Sep 16, 2020 | Featured Web Article

Hummingbirds and Torpor

Hummingbirds have evolved a unique ability to go into a trancelike state called torpor to help them survive cool temperatures and periods of inactivity, especially while sleeping at night. The birds are unresponsive and appear to be dead when in this state.
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Most of the hummingbirds that nest in North America migrate to tropical habitats during the nonbreeding seasons, because regions with icy winters cannot support birds that require nectar and flying insects. Of course, some areas of the United States are temperate enough to provide insects and flowers all year, and these places may have hummingbirds throughout the year.

Most North American hummers can tolerate cold for periods ranging from a few hours to a couple of days as long as there is sufficient food available. They do not ordinarily build quantities of body fat except during migration. Hummingbirds have evolved a unique ability to go into a trancelike state called torpor to help them survive cool temperatures and periods of inactivity, especially while sleeping at night. The birds are unresponsive and appear to be dead when in this state.

A hummingbird's body temperature drops significantly during torpor, its heart rate slows, and its metabolism runs at a greatly reduced rate. This saves a hummingbird from burning up all of its energy and starving because the metabolic rate of a torpid hummingbird is as little as 1/50 the rate of an active bird. A hummingbird's heart normally beats from 1,000 to 1,200 times per minute, but in torpor, it beats only 50 times per minute! Arousal from torpor usually requires 30 minutes or more until body temperature returns to normal.

New research published this month in the journal Biology Letters suggests that in certain conditions hummingbirds can lower their temperature to near freezing. The study observed 26 hummers consisting of six different species of various sizes at high elevations in the Andes mountains. All of the hummers entered torpor at night, with one black metaltail hummingbird's temperature dropping to a surprising 3.3 degrees Celsius (37.9°F)—the lowest recorded temperature for a bird and non-hibernating mammal. (The hibernating Arctic ground squirrel can lower its temperature to below freezing.) The depth and duration of the topor varied by species.

Torpor has its drawbacks, though, as a motionless bird makes for easy prey. This is a bigger risk for hummers in lower elevations, as the Andean hummingbirds in the study have relatively few predators.



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