Hummingbirds exert fine control over body heat
Study finds state of torpor is flexible and variable

Date:
February 2, 2022
Source:
Cornell University
Summary:
Scientists from multiple universities now find there's more than one
level of hummingbird torpor: shallow and deep, plus the transition
stage between levels of torpor and the normal sleep state.



FULL STORY ==========================================================================
At night, hummingbirds lower their body temperature and metabolism
drastically by dropping into an energy-saving state of inactivity called torpor. Scientists from multiple universities now find there's more than
one level of torpor: shallow and deep, plus the transition stage between
levels of torpor and the normal sleep state. Their findings have been
published in the Journal of Experimental Biology.


========================================================================== "There have been a few hints that this ability to fine-tune
thermoregulation was possible," said lead author Anusha Shankar, currently
a Rose Postdoctoral Fellow at the Cornell Lab of Ornithology. "But the
studies were done under laboratory conditions, not the conditions a bird
would encounter in the wild.

It was really exciting to see that hummingbird torpor could be variable
and flexible." Shankar and colleagues from Stony Brook University,
the Swiss Federal Research Institute, and George Fox University, used
infra-red thermal imaging to track the body temperatures of three
hummingbird species in Arizona: Blue-throated Mountain-gem, Rivoli's Hummingbird, and Black-chinned Hummingbird. They measured the temperature emitted from the skin around the eyes of the birds.

The differences in heat generation at various stages are stark.

The normal daytime body temperature of a hummingbird is more than 100
degrees Fahrenheit, even in colder weather. During shallow torpor, their
body temperature drops by about 20 degrees Fahrenheit. In deep torpor,
the bird maintains a body temperature 50 degrees Fahrenheit below its
normal daytime temperature. If human body temperature were to drop a
mere 3 degrees from the standard 98.6 degrees Fahrenheit, we'd be in a
state of hypothermia and, unlike hummingbirds, would need outside help
to get warm again. Size also matters.

"In this study we found that the smallest bird used deep torpor every
night," said Shankar. "The bigger birds sometimes use deep torpor and
sometimes shallow torpor, and sometimes no torpor at all!" Shallow
torpor may have developed to balance energy-saving with the costs of
deep torpor. In deep torpor a hummingbird is probably more vulnerable to disease because its immune system shuts down, plus it is vulnerable to predation and sleep deprivation. This ability to use torpor at varying
depths indicates that at least some birds are capable of fine-tuning
how cold they get in torpor, and how they manage saving energy with the potential costs of dropping way down into deep torpor.

Torpor is not the same as sleep. Sleep uses much more energy and
serves many restorative functions. Daily torpor also differs from
hibernation. Hibernating animals enter a low-energy state for weeks or
months at time while hummingbirds can enter torpor every night.

Many questions remain about what is taking place in the brain during
torpor, how levels of body fat may trigger torpor, and how the
hummingbirds can generate the heat to warm up again -- a process that
can take up to 30 minutes.

"At least 42 bird species use torpor," said Shankar, "But only
hummingbirds, nightjars, and one species of mousebird go into deep
torpor. Studying the range of torpor could help us understand the
evolution of thermoregulation in birds." Support for this research
comes from NASA, the Tinker Foundation, National Geographic Society,
American Philosophical Society, European Research Council, the Swiss
Federal Research Institute for Forest, Snow, and Landscape Research,
George Fox University, the Swiss National Science Foundation, and two
crowd- funded grants from experiment.com.

========================================================================== Story Source: Materials provided by Cornell_University. Original written
by Pat Leonard, courtesy of the Cornell Chronicle. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Anusha Shankar, Isabelle N. H. Cisneros, Sarah Thompson,
Catherine H.

Graham, Donald R. Powers. A heterothermic spectrum in hummingbirds.

Journal of Experimental Biology, 2022; 225 (2) DOI:
10.1242/jeb.243208 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220202111806.htm

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