If hummingbirds slept, they’d die of starvation. So how do they get rest?

In between sleep and death exists a state called torpor, and it’s the magical way hummingbirds sleep!

In our previous newsletter, we learnt many cool things about hummingbirds - like the fact that while they can fly in all directions, they can’t walk! Or the fact that their metabolic rate forces them to refuel every 10-15 minutes, which means they have no time to sleep.

Image credit: The Spruce. This is a photo of a hummingbird, still awake… sorry! But anyway, Hummingbirds can go into torpor any time of the year when temperature and food conditions demand it. (If torpor lasted for long periods, we would call it hibernation.)

Today, we’ll explore how they get any rest at all, if sleeping meant certain death.

Torpor - a state in between sleeping, and death

Think of torpor as the hummingbird version of a bear’s hibernation. Actually - it turns out bears don’t really hibernate; they go through torpor, too! The difference between the two: torpor typically lasts for several hours while hibernation lasts several months. Because bears actually do wake up from time to time, what they go through is technically called torpor, and not hibernation.

Image credit: North American Bear Centre. Bears technically hibernate during winter, but aren't sleeping the whole time. Hibernation for bears simply means they don't need to eat or drink, and rarely urinate or defecate (or not at all). There is strong evolutionary pressure for bears to stay in their dens during winter, if there is little or no food available.

In simple terms, torpor (or hibernation) is a state of decreased physical activity so extreme that an animal’s heart beat, metabolic rate and body temperature drops to unfathomable amounts.

What does torpor do to a body?

Firstly: a drastic drop in body temperature!

A hummingbird’s daytime body temperature measures at around 38C - since that’s close to ours, you probably know what that feels like. Now imagine dropping your body temperature to just 10C: that’s how low a hummingbird’s nighttime torpor temp can get down to!

Secondly: a heart rate that slows down by 95% - 99.96%!

In conjunction with a lowered body temperature, their heart rate also becomes extremely sluggish. Hummingbird hearts beat at around 1200bpm while awake and active, at 500bpm when resting - and then at fewer than 50bpm while in torpor! That’s a drop of 95% - 99.96%.

Thirdly: an inability to wake up, even when prodded!

If you ever get the chance to poke at a hummingbird sleeping at night, you’ll be forgiven for thinking they’re actually dead, and not asleep. While in a state of torpor, hummingbirds lower their vital signs to a point where one would barely think they are even alive.

When in this state of suspended animation, hummingbirds can’t be awakened, even by outside stimuli! Poke and prod them all you like (but gently please, they are so tiny) and they won’t react.

While this does mean that hummingbirds are easy prey for predators while they sleep, they don’t seem to have many natural predators who go out of their way to hunt them.

Somehow, their bodies know to begin ‘recharging’ a few hours before the sun rises. It takes them 20 minutes to an hour, but eventually, hummingbirds begin revving back to life: their body temperature and heart rate begin to rise, and their wings start vibrating, which helps to warm the blood. Once the warm blood is completely circulated throughout its body, hummingbirds are ready to go out and repeat their exhausting day of binge-eating again!

Image credit: Good Housekeeping

While it seems extreme, all these adaptations help a hummingbird burn less calories while sleeping, so they don’t die of starvation by the next morning. I love how these birds are just so unique!

More on next week’s post: how violently territorial hummingbirds sometimes fight to their deaths! :O

Image credit: The New York Times