By Vidya Rajan, Columnist, The Times
In summer, I feel like life slows down, just a little, despite plants growing willy nilly. Maybe it’s because there are more hours of daylight, or maybe because bug-watching or gardening makes one lose track of time. But I fell asleep sitting outside one day last week and got thinking about the sorts of metabolic slowdown that occur when animals turn in for whatever reason. As usual, there are terminology words. They are somewhat related, but have different nuances, based on the body temperature (heterothermic and poikilothermic). Heterotherms maintain a set body temperature irrespective of the outside temperature, whereas poikilotherms’ body temperatures fluctuate and with the ambient temperature. The second consideration is the source of energy (endothermic and ectothermic). Many endotherms are also homeotherms (birds, mammals) and many endotherms are also poikilotherms (reptiles, fish). But there are exceptions where ectotherms can maintain a fairly steady body temperature if their outside temperature seldom varies. Many of the animals we will consider in this article (e.g. birds) are ectotherms which can temporarily lower their body temperature to match the environment as an energy-saving mechanism.
Figure 1: Illustration of two distinct scales used to categorize animals based on their thermal physiology. (A) The top scale differentiates organisms based on their primary source of body heat, ranging from endotherms to ectotherms. (B) The bottom scale categorizes organisms based on the stability of their internal body temperature, ranging from poikilotherms to homeotherms. Figure and legend reproduced from Reference [1] under Creative Commons License 4.0.
Saving energy is important. Energy is the currency of life, and it is prudent to conserve it, especially if it is used to make heat, which is the most inefficient form of energy and simply diffuses into the environment. As a result, almost all organisms show dormancy: a period where metabolism slows down to save energy. For example, many plants drop their leaves and slow down their metabolism. Since animals have such varying energy needs, their dormancy periods are different: summer dormancy is called aestivation; dormancy in cold temperatures can be brumation or hibernation; brief periods are called torpor; and diapause fittingly pauses physiology and development. I will examine each of them briefly.
Aestivation is sleep to overcome hot temperatures and shortages of water. This process is often seen in animals that live in water bodies which may dry up. For example, sea cucumbers (Apostichopus japonicus) enter aestivation when temperatures rise above 25oC with an impressive series of physiological changes: they stop feeding, lose weight, and their digestive system atrophies. They can remain in this state for over three months in the laboratory, but, eventually when temperatures drop they start feeding and their digestive system regenerates. African lungfish (Protopterus sp.), which are remarkable animals which can breathe air and endure the drying up of their ponds by burrowing down and secreting a mucus cocoon around themselves; their metabolic rate drops and they switch from making toxic ammonia as a waste to storing less toxic urea. Other lungfish[2] can endure low water but not drying out. Other animals which aestivate are African clawed toads (Xenopus laevis) which deal with drying out of their ponds by drying themselves out. This causes their blood to thicken and become less efficient at transporting oxygen, but since metabolic activity is also depressed, they somehow endure.[3] The challenges of aestivation are dehydration, degeneration of tissues and the suppression of metabolism and the immune system. Curiously both endotherms (e.g. dormouse) and ectotherms (reptiles, echinoderms, snails, amphibians, worms and many more) regularly aestivate. However, the difference is that the homeotherm has to occasionally wake up to stabilize its body temperature by physiological processes whereas a ectothermic poikilotherm generally endures until conditions get better.
The next dormancy process include brumation vs. hibernation. These again depend on whether an animal is ectothermic or endothermic. Ectotherms undergo brumation and their body temperature drops to match that of the environment,[4] whereas endothermic animals hibernate. The key here is that the ambient temperature determines the activity of the animals that brumate. Even during their inactive period, increase in ambient temperature causes them to become active, and seek water and warmth. Another curious discovery was the effect of brumation on memory. Fire salamanders which had been trained to a task and then forced into brumation remembered their training due to a rapid drop in temperature which prevented the enzymatic pruning of neurons.[5] But the brain needed a little time to consolidate the memory. Snails which were trained to a task and then rapidly chilled lost the memory, probably because the memories were not yet consolidated by proteins which failed to be synthesized in cold temperatures. On the other hand, brumation caused its brain cells to die through apoptosis.[6]
Hibernation (a type of multi-day torpor) is familiar and it’s seen with bears, marmots, squirrels, and even some insects such as wasps and ladybugs[7]. Whereas marmots, ground squirrels and some bats show true hibernation where their body temperatures plummet, bears show a type of hibernation called “denning” in which their body temperature may drop a little, but they can quickly warm up. In all hibernating animals, heart rate, metabolic rate and other energy-demanding biochemistry slows down.[8] But since their metabolism still ticks over, energy is still consumed and is provided by food stores, such as fat, or seeds and nuts that they store. Such animals may emerge from hibernation to sleep or to activate their immune function. Bears do not usually eat or excrete, for as long as six months. This is the reason why bears feed single-mindedly in the Fall months. (In this context, the US National Parks Service runs a “Fat Bear Week” contest out of Katmai National Park in Alaska. Images of bears emerging from hibernation and preparing for hibernation are gathered and the fattest bear is voted on. Scroll down the NPS page to see the contenders for this year.)
Diapause is the most curious type of dormancy of them all. It is an adaptation to overcome stressful or unfavorable environmental conditions by going into a sort of arrest state, much like the game of “Freeze” that children play. Insects, fish, reptiles, birds and mammals show diapause, although in vertebrates it occurs in the embryonic stage.[9] Monarch butterflies (Danaus plexippus) can show diapause at any stage – egg, pupae or adult – which makes them even more amazing than they already are, and fruit fly (Drosophila melanogaster) moms can arrest egg development in their ovaries until conditions improve.[10] Diapause is very common in mammals with over 100 animals (rodents, kangaroos and mustelids, roe deer) exhibit it.[11] This causes the egg or embryo to arrest its biochemistry, including its growth and development until conditions improve. The only mammalians who do not show diapause are primates.
But who wants a pregnancy that lasts longer than it already does?
[1]. Hoey, M. (2025). 11.3 Thermoregulation. [online] Concepts in Biology. Available at: https://lmu.pressbooks.pub/conceptsinbiology/chapter/thermoregulation/ [Accessed 25 Aug. 2026].
[2]. A random curious fact is that the African marbled lungfish has the largest genome of any vertebrate at 133 billion base pairs (humans have 3 billion base pairs.)
[3]. Link to external site (2023). Aestivation in Nature: Physiological Strategies and Evolutionary Adaptations in Hypometabolic States. ProQuest, [online] 24(18), p.14093. doi:10.3390/ijms241814093.
[4]. Brumation occurs largely in animals that live in moderately warm areas, such as reptiles and amphibians. The danger of freezing and consequent death due to the disruption of cells by ice crystals restricts the lower temperature range animals brumate at.
[5]. Wilkinson, A., Hloch, A., Mueller-Paul, J. and Huber, L. (2017). The effect of brumation on memory retention. Scientific Reports, [online] 7(1). doi:10.1038/srep40079.
[6]. A type of cellular suicide where a cell undergoes “programmed” death to protect other cells in the organism due to internal or external stresses.
[7]. Although the technical difference between brumation and hibernation and torpor in these animals is not clearly delineated in any of the papers I read.
[8]. Geiser, F. (2013). Hibernation. Current Biology, [online] 23(5), pp.R188–R193. doi:10.1016/j.cub.2013.01.062.
[9]. Ewert, M.A. (1991). Cold torpor, diapause, delayed hatching and aestivation in reptiles and birds. Egg Incubation, [online] pp.173–192. doi:10.1017/cbo9780511585739.012.
[10]. Denlinger, D.L. (2023). Insect diapause: from a rich history to an exciting future. The Journal of Experimental Biology, 226(4). doi:10.1242/jeb.245329.
[11]. The only artiodactyl (even-toed ungulate; ruminant) in the group.
