Torpor Conserves Energy

a small fluffy brown common poorwill sits on gravel at night

Biological Strategy

Common poorwill

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Image: Frank D. Lospalluto / Flickr / Some rights reserved

Maintain Homeostasis

When a living system is in homeostasis, it means that internal conditions are stable and relatively constant. For example, a human’s internal temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit) unless there’s an illness. The human body maintains this temperature despite external ambient temperature. However, as with all physiological processes, maintaining homeostasis requires communication and coordination. So living systems have ways to detect changes from the norm, mechanisms to cause an adjustment, and negative feedback connections between the two. A desert lizard called the Gila monster offers a good example of maintaining homeostasis. The lizard goes from eating large meals to fasting for extended time periods. To maintain its blood sugar levels at a steady level, when food is scarce, its endocrine system releases a hormone that raises its blood sugar levels.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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The common poorwill conserves energy when food is limited by entering a state of torpor for extended periods of time.

All animals need energy to survive, and they get this energy from the food they eat. When food is limited, animals need to conserve their energy until food is readily available again. One way to conserve energy is to exert less throughout the day and/or night. Because all major life functions (breathing, moving, pumping blood, maintaining body temperature, etc) require energy, reducing body temperature or slowing heart rate is one way to conserve energy. Many animals do this during hibernation, but other animals, such as birds, do this for shorter periods of time (anywhere from 24 hours to a few days), which is known as torpor.

Many birds enter torpor when it is difficult to hunt for prey and find food, for example during moonless and/or cloudy nights, when ambient light levels are low. The birds that most commonly engage in torpor are small-bodied specialized foragers, such as hummingbirds. Although there is considerable variation in patterns of torpor among birds, one species, the common poorwill (Phalaenoptilus nuttallii), is the only bird known to remain torpid for extended periods of time. Poorwills feast mainly on insects, and experience substantial seasonal fluctuations in food availability, and thus are much less active in the winter. To cope with the decrease in energy availability, common poorwills reduce their body temperature during winter, entering a hibernation-like state. This ability to engage in prolonged periods of torpor allows the common poorwill to conserve energy during times of limited food availability so it can forage with an increased likelihood of success.

Image: Connor Long / CC BY SA - Creative Commons Attribution + ShareAlike

Common Poorwill (Phalaenoptilus nuttallii)

Image: Dominic Sherony / CC BY SA - Creative Commons Attribution + ShareAlike

Last Updated May 29, 2018

References

Common Poorwills, small nocturnal insectivorous birds found across western North America, are seemingly unique because of their alleged ability to remain torpid for extended periods during winter. We used temperaturesensitive radio transmitters to assess patterns of torpor use at sites in the Sonoran desert of southern Arizona. Poorwills used torpor extensively whenever ambient temperature (Ta) dropped below 10° C, and there was little evidence for thermoregulation when Ta was above 5° C. During the winter months (December through February), birds remained entirely inactive on 72% of bird-nights, and continuously inactive periods of 10 days or longer were common. The extent of inactivity is similar behaviorally to that of hibernating small mammals. Roost selection, however, facilitated routine passive solar warming, and inactive birds exhibited a regular pattern of arousal on sunny days, followed by reentry into torpor at sunset. We argue that daily arousals are likely an adaptation to the circumstances that characterize surface dormancy. We hypothesize that the relationship between Ta and availability of flying insects at night, in combination with unique ecological aspects of arid regions, contributed to the evolution of multiday torpor use by poorwills.” (Woods et al. 2004:231)

Journal article

The Avian Enigma: “Hibernation” by Common Poorwills

Twelfth International Hibernation Symposium. Institute of Arctic Biology, University of Alaska Fairbanks, USA (pp. 129-138). | Christopher P. Woods and Mark Brigham

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