When a natural environment becomes depleted of some essential factor such as food, water, heat, or oxygen, the animals living in that environment tend to do as you and I would: they leave. This is why birds go south, whales go north, and house mice usually show up in winter. Not all animals are able to leave, however, and for those that don’t, the most effective survival strategy is a depressed metabolic rate. Metabolic depression is effective because it reduces the rate at which the animal uses the dwindling essential factor, extending the time period over which that factor lasts. If the time extension is long enough, the animal may live to see its environment replenished with that factor. Not all animals are capable of metabolic depression, but those that are tend to survive extremely harsh conditions. Think hibernating bears, aestivating lungfish, and cryptobiotic Sea-Monkeys.
I study animals that use metabolic depression. Specifically, I investigate the mechanisms that underlie metabolic depression and the ways animals use metabolic depression to survive harsh environments. One theme of my work explores these traits in hibernating mammals, while a second theme explores them in hypoxia-tolerant fishes.
Metabolic strategies of hibernation
Hibernation is a complex trait that enables animals—mammals in particular—to survive harsh winter months in the absence of food. It involves numerous mechanisms spanning the levels of biological organization, from behaviour to molecular biology. The most important of these—the one that is synonymous with hibernation itself—is torpor. Torpor is the term used for a metabolically depressed state, and it is these states of torpor that enable the hibernating mammal to fast for up to nine months with no detrimental effects.
Torpor is the combined effect of a regulated metabolic depression within the cells and a reduced body temperature set point in the hypothalamus. Because body heat is generated by metabolic processes, the depressed metabolic rate reduces the mammal’s body temperature. And because the temperature set point has been reduced, body temperature remains hypothermic (as opposed to non-hibernating mammals, which elevate their metabolic rates to counter hypothermia and return body temperature to 37ºC). The degree to which body temperature is reduced varies among hibernating species. Bears reduce to approximately 31ºC, while arctic ground squirrels have been measured as low as -2.9ºC. The chemical reactions that comprise metabolism, like all chemical reactions, proceed slower at low temperatures than high temperatures. So, generally speaking, the lower the body temperature, the deeper the metabolic depression and the longer the time period over which the mammal can survive without food.
Despite torpor’s central role in hibernation, hibernating mammals are not in a constant state of torpor throughout the winter. Every two to 21 days, a hibernator will arouse by rewarming its body temperatures to 37ºC for 12 to 24 hours before falling back into a torpid state. These inter-bout arousals benefit the mammals physiologically by clearing metabolic wastes, replenishing metabolic fuels, rebuilding synaptic connections, and even repaying slow wave sleep debt (torpor and sleep are different physiological states).
We know a lot about torpor and hibernation, largely because our longstanding curiosity with dormant animals has led to much research on, and consequently much knowledge of, hibernation. But hibernation’s multifaceted complexity means there is much we have yet to understand.
Our hibernation research
We explore the metabolism of torpor using the 13-lined ground squirrel, Ictidomys tridecemlineatus, a North American squirrel that is among the most extreme of hibernating mammals. Specifically, we investigate how the squirrel’s gut microbiome—the population of microbial species that live in its intestine—influences the metabolism of torpor. There are two reasons we believe the microbiome may influence torpor: first, the gut microbiome is known to influence the host’s physiology in various ways, including expanding their metabolic capabilities; second, the squirrel’s microbiome sees massive changes in microbial abundance and species diversity over the hibernation season due to long-term fasting and large temperatures swings. Gut microbes affect host physiology by producing chemicals called metabolites that the host absorbs. The metabolites are the products of microbial metabolism and vary among microbial species. Therefore, because these species change over the hibernation season, so too do the metabolites that are made available to the squirrel. These different metabolites may influence the squirrel’s metabolism in different ways.
Why this research is important
A fundamental understanding of torpor’s metabolic mechanisms will accelerate the development of technologies to safely depress the metabolic rates of animals that have not evolved the natural ability to do so, including humans. This technique, called synthetic torpor, has tremendous applications. For example, victims of stroke, heart attack, and other ailments that reduce oxygen supply to critical tissues would benefit from being placed in synthetic torpor because this would slow the rate of tissue damage and provide doctors with more time to administer treatment. Similarly, synthetic torpor would prolong the survival time of a transplanted organ when isolated from the body. Finally, major health-related challenges of long-duration human spaceflight, including crewmembers’ continuous exposure to microgravity, space radiation, and a small, psychologically demanding physical space, would simultaneously be mitigated if crewmembers were placed in synthetic torpor. This would also reduce crewmembers’ food consumption and waste production rates, thus relaxing demands on the spacecraft’s limited mass, volume, and power capacities.
Part of my work, in collaboration with colleagues at Northern Arizona University, NASA Ames Research Centre and elsewhere, explores synthetic torpor. These technologies are at relatively early stages of development, but there is ample evidence to show that synthetic torpor is a viable idea and one with very real benefits to biomedicine and human spaceflight programs. Needless to say, this is exciting.
Metabolic strategies of hypoxia tolerance
To best understand how torpor aids the hypoxic survival of animals, we should first discuss why oxygen is important to animal survival, and then describe what strategies besides torpor may help an animal survive in hypoxic environments. We’ll start with the big picture.
Life is defined as the avoidance of decay into disorder and equilibrium, the natural state of matter in the universe. Living things manage this by spending energy to maintain their required state of order and disequilibrium with respect to the universe, and this is what separates them from non-living things. The required energy comes in the form of adenosine triphosphate (ATP), a molecule that stores and transfers the energy extracted from ingested food. Large quantities of ATP are required to maintain order, and animals supply these quantities through a process called oxidative phosphorylation (OxPhos), which is oxygen-dependent. While oxygen-independent energy supply pathways exist (e.g., anaerobic glycolysis), OxPhos is more efficient than these at extracting energy, producing up to 18 times as much ATP from each unit of food. Because of this, almost all animals on earth have evolved an ultimate reliance on oxygen to supply their cells with the energy needed to sustain an orderly state, and subsequently, life.
But there are many environments on earth that are low in oxygen (i.e., hypoxic), and despite the critical importance of oxygen in supplying cellular energy, animals can be found living in most of them. These environments include the subterranean, the high-altitude, and the aquatic. Animals living here risk their ability to supply ATP via OxPhos, and subsequently, risk upsetting the balance of energy supply and demand. When this scale tips too heavily towards the demand side, the animal’s orderly state becomes compromised and death is sure to follow. But as animals are found living—even thriving—in these hypoxic environments, they are maintaining the balance somehow.
There are three general strategies animals use to balance energy supply and demand in hypoxic environments. The first two—improved oxygen uptake and upregulated anaerobic (i.e., oxygen-independent) ATP production pathways—aid in supplying ATP, while the third—metabolic depression—aids in reducing ATP demand. Animal species differ in not only their reliance on each strategy, but also their ability to make use of each strategy. For example, metabolic depression is an effective survival strategy for goldfish in severely hypoxic environments, but the related zebrafish is unable to depress its metabolic rate under the same conditions. In the end, different combinations of these three strategies are able to maintain cellular energy balance, and it is probably true that different species use different combinations to achieve hypoxia tolerance. We call a species’ particular combination its total hypoxic response (THR).
Our hypoxia research
We explore how animals concurrently use aerobic metabolism, anaerobic metabolism, and metabolic depression (i.e., their THRs) to survive hypoxia, and how and why this varies among different species. We are especially interested in metabolic depression, the least studied (but hypothesized to be the most effective) of the hypoxic survival strategies. We address these questions using fishes because hypoxia tolerance has evolved numerous times independently among the fishes owing to environmental hypoxia being highly prevalent in aquatic habitats. Studying hypoxia tolerance using fishes therefore allows for effective comparisons that aid in understanding the general concepts of hypoxia tolerance (e.g., a comparison of two fish species that have independently evolved tolerances of hypoxia may shed light on universal traits enabling that tolerance).
To measure the THR, we have built an apparatus called a calorespirometer that simultaneously measures a fish’s oxygen uptake, anaerobic ATP production rate, and metabolic depression under a variety of environmental conditions. These studies have revealed the THR to be a plastic response, changing with the biology of the animal and the abiotic aspects of its hypoxic environment.
Why this research is important
Besides a better grasp of nature and its workings, there are two important areas that directly benefit from an understanding of how animals can survive low-oxygen environments. First, anthropogenic increases in global temperature and agricultural runoff are increasing the prevalence of aquatic hypoxia in both marine and freshwater environments. This poses a significant threat to the survival of innumerable species and populations. An improved understanding of the mechanisms underlying hypoxic survival can therefore help mitigate this threat, benefitting predictive models and conservation efforts that help identify and protect these potentially vulnerable animals. Second, the damaging effects of stroke and heart attack are the result of tissues—brain and heart, respectively—being starved of oxygen. Because the tissues of animals that have adapted to low-oxygen environments have evolved ways of mitigating this damage, understanding how these mechanisms work and how they might be applied to patients could present obvious benefits to the treatment of these ailments.