through calories and fat nearly as quickly. Their bodies operate on minimal energy requirements.
Lowered heart rate
With their heart beating slowly, less energy is needed for cardiac function. This helps preserve their fat stores longer.
Reversing torpor
Hummingbirds have the remarkable ability to quickly reverse torpor once they find a food source. They can raise their heart rate and temperature back to normal within minutes to capitalize on available food.
Do hummingbirds eat during torpor?
No, hummingbirds do not eat or drink while in torpor. They rely solely on their fat reserves and limited liver glycogen stores to survive.
They need to regain normal body temperature and heart rate before they can resume eating. Once aroused from torpor, they require quick access to a concentrated sugar source to refuel and recover their energy.
Dangers of prolonged torpor
While torpor allows hummingbirds to survive periods of low food availability, remaining in torpor too long can be dangerous or fatal:
- If fat stores are depleted, they may starve
- Long-term metabolic slowdown can impair organ function
- Immune system may become compromised
- Spending too long in hypothermic state can be detrimental
Therefore, hummingbirds aim to balance the energy conservation benefits of torpor with the risks of remaining in torpor too long. Migrating hummingbirds must locate food sources to feed periodically and restore their energy.
Adaptations for fast recovery from torpor
Hummingbirds need the ability to quickly reverse torpor when food becomes available again. They have several key adaptations that aid their fast recovery:
Rapidly increasing heart rate
Hummingbird hearts can accelerate from around 50 bpm to over 500 bpm within a matter of minutes after arousal from torpor.
Fast metabolite switching
They can rapidly switch their main energy source from fats to carbohydrates once they resume feeding. This supports the energy demands of returning to normal body temperature and activity.
Effective fat storage
Storing excess fat before entering torpor ensures they have enough energy reserves to fully arouse and restore normal functions.
Do all hummingbirds exhibit torpor?
Torpor capability varies across different hummingbird species. Species that endure periods of limited food, such as migrating species and those in cold climates, are most likely to use torpor.
Some species known to regularly exhibit torpor include:
- Ruby-throated hummingbird
- Rufous hummingbird
- Allen’s hummingbird
- Calliope hummingbird
- Broad-tailed hummingbird
Meanwhile, tropical hummingbird species with regular food access, like the aptly named Jamaican Mango, have less need for torpor and therefore may not exhibit the behavior.
Unique torpor adaptations in migrating hummingbirds
Hummingbirds that migrate long distances have evolved unique physiological adaptations to optimize their use of torpor:
More efficient fat storage
Migrating hummingbirds can store up to 30-40% of their body weight as fat before migrating. This provides more energy reserves during migration.
Better kidney function
Their kidneys can function at cooler temperatures than other birds. This allows them to remain in torpor longer without kidney damage.
Enhanced skeletal muscles
Migrating species have enhanced shivering muscles that allow them to raise their body temperature faster after torpor.
Larger hearts and lungs
Larger cardio-respiratory systems support their rapid heartbeat acceleration and high oxygen needs during recovery.
Do hummingbirds sleep while in torpor?
No, hummingbirds do not sleep deeply during torpor. They maintain a state of lethargy but can still be alerted by noises or other disturbances. This allows them to spot any potential predators or food opportunities.
However, some studies show hummingbird brains may undergo slow-wave sleep cycles while in torpor. But they do not display the typical total muscle relaxation seen in deep sleep.
Torpor through the seasons
Hummingbirds enter torpor at various times throughout the year depending on environmental conditions and food availability:
Winter
Use torpor to conserve energy on cold nights when nectar sources are limited.
Migration
Employ torpor while flying long distances when food is scarce.
Rainy or stormy weather
Lowered food availability may lead them to use torpor.
Overnight
Often the easiest time to enter torpor since main energy source (flower nectar) is unavailable at night.
Tips for supporting hummingbirds coming out of torpor
Here are some tips if you notice hummingbirds coming out of torpor in your yard:
- Offer a concentrated nectar or sugar solution – They need quick energy.
- Supply clean water – Rehydration is important after torpor.
- Let them rest – Give them a safe place to fully recover their energy.
- Hummingbird feeders – Having a feeder handy helps them refuel fast.
With some extra care and feeding, you can help hummingbirds recover their energy reserves quickly after arousing from torpor.
Key facts about torpor in hummingbirds
- Torpor is a hibernation-like state that decreases their metabolism and body temperature.
- It allows them to conserve huge amounts of energy when food is scarce.
- Their body temperature can drop from 105°F to as low as 50°F.
- Heart rate slows from 500 bpm down to 50-180 bpm.
- They can stay in torpor up to 14 hours overnight or 2-3 weeks during migration.
- Fat stores provide energy for them to survive torpor.
- They can quickly reverse torpor when food becomes available again.
- Some species use torpor more than others.
- Migrating species have special adaptations for optimizing torpor use.
Conclusion
A hummingbird’s ability to enter torpor is an incredible survival adaptation. By lowering their metabolic rate and temperature, they can endure periods of low food availability for surprisingly long stretches of time. While extended torpor can be dangerous, their specialized physiology allows them to minimize risks and recover quickly once feeding resumes.
Understanding torpor gives us appreciation for the amazing feats hummingbirds can achieve. Their dynamic use of torpor enables them to thrive in diverse environments across much of the Americas.
