What Is Torpor? Animals Turning Themselves Down to Save Energy
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Some animals let their body temperature and metabolism drop far below normal for hours or days, which saves an enormous amount of energy. It is a controlled state rather than a failure, and it is more widespread among animals than hibernation.
What happens in the state
An animal entering torpor allows its body temperature to fall towards the surrounding temperature rather than defending the usual value, and its metabolic rate drops accordingly, in some species to a few percent of the normal level. Heart rate and breathing slow dramatically. The animal becomes unresponsive and cannot react quickly to disturbance, which is the cost. Entering and leaving are both actively controlled rather than passive, and rewarming is achieved by burning stored fat, particularly a specialised tissue adapted for generating heat directly rather than by shivering. That rewarming is expensive, which means the state pays only when the savings during the bout exceed the cost of getting out of it, and short frequent bouts are therefore worse value than long ones.
The forms it takes
The state occurs over different durations and the names reflect that:
- •Daily torpor, lasting hours, used by small birds and mammals overnight or during the day depending on when they are active
- •Hibernation, a series of long bouts through winter interrupted by periodic rewarming that is itself unexplained and expensive
- •Aestivation, the equivalent response to heat and drought rather than cold
- •Torpor during food shortage regardless of season, used opportunistically by several species
- •Torpor in nestlings and in incubating birds, which is more common than was recognised
- •Facultative use, where an individual enters the state only when conditions require it, as opposed to obligate seasonal patterns
Who does it
The distribution is wider than the familiar examples suggest. Small mammals dominate, since a small body loses heat fast and the savings are proportionally largest, and bats, rodents and small marsupials use it extensively. Hummingbirds enter it nightly, which is necessary given that their metabolic rate while active is among the highest of any animal and they cannot store enough fuel to last a night. Several bird groups use it, including nightjars, one of which is the only bird documented hibernating for extended periods. Primates were thought not to until a Madagascan lemur was found hibernating, which was a surprise. Large mammals mostly do not, since the savings scale poorly with size, and the winter dormancy of bears involves a modest temperature drop and is a different arrangement from deep torpor.
The costs of doing it
The state is not free and the drawbacks explain why animals do not use it more. A torpid animal cannot respond to a predator and is essentially defenceless, which is why the state is generally entered in a burrow, a roost or somewhere else protected. Rewarming takes time as well as energy, so an animal disturbed cannot simply wake, and repeated disturbance can exhaust the reserves the state was conserving. Immune function is suppressed during deep bouts, which is one proposed explanation for the periodic rewarming that punctuates hibernation and which costs a large share of the total energy budget. Memory appears to be affected in some species, with evidence of losses over hibernation. And reproduction generally cannot proceed during it, so the state constrains the timing of breeding in species that use it seasonally.
Why it interests researchers
Work on the state has attracted attention well outside ecology. The tissue damage that would be expected from such extreme changes in temperature, blood flow and metabolism does not occur, and understanding the protective mechanisms has obvious medical relevance for organ preservation, for stroke and cardiac arrest and for trauma care. Muscle and bone do not waste during long dormancy in the way they do during comparable inactivity in humans, which is studied for the same reasons and for spaceflight. Inducing a comparable state in animals that do not naturally enter it has been achieved experimentally by several routes, which raises the possibility of applications and a long way from any use in people. The seasonal timing mechanisms are studied as examples of long-period biological clocks. Each of these is a case of a natural solution being examined for what it can be borrowed for.
The takeaway
Body temperature and metabolism fall far below normal, in some species to a few percent of the usual rate, under active control rather than as a failure. Rewarming is expensive, so short bouts are poor value. Hummingbirds enter it nightly because they cannot store enough fuel, and the absence of the expected tissue damage is why the state is studied medically.