Doug Bonderud

Jan 11th 2021

Live Fast, Fly Hard, Sleep Frosty: The Science of Suspended Animation


Hummingbirds already feel like avian outliers. With their hovering ability that’s more akin to insect-like flapping than familial feathery frameworks and their hearts racing at a ridiculous 1,200 beats per minute, it’s no surprise that many species require up to 500 flowers worth of nectar simply to survive each day. Now, researchers have added a new evolutionary twist to the already-bonkers narrative of this live-fast, fly-hard bird: suspended animation to solve for cool, high-altitude climates.

That’s right — when the going gets cold, these the tiny tweeters get even colder, dropping their body temperatures to a near-freezing 38 degrees Fahrenheit. Here’s how.

Everybody, Just Cool It!

The high altitudes of the Peruvian Andes offer an ideal hummingbird environment rich in nectar and short on predators — at least during the day. At night, temperatures quickly tank, often coming close to the freezing mark. And for a bird that survives with a mystical combination of speed, nectar supply and sheer force of will, this is a serious problem.

The solution? Suspended animation. Researchers from the University of Pretoria in South Africa and the University of New Mexico captured six species of Andean hummingbirds and found that all were capable of entering a state of “torpor,” which saw their body temperatures and heart rates significantly lowered, in turn making it possible for them to survive chilly mountaintop nights. And while some species like the sparkling violetear reduced their bodies to a set temperature of around 46 degrees Fahrenheit, others like the black metaltail displayed adaptive climate controls — one bird, in particular, reached a low point of 38 degrees Fahrenheit, the lowest temperature ever recorded for a hummingbird.

The Science of Suspended Animation

So, why suspended animation? Put simply, the lower the temperature and the slower the heartbeat, the less energy needed to survive. This is a big deal for small creatures like hummingbirds, which often run at 96 degrees or more during the day. Blair Wolf, professor of biology at the University of New Mexico, notes that this extreme temperature differential could allow hummingbirds to cut their energy usage by 95% overnight. It also makes them “cold as a rock,” according to Wolf. “If you didn’t know better, you’d think they were dead.”

But they’re not — when sunrise starts, the birds begin warming by around one degree per minute, vibrating quickly to warm up chilly muscles and get systems back up to speed. “You see the bird quivering there,” says Wolf, “then all the sudden its eyes pop open and it’s ready to go.”

Tracking the Torpor Trail

If this cooling off and chilling out sounds familiar, that’s no surprise — many species enter the long-term low-temperature state of hibernation during the winter months.

But that’s not what’s happening here. Instead, the birds are conquering climate concerns by leveraging what’s known as “torpor”. While hibernation and torpor share the characteristic of heterothermy (hypermetabolic states associated with low body temperature), hibernation lasts consecutive days or weeks and relies on food stores, whereas torpor lasts less than 24 hours and relies on daily food foraging.

The length of hummingbird torpor times is also relevant; the researchers found that birds who used torpor states only briefly could lose up to 15% of their body weight, while hummingbird torpor that lasted overnight caused a weight loss of just 2%. There’s also anecdotal evidence to suggest that some hummingbirds may even enter a state of true hibernation.

The Cold and the Beautiful

Hummingbirds are fascinating, fragile creatures that somehow manage to survive with super-speed heart rates and a need for nectar that can’t be sated. As it turns out, they’re also capable of extraordinary organism action — enduring Andes mountain nights by turning off the motor and masquerading as tiny, low-temperature feathered rocks until the sun rises again.

Live fast. Fly hard. Sleep frosty.