Tuesday, January 29, 2019

Why You Can’t Hibernate the Winter Away

A reposting of an original article from January, 2015.

You open your eyes, slap the alarm, and pull the covers a little tighter around your shoulders. It’s still dark outside and you dread the moment that you step out from under the warm comforter and the cold sucks your breath out. Can’t you just hibernate and sleep the winter away?

A dormouse in his snuggly hibernation state.
Image by Krysztof Dreszer at Wikimedia.
Actually, no. Hibernation and sleep are two completely different physiological processes (shown by studies of brain function). And chances are, you don’t have the physiological bits needed to hibernate safely.

Hibernation has more to do with energy and body temperature than it does with sleep. Hibernation is defined as a process in which an animal allows its body temperature to approximate the environmental temperature for several days or longer. It is a strategy that some animals use during periods of food shortage to conserve the energy that would normally be used to generate body heat. When food is scarce in the winter, the animal will lower its metabolism (the burning of food molecules to create energy and heat), which will result in the animal having less energy (and entering a sleep-like state) and less heat (until the body approaches the environmental temperature). So really, hibernation is the reduction of metabolism when food is scarce. Lack of activity and cold body temperatures are just the by-products.

Almost all species that hibernate are small mammals, including some hamsters, dormice, jumping mice, ground squirrels, marmots, woodchucks, bats, marsupials and monotremes. Bears, common examples of hibernating species, are actually debated by scientists as to whether they should even be considered hibernators due to the fact that their metabolisms and body temperatures do not decline as much as those of other hibernating species. The only bird species known to hibernate is the poorwill.

Each hibernating species has a specific range of body temperatures that their body can endure. Their first line of defense is to find a hibernaculum (a chamber or cavity in which to hibernate that is more insulated than the exposed environment). If the hibernaculum becomes so cold that the animal’s body temperature drops below its minimum endured range, it will either increase its metabolism slightly to raise its body temperature or it will arouse (wake up). Arousal is the process of increasing metabolic heat production to near-normal levels. All hibernating species seem to undergo multiple periods of temporary arousals during hibernation and scientists are still unsure why. Increasing the metabolism and body temperature from lower levels is an energetically costly process (similar to how your car uses more gas to accelerate than to maintain a higher speed). In most hibernating species, the process of increasing the metabolism uses a specialized tissue called brown fat.

Fat cells come in two main types: white fat and brown fat. White fat, the squishy stuff that we constantly try to diet and exercise away, is filled with lipids (fats) that we store to generate energy in the future. Brown fat cells also contains lipids, but they are specialized to break them down faster. Brown fat is found in newborn mammals and adult hibernators and is commonly located on the upper back, neck, chest and belly (like a vest) and around major arteries. Brown fat cells have lots of mitochondria (the metabolic parts of the cell that break down food molecules like lipids to generate energy). Brown fat mitochondria is specialized in that they have a protein called uncoupling protein 1 that causes them to generate heat rather than energy when they break down lipids. When the body becomes stressed, it releases norepinephrine, a stress hormone, which causes brown fat cells to increase the rate at which they break down lipids to generate heat. This heat warms the major arteries and increases blood flow, which then distributes the heat throughout the body.

A PET scan shows brown fat in a human.
Image by Hellerhoff at Wikimedia.
Although humans are born with a fair amount of brown fat, we lose it as we age. More specifically, it converts to white fat. We used to think that we lost it completely, but in recent years we have learned that some lean adults maintain a few pockets of brown fat in their necks and chests that obese people are more likely to lose. Researchers are currently exploring if and how we can convert some of our adult white fat to brown fat in order to increase our metabolisms and potentially combat obesity and diabetes.

So for now, we can’t hibernate the winter away. But continuing research into hibernating animals may hold an important secret to our own health.

Tuesday, January 22, 2019

Nature Shapes Faithful and Unfaithful Brains

A reposting of an original article from January 22, 2017.

Among monogamous animals, some individuals are more faithful than others. Could these differences in fidelity be, in part, because of differences in our brains? And if so, why does this diversity in brain and behavior exist?

A snuggly prairie vole family. Photo from theNerdPatrol at Wikimedia Commons.

Prairie voles are small North American rodents that form monogamous pair bonds, share parental duties, and defend their homes. Although prairie voles form monogamous pairs, that does not mean they are sexually exclusive. About a quarter of prairie vole pups are conceived outside of their parents’ union.

Not all male prairie voles cheat on their partners at the same rates. In fact, some males are very sexually faithful. It turns out, there are both costs and benefits to being faithful and to cheating. Mariam Okhovat, Alejandro Berrio, Gerard Wallace, and Steve Phelps from the University of Texas at Austin, and Alex Ophir from Cornell University used radio-telemetry to track male prairie voles for several weeks to explore what some of these costs and benefits might be. Compared to males that only sired offspring with their own partner, unfaithful males had larger home ranges, intruded on more territories of other individuals, and encountered females more often. However, these unfaithful males were also more likely to be cheated on when they were away (probably because they were away more). I guess even rodents live by The Golden Rule.

Maps of how paired male voles in this study used space. The solid red/orange/yellow peaks show where a faithful male (in the left map) and unfaithful male (in the right map) spent their time in relation to where other paired males spent their time (showed by open blue peaks). Image from the Okhovat et al. Science paper (2015).

Vasopressin is a hormone that has been found to affect social behaviors such as aggression and pair bonding when it acts in the brain. Mariam, Alejandro, Gerard, Alex, and Steve all set out to determine how vasopressin in the brain may relate to sexual fidelity in prairie voles. They found that faithful males had lots of a particular type of vasopressin receptor (called V1aR) in certain brain areas involved in spatial memory. Surprisingly, faithful males did not have more V1aR in brain regions typically associated with pair bonding and aggression. A male that has more V1aR in spatial memory regions might better remember where his own mate is and where other males have been aggressive, which would decrease the chances that he would intrude on other territories in search of other females and increase the time that he spends home with his own mate. A male that has less V1aR in spatial memory regions might be less likely to learn from his negative experiences and more likely to sleep around.

Photos of a brain section from a faithful male (left) and unfaithful male (right). The dark shading shows the density of V1aR vasopressin receptors. The arrows show the location of the retrosplenial cortex (RSC), a brain area involved in spatial memory. Faithful males had significantly more V1aR receptors in the RSC compared to unfaithful males. Image from the Okhovat et al. Science paper (2015).

The research team then found genotype variations that related to having lots or not much V1aR in one of these spatial memory regions (called retrosplenial cortex … but we’ll just call it RSC). They confirmed these findings with a breeding study, in which they reared siblings that were genetically similar, but some had the genotype they predicted would result in lots of V1aR in RSC and some had the genotype they predicted would result in very little V1aR in RSC. They confirmed that these genetic variations correspond with the amount of vasopressin receptor in this specific spatial memory area.

The researchers then looked closer at the different versions of this vasopressin receptor gene in the RSC brain region to see if differences in the amount of vasopressin receptors in RSC may be caused by the epigenetic state of the gene (i.e. how active the gene is). They found that the genotype that results in very little V1aR in RSC had many more potential methylation sites, which can repress gene activity.

All of this data together tells a very interesting story. Male prairie voles that have the genotype for more V1aR vasopressin receptors in their RSC part of their brain are more likely to remember where their home and mate are and to remember where other aggressive prairie voles are, which will make them more likely to spend more time with their partner, to be sexually faithful and to have sexually faithful partners. Male prairie voles that have the genotype for less V1aR in their RSC are more likely to forget where their home and mate are and where other aggressive prairie voles are, which will make them more likely to cheat and to be cheated on. Overall, faithful and unfaithful male prairie voles have roughly the same number of offspring, but advantages may emerge with changes in population density. Prairie vole populations vary anywhere from 25 to 600 voles per hectare from year to year. When population densities are high, you (and your partner) are more likely to encounter more potential mates and it may benefit you to cheat (and have a “cheater’s brain”). When population densities are low, you (and your partner) are less likely to encounter more potential mates and it may benefit you to be faithful (and have a “faithful brain”). But when populations fluctuate between high and low densities, both faithful and unfaithful genotypes will get passed along from generation to generation.

Want to know more? Check this out:

Okhovat, M., Berrio, A., Wallace, G., Ophir, A., & Phelps, S. (2015). Sexual fidelity trade-offs promote regulatory variation in the prairie vole brain Science, 350 (6266), 1371-1374 DOI: 10.1126/science.aac5791