Depending on where you live, you may either love or hate the idea of a night’s sleep spent under cold, frigid conditions. For some, this may seem like an attractive alternative to midnight heat and humidity, while for others, it’s a teeth-chattering nightmare.
As humans, we can regulate our body temperatures just like every other mammal species, using a range of physiological adaptations. Shivering, blood vessel constriction and hair erection all help to warm our bodies when conditions get cold. We also have many behavioural responses (a bar of hot chocolate and a blanket can work wonders). These actions – along with others – help us to survive in some of the most challenging environments. But what do you do if you’re one of the smallest birds in the world?
Some species live in the high Andes of South America, where overnight temperatures drop to around freezing.
As a group, the hummingbirds are some of the most recognisable birds, well known for their bright colours and energetic flight. As they zip from flower to flower, their wings can flap up to 80 times a second, a seemingly impossible feat. To maintain this level of hyperactivity, hummingbirds have an incredibly high metabolic rate - the highest of any homeothermic animal. As a result, they burn through energy at a staggering rate and need to spend most of the day foraging for sugar-rich nectar.
Some species live in the high Andes of South America, where overnight temperatures drop to around freezing.1 So as night falls and the hummingbirds retreat to the trees to roost, they are faced with a problem – having to go hours without food to make up for their rampant energy usage, they will almost certainly die.
Luckily, evolution has come up with a powerful solution for this hummingbird headache. To conserve energy overnight, the birds turn down their metabolic rates and enter a state known as torpor2. Torpor is similar to hibernation. However, it differs in that it is a temporary state, maintained only for a short period. During this state, the body temperature of some hummingbirds drops from around 40°C (104°F) to as low as 3.3°C (38°F). This particular temperature was recorded in a black metal tail hummingbird, a species native to Peru. The figure - appearing in a 2020 study published in the journal Biology Letters set a new record for the lowest temperature recorded in a bird or non-hibernating mammal.
During torpor, hummingbirds can often appear dead. Their heart rates and breathing rates fall dramatically, with few stimuli capable of rousing them. But even with this extreme adaptation, the birds can lose as much as 10% of their body mass overnight, making the early morning period a critical one. It takes less than half an hour for a hummingbird to awaken from torpor, during which time they shiver and vibrate their wing muscles to generate heat before taking to the skies with just enough energy reserves to fuel their first few feeding trips.
As torpor is an adaptation to environmental conditions, different hummingbird species use it differently, depending on their distribution. Those in colder environments require more profound, more extended forms of passivity to survive. In contrast, species in warmer climates generally don’t have to go to such extremes, reducing their body temperatures in a less pronounced way.
Larger hummingbirds are more plastic in their use of torpor, entering the stateless frequently than smaller species. This interspecific flexibility is likely to be a factor behind the success of hummingbirds across a range of geographical and environmental regions. Although they are restricted to the Americas, hummingbirds can be found as far north as central Alaska and as far south as Tierra del Fuego in habitats such as mountains, rainforests and deserts.
Other birds such as swallows and poorwills are also known to enter torpid states, but hummingbirds are indeed the masters. Their impressive ability to drastically reduce body temperature is put into context when you consider that humans can only tolerate a drop of just 2°C in core body temperature before physiological problems arise. Hummingbirds, in comparison, simply shake off the effects of their overnight chills, with it also helping them during times of food shortages.
For this reason, hummingbird torpor has even caught the attention of NASA, who in 2013 began researching whether it would be possible to induce a torpor-like state in humans, potentially making it easier to journey to Mars. While it is unlikely that humans will ever compete with the record-breaking black metal tail hummingbird, it is a fascinating example of how the ingenuity of mother nature can inspire technological advances.
Wolf, B.O., McKechnie, A.E., Schmitt, C.J., Czenze, Z.J., Johnson, A.B. and Witt, C.C., 2020. Extreme and variable torpor among high-elevation Andean hummingbird species. Biology Letters, 16(9), p.20200428. ↩
Spence, A.R. and Tingley, M.W., 2021. Body size and environment influence both intraspecific and interspecific variation in daily torpor use across hummingbirds. Functional Ecology. ↩