Hibernation, estivation, and torpor are all similar behaviours found in nature, but what are they, how are they similar, and more importantly, how are they different?
Hibernation is probably the easiest to define. Most famously, bears go through hibernation annually during winter. Hibernation is characterised by a significant drop in metabolic rate. It is a strategy for animals to survive colder weather when food levels in the environment are low. There are two levels of hibernation. Bears fall into the "light hibernation" group as their body temperature only drops a few degrees and their body weight can drop by up to 40%. They are also able to be woken relatively easily, although this is dangerous for the bear because it means they might not have enough fat storage to last the rest of the winter if they wake up for too long. Other species undergo "true hibernation" like the Common Poorwill of North America. It is a bird that hibernates and its body temperature plummets to a very chilly 5C, or 41 f. Since their body temperature drops so significantly, their metabolism plummets and they end up using only about 10% of the oxygen compared to what is required during while awake.
Estivation is very similar to hibernation. Hibernation is the response by some animals to low food availability in the cold winter. estivation is the response by some animals when there is less food during hot and dry times of the year. It is found in ectothermic species around the equator. We spoke about it in Episode 2 of the Biome Podcast, which you can listen to here.
When an animal estivates, they generally bury themselves so that their temperature stays relatively constant. Some common estivating animals are snails, lungfish, earthworms, amphibians, and even reptiles. The massive Nile crocodile has been known to estivate when necessary. A real, concern, especially for species like lungfish that need water eventually, is that the drought of citation causing the animal to go through estivation is prolonged, in which case, animals can still die.
Torpor is similar to hibernation and estivation in that it is a period of lowered metabolic activity. However, unlike the other two which can last months, torpor is generally much shorter time period. Animals that go through torpor usually do it daily. Usually t is the smaller endotherms, so animals that can regulate their body temperature that undergo torpor. This is because this creatures need a high level of constant energy to maintain their body temperature as their slight mass will lose heat extremely fast. Animals like hummingbirds will generally go into this state during the night so that they don't use all of their nectar stores to keep their body temperature up when they aren't looking for flowers. Bats and some small rodents are known to undergo torpor during cold nights.
A danger with torpor is that the animal becomes very sluggish during the cold night and becomes easy prey for predators.