Yes, many species of hummingbirds are capable of going into torpor. Torpor is a state of decreased physiological activity that hummingbirds can enter to conserve energy when food is scarce or environmental conditions are challenging.
What is torpor?
Torpor is a state of decreased metabolic rate and lowered body temperature adopted by some animals in order to conserve energy. It is similar to hibernation, but differs in the duration and depth of the dormant state. While hibernating animals experience deep drops in metabolism for weeks or months, torpid animals experience shallower, shorter-term metabolic suppression lasting less than 24 hours.
During torpor, an animal’s metabolic rate can drop to just 5-30% of normal resting levels. Their body temperature decreases as well, in some cases reaching ambient temperature. Breathing, heart rate, and other physiological processes slow dramatically. The animal is largely immobile and unresponsive to external stimuli.
By lowering their energy requirements during times of environmental stress or shortage of food, animals that undergo torpor are able to survive conditions that would otherwise lead to starvation and death. Torpor allows them to wait out the unfavorable conditions until circumstances improve and normal activity can resume.
Torpor in hummingbirds
Many hummingbird species have been observed to enter nightly torpor. During their usual active period in the daytime, hummingbirds have extremely high metabolic rates to power their rapid wing-beating and busy foraging. They must consume large quantities of flower nectar to meet these high energy demands. At night when they are roosting, their metabolic rate naturally decreases somewhat, but many species take this further by going into torpor to conserve even more energy.
Research has shown that up to 12 species of hummingbirds in North America regularly employ torpor as part of their daily energy budgeting strategy. These include Anna’s hummingbird, Allen’s hummingbird, Costa’s hummingbird, and Rufous hummingbird. The precise torpor strategies vary by species and populations, with some species more likely to use torpor in certain geographical regions or seasons.
Entry into torpor usually occurs at night, but hummingbirds may also become torpid during the daytime if conditions necessitate. Cold temperatures, rain, or food shortages can all trigger daytime torpor. Their body temperature can decrease by up to 20-30??C and metabolic rate can diminish to just 5-10% of normal resting rates. This allows substantial energy savings. Torpor bouts typically last a few hours, with birds arousing once external conditions improve or to forage.
Adaptations for torpor
Hummingbirds possess several key adaptations that allow them to utilize torpor:
- High capacity for fat storage – Hummingbirds can store large amounts of fat, primarily around the furcula (wishbone). This provides an energy reserve to tap into when food is scarce.
- Low body temperature – Their normal body temperature of around 40??C is lower than many similar sized birds, facilitating metabolic suppression.
- Reduced organ mass – Their hearts, livers, kidneys and other organs are smaller relative to body size compared to other birds. This also aids metabolic reduction.
- Rapid warming ability – Specialized physiology allows hummingbirds to raise their body temperature quickly upon arousal from torpor so they can resume activity rapidly.
These adaptations allow hummingbirds to minimize energy expenditure through torpor while avoiding negative effects such as physiological damage from extreme temperature reductions.
Ecological significance
The use of torpor grants hummingbirds a valuable survival advantage in their specialized ecological niches.
As small, high-metabolism animals, hummingbirds are constantly faced with potential energy shortfalls. Their primary food source, nectar, can be temporally and spatially patchy. By suppressing their metabolism during overnight roosting or daytime periods of inclement weather or low food availability, hummingbirds can withstand these energetic bottlenecks. Without torpor, they would be much more vulnerable to starvation under such conditions.
Torpor may also facilitate hummingbirds expanding their geographical ranges into cooler climates. By decreasing their metabolism and body temperature at night, they are better able to cope with lower ambient temperatures. Species such as Anna’s hummingbird have successfully expanded their range northwards in North America, aided by their use of torpor.
In warmer climates, torpor allows hummingbirds to conserve energy even when food is continuously abundant. This frees up energy to allocate towards reproduction and other physiologically demanding activities.
The strategic use of torpor likely played an important role in hummingbirds diversifying to fill their unique ecological niche as specialized nectar-feeding birds.
Physiological regulation
Entry into torpor is precisely regulated by physiological mechanisms that integrate environmental cues of energy balance and initiate metabolic suppression at appropriate times.
Falling ambient temperature is one external trigger that can initiate torpor. But endogenous cues also play an important role. Declining fat stores provide a signal to start conserving energy through torpor. Regulation of ghrelin and leptin, hormones that control appetite and fat storage, initiate torpor when the animal is in negative energy balance.
The central nervous system regulates torpor through specialized neurons that integrate internal and external signals. These neuronal populations then initiate cascades which ultimately slow metabolic processes. Suppressed sympathetic nervous system activityrestricts heat production, allowing body temperature to fall.
Arousal from torpor is controlled by separate mechanisms that reactivate normal metabolism once conditions necessitate. Triggers for arousal can include warming external temperatures, a circadian timer, or increasing hunger signals as energy stores decline.
Comparison to hibernation
Torpor differs from true hibernation in mammals in several key ways:
| Factor | Torpor | Hibernation |
|---|---|---|
| Metabolic rate reduction | 5-30% of normal resting rate | 2-5% of normal resting rate |
| Body temperature decrease | 10-30??C drop | More than 30??C drop, near ambient |
| Duration | Under 24 hours | Days to months |
| Frequency | Daily/regular bouts | Once annually or sporadically |
The shallower metabolic suppression over shorter time periods allows birds and other torpid heterotherms to avoid extreme physiological disruptions while still obtaining energy savings. However, the duration and depth of hibernation provides substantially greater energy conservation for hibernators when food is unavailable for extended periods.
Conclusion
Many hummingbird species employ short-term torpor as an essential energy saving strategy. By reducing their high metabolic rates and body temperature, mainly overnight but sometimes during the day, they are able to survive periods of energetic hardship and thrive in challenging environments. Torpor provides hummingbirds a key advantage, allowing them to occupy their unique ecological niche as non-migratory, nectar-feeding birds.
