The largest bird known to employ torpor, it goes into this energy-saving state at night or in the early morning, especially on cold winter days, between which it feeds and functions as usual. In fact, many birds enter daily or nightly torpor, including kingfishers and owls, as do many small mammals.
So how does this relate to hibernation? Well, hibernation is essentially a series of bouts of torpor that each last for many days. Hibernation differs from daily torpor in that it usually involves much lower body temperatures and metabolic rates, and is often seasonal.
In addition, while animals who go into daily torpor wake up and forage or feed in the usual way, hibernating animals either feed off their body fat or on specially stored food. Like many other hibernators, they pig out in the warmer months, gaining up to 13 kilos a week on carb-rich berries and other food. They also prepare a special place to hibernate a hibernaculum, if you want to get technical —a den lined with leaves and twigs.
When winter sets in, the bears hole up in their dens and go without eating, drinking, exercising, urinating or defecating for as long as days, waking up somewhat lighter but apparently none the worse for wear. Hibernation aficionados, however, disagree about whether bears actually hibernate.
Nevertheless, the metabolic rates of bears during this time are comparable to those of other hibernators; their heartrates slow to around just 4 beats per minute, and oxygen consumption drops by around 75 per cent , with the bears taking only one or two breaths per minute.
This suggests that the same physiological processes are at play in overwintering bears as in hibernating animals. Interactive Who hibernates? Test your knowledge or take a guess to discover which of these five animals hibernate.
Found in the Americas, hummingbirds do not hibernate, but, like many birds, they go into a daily or in their case, nightly state of torpor.
They do this during migration to save energy for the next leg of the journey. This is because their body temperature only drops by around 3 to 5 per cent. Did you know some Australian animals hibernate? Found in the United States and Mexico, these are the only birds known to hibernate. Their body temperature drops from 38 degrees Celsius to about 5 degrees Celsius. Do you dream of spending winter under the doona? So, who knows—maybe we will be able to one day.
Hummingbirds do not hibernate, but, like many birds, they go into a daily or in their case, nightly state of torpor. View as an infographic.
Secondly, hibernation is one of the only survival strategies available to tiny creatures who are unable to migrate to escape deadly weather or to find an alternative source of food. And thirdly, smaller animals have a higher surface-area-to-volume ratio, meaning that they are more susceptible to losing heat from their skin. Well, again, not exactly. Insectivorous bats, for example, often wake up from hibernation to forage on warmer winter days before entering another period of multiday torpor.
And hibernating echidnas in the Australian Alps typically wake up in mid-winter to mate. The Arctic ground squirrel of Alaska, Siberia and Canada, for instance, hibernates through seven months of freezing weather, its body temperature dropping to almost minus 3 degrees Celsius!
But hibernation is not, as used to be thought, only restricted to just a few mammals and birds in very cold climates, but occurs in a range of animals living from the arctic to the tropics. The fat-tailed dwarf lemur for example, is a hibernating tropical mammal living in western Madagascar. It hibernates during the long dry season, a time when food and water are scarce. The fat-tailed lemur was not just the first tropical mammal found to hibernate, but the first primate known to do so, with other hibernating lemur species having been found since.
Yet, surprisingly, these European mammals manage to live to a ripe old age of up to nine years—a very long time for a small rodent. How do they manage such remarkable survival rates? Researchers looking at edible dormice in captivity found that, during non-reproductive periods, large fractions of the population disappeared into underground burrows to hibernate. It is identified by metabolic suppression, a drop in body temperature, and torpor — a sleep-like state — interspersed with brief bouts of wakefulness.
Though certain species of fish, amphibians, birds and reptiles are known to lie dormant during cold winter months, hibernation is generally associated with mammals, according to Don Wilson, a curator emeritus of vertebrate zoology at the Smithsonian National Museum of Natural History.
Endothermic mammals — "warm-blooded" animals that generate body heat internally — need a constant energy source to keep their engines running, Wilson told Live Science. And when that energy source becomes difficult to find, hibernation can help them weather harsh conditions.
A special type of fat called "brown fat" accumulates in hibernating mammals, Wilson said. Bats that hibernate develop brown fat on their backs between their shoulder blades, but mammals can also store brown fat in their bellies and elsewhere in their bodies, Wilson said.
Related: 5 fascinating facts about brown fat. Brown fat goes a long way because the hibernating animal draws on it very slowly, reducing their metabolism to as little as 2 percent of their normal rate, according to a study published in the Journal of Neurochemistry. Their core body temperature is also greatly reduced. It generally hovers close to the air temperature in the animal's den but can sometimes fall as low as 27 degrees Fahrenheit minus 3 degrees Celsius in Arctic ground squirrels , according to Kelly Drew, a neurochemist and professor with the Institute of Arctic Biology at the University of Alaska Fairbanks.
Arctic ground squirrels' bouts of torpor last about two to three weeks, Drew told Live Science, and the animals rouse "pretty consistently" for about 12 to 24 hours, before resuming their winter sleep. They repeat this process for up to eight months. But even though Arctic squirrels maintain a lower body temperature than any other hibernating mammal, the changes in their bodies overall aren't that different from those that occur in other hibernating mammals, Drew said.
Related: Sleep tight! Snoozing animals gallery. Once they wake up, the animals are weak and must recover quickly if they hope to survive. Bears , on the other hand, experience less severe changes their body temperature stays within 12 degrees Fahrenheit of normal , allowing them to react to danger throughout the winter.
Unlike bears, small mammals like chipmunks lose heat quite easily, forcing them to wake up every few days to warm up, urinate, and eat. Amphibians and reptiles experience a hibernation-like state of dormancy called brumation to escape the cold, but will move about on warmer days to find water. Even more curious, some species of insects and amphibians, like wooly bear caterpillars and wood frogs produce a natural antifreeze that stops their cells from freezing completely in cold temperatures.
A similar procedure, known as therapeutic hypothermia, is already used as a medical treatment after heart attacks or traumatic brain injury. Leave them be. Bears and other animals are gorging themselves on food this month before bedding down for the winter, so hold off on putting the bird feeders out for just a few more weeks.
Join Rick Sammon — a tireless, prolific, and inspirational image-maker and Teatown neighbor and friend — as he takes you around the world to share his favorite bird photographs and bird photography tips.
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