Shallow Torpor and Deep Torpor in Hummingbirds

You have probably heard that hummingbirds have such a high metabolism that they must eat constantly. On a human scale, a hummingbird would have to drink a can of Coca-Cola every minute just to stay alive. However, it isn’t just the rapid wing beat that requires so much energy, but hummingbirds must maintain their body heat so that organs like the liver and heart can function. A research team has studied the design built into hummingbirds to address this problem. It has to do with shallow torpor and deep torpor in hummingbirds.

Anusha Shankar at Cornell University and her team found that the smallest hummingbirds cool down at night to as low as 3 degrees Celsius. Shankar called that “an incredible ability.”
Hummingbirds can fall into deep torpor at night, something analogous to hibernation. They also have a shallow torpor they can use if they need to wake up quickly. In deep torpor, the hummingbirds can save an average of 60% of their energy relative to their basal metabolic rates. In addition to saving energy, when the hummingbirds are in deep torpor, they are invisible to temperature-sensing predators like snakes.

Hummingbirds sleep with their bills turned up and their eyes closed. While in deep torpor, their breathing is greatly reduced with ten-second periods when they don’t breathe at all. Researchers were impressed with the hummingbird’s energy flexibility. The birds not only have the options of shallow torpor and deep torpor while they sleep, but during the day, they can spend 80% of their time hovering or 80% of their time perching. Studies of gene expression show that genes are being turned off and on in hummingbird tissues in shallow torpor and deep torpor and when the bird is awake.

Hummingbirds are amazing creatures that show incredible complexity in their design. As we watch them around our feeders, we need to be impressed with how their bodies maintain their activity and survive the range of temperatures in their environment. It is just a reminder that “we can know there is a God through the things He has made” (Romans 1: 20). Hummingbird design speaks loudly of the truth of that statement.

— John N. Clayton © 2022

Reference: American Scientist, March/April 2022 page 70.