Does Social Status Change Brains?

A reposting of an original article in The Scorpion and the Frog.

Photo by The Grappling Source Inc.
at Wikimedia Commons

Being subordinated is stressful. The process of one individual lowering the social rank of another often involves physical aggression, aggressive displays, and exclusion. In addition to the obvious possible costs of being subordinated (like getting beat up), subordinated individuals often undergo physiological changes to their hormonal systems and brains. Sounds pretty scary, doesn’t it? But what if some of those changes are beneficial in some ways?

Dominance hierarchies are a fact of life across the animal kingdom. In a social group, everyone can’t be dominant (otherwise, life would always be like an episode of Celebrity Apprentice, and what could possibly be more stressful than that?). Living in a social group is more peaceful and nutritive when a clear dominance hierarchy is established.

Establishing that hierarchy often involves a relatively short aggressive phase of jostling for position, followed by a longer more stable phase once everyone knows where they fall in the social group. Established dominance hierarchies are not always stable (they can change over time or from moment to moment) and they are not always linear (for example, Ben can be dominant over Chris, who is dominant over David, who is dominant over Ben). But they do generally help reduce conflict and the risk of physical injury overall.

Nonetheless, it can be stressful to be on the subordinate end of a dominance hierarchy and these social interactions are known to cause physiological changes. Researchers Christina Sørensen and Göran Nilsson from the University of Oslo, Cliff Summers from the University of South Dakota and Øyvind Øverli from the Norwegian University of Life Sciences investigated some of these physiological differences among isolated, dominant, and subordinate rainbow trout.

A photo of a rainbow trout by Ken Hammond at the USDA.
Photo at Wikimedia Commons.

Like other salmonid fish, rainbow trout are aggressive, territorial and develop social hierarchies as juveniles. Dominant trout tend to initiate most of the aggressive acts, hog food resources, grow larger, and reproduce the most, whereas subordinate trout display less aggression, feeding, growth, and reproduction. The researchers recorded the behavior, feeding and growth rates in three groups of fish: trout housed alone, trout housed with a more subordinate trout, and trout housed with a more dominant trout. The researchers also measured cortisol (a hormone involved in stress responses), serotonin (a neurotransmitter involved in mood, the perception of food availability, and the perception of social rank, among other things) and the development of new neurons (called neurogenesis) in these same fish.

This video of two juvenile rainbow trout was taken by Dr. Erik Höglund. Here is Christina Sørensen’s description of the video: “What you see in the film is two juvenile rainbow trout who have been housed on each side of a dividing wall in a small aquarium. The dividing wall has been removed (for the first time) immediately before filming. You will see that the fish initially show interest for each other, followed by a typical display behaviour, where they circle each other. Finally one of the fish will initiate aggression by biting the other. First the aggression is bidirectional, as they fight for dominance, but after a while, one of the fish withdraws from further aggression and shows only submissive behaviour (escaping from the dominant and in the long run trying to hide... and as is described in the paper, depressed feed intake). The video has been cut to show in quick succession these four stages of development of the dominance hierarchy”.

The researchers found that as expected, the dominant trout were aggressive when a pair was first placed together, but the aggression subsided after about 3 days. Also as expected, the dominant and isolated trout were bold feeders with low cortisol levels and high growth rates, whereas the subordinate trout did not feed as well, had high cortisol levels and low growth rates. Additionally, the subordinate trout had higher serotonin activity levels and less neurogenesis than the dominant or isolated trout. These results suggest that the subordination experience causes significant changes to trout brain development (Although we can’t rule out the possibility that fish with more serotonin and less neurogenesis are predisposed to be subordinate). In either case, this sounds like bad news for subordinate brains, right? Maybe it is. Or maybe the decrease in neurogenesis just reflects the decrease in overall growth rates (smaller bodies need smaller brains). Or maybe something about the development of these subordinate brains improves the chances that these individuals will survive and reproduce in their subordination.

A crayfish raising its claws. Image by Duloup at Wikimedia.

Research on dominance in crayfish by Fadi Issa, Joanne Drummond, and Don Edwards at Georgia State University and Daniel Cattaert at the University of Bordeaux helps shed light on this third possibility. Crayfish (which are actually not fish at all, but are freshwater crustaceans that look like small lobsters) form long-lasting and stable social hierarchies. If you poke a crayfish in the side, an isolated or dominant crayfish will turn towards whatever poked it and raise its posture and claws to confront it; A subordinate crayfish will do one of two maneuvers that involves lowering the posture and backing away from whatever poked it. Furthermore, dominant and subordinate crayfish have different neuronal activity patterns in response to being poked, and part of this difference involves differences in the activity of serotonergic neurons.

It appears that the brains of dominant and subordinate individuals function differently and part of this difference involves serotonin. This may help dominant animals to continue to behave in a dominant fashion and subordinate individuals to continue to behave in a subordinate fashion, thereby preserving the peace for the whole social group.

Want to know more? Check these out:

1. Sørensen, C., Nilsson, G., Summers, C., & Øverli, �. (2012). Social stress reduces forebrain cell proliferation in rainbow trout (Oncorhynchus mykiss) Behavioural Brain Research, 227 (2), 311-318 DOI: 10.1016/j.bbr.2011.01.041

2. Issa, F., Drummond, J., Cattaert, D., & Edwards, D. (2012). Neural Circuit Reconfiguration by Social Status Journal of Neuroscience, 32 (16), 5638-5645 DOI: 10.1523/JNEUROSCI.5668-11.2012

3. Yeh, S., Fricke, R., & Edwards, D. (1996). The Effect of Social Experience on Serotonergic Modulation of the Escape Circuit of Crayfish Science, 271 (5247), 366-369 DOI: 10.1126/science.271.5247.366

4. Issa, F., & Edwards, D. (2006). Ritualized Submission and the Reduction of Aggression in an Invertebrate Current Biology, 16 (22), 2217-2221 DOI: 10.1016/j.cub.2006.08.065

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

Reindeer Games: 8 Surprising Facts About Reindeer

A reposting of an original article from December, 2017.

A Swedish reindeer watches you. Photo by Alexandre Buisse at Wikimedia Commons.

1. Reindeer are caribou (kinda): Reindeer are the same species as caribou (with the scientific name Rangifer tarandus), but the terms are not completely interchangeable. Rangifer tarandus is a species of deer that is native to Northern regions of Europe, Siberia and North America, which includes many different habitat types, like arctic, subarctic, tundra, snow forest and mountains. These variations in harsh environments have led to variations among populations, resulting in multiple subspecies. The Rangifer tarandus subspecies that live in North America are commonly called caribou and the subspecies that live in Europe and Siberia are commonly called reindeer. We also often refer to domesticated populations as reindeer, regardless of where they are.

A map of reindeer and caribou distributions. Image by TBjornstad at Wikimedia Commons.

2. Rudolf’s red nose was an adaptation: Technically, reindeer don’t have red noses, but they do have lots extra blood flow in them. The inside of their noses are twisted and vascularized so the warm blood can heat up the frigid Arctic air before it gets into the lungs.

3. Santa’s reindeer were probably girls: Not only do reindeer have the biggest antlers of all deer species (relative to body size), but they are the only deer species in which both males and females grow antlers. Both males and females use their antlers to scrape through the snow and look for food, but males also use their antlers to compete with one another and impress the ladies during the breeding season. Unlike horns, antlers shed and regrow every year, and this process is regulated by sex hormones. When the new antlers grow in spring, they are made up of cartilage and lots of blood vessels and are covered in a furry skin called velvet. The blood carries lots of calcium into the antlers, which helps them to grow and harden into bone. When testosterone levels drop in males at the end of their breeding season in early December, their antlers fall off. Females, however, generally keep their antlers until March or April. So, if Santa’s reindeer had antlers at the end of December, they were probably female!

4. If you’re going to pick an animal to travel the world in one night, reindeer are a good choice: Some North American caribou migrate over 3,000 miles a year (more than any other land mammal). They can run up to 50 miles per hour and swim over 6 miles per hour. Migration herds can be up to 500,000 animals and baby reindeer learn to run within two hours of birth!

A swimming caribou herd. Photo by Lestar Kovac at Wikimedia Commons.

5. Reindeer eat weird stuff: Like cows, reindeer are ruminants, which means their stomachs have multiple compartments, some of which specialize in maintaining microbial communities to help them digest. Unlike cows, reindeer predominantly eat lichen, which are combinations of algae and fungi that are typically high in carbohydrates and low in proteins. To make up for this low amount of protein in their diet, reindeer may occasionally eat rodents and bird eggs.

6. They have the coolest feet: Their hooves have four toes: two that splay out like snow shoes and two dew claws. Their hooves have sharp edges to dig for food and are paddle-shaped for swimming. Their hooves even change with the seasons to provide the best traction, being softer in the summer when the ground is soft and hard in the winter to walk on slippery snow and ice.

7. Some reindeer use clicking knees to communicate: Some subspecies have knees that click when tendons slip over bone extensions in their feet. They use this sound to stay with their herd, even when weather conditions limit visibility. But because larger reindeer have larger legs and therefore make louder knee-clicks, they also use these sounds to establish dominance.

8. Reindeer are the only mammals that can see UV light: They have a reflective layer in the back of their eyes that is golden in summer and blue in winter. When it is blue, this allows reindeer to see contrasts in UV light, such as lichen (which absorbs UV) versus snow (which reflects UV).

Mr. Nanny Makes Mr. Right

A reposting of an original article from November 28, 2012.

Quick! Introduce yourself to this guy before
his baby-high wears off! Photo by David
Castillo Dominici at FreeDigitalPhotos.net. 

What happens if you take a wrestler or action star and force him to babysit obnoxious but lovable kids? Well, if you’ve seen movies like The Pacifier with Vin Diesel, The Tooth Fairy with Dwayne ‘The Rock’ Johnson, Kindergarten Cop with Arnold Schwarzenegger, or The Spy Next Door with Jackie Chan, you know that he will fall madly in love both with his young charges and with the closest available woman. Hollywood is so sure of this phenomenon that they have based a whole genre of family movies on it. Now, scientists are finding that Hollywood may be on to something.

Prairie voles are one of the only 3-5% of mammals that are monogamous and in which both parents help take care of young. In females, maternal care is regulated in part by the hormones associated with pregnancy, birth and lactation. The fact that males don’t do those things and they still provide paternal care is curious. The fact that male prairie voles will often provide care to offspring that aren’t even their own is even more curious.

Will Kenkel, Jim Paredes, Jason Yee, Hossein Pournajafi-Nazarloo, Karen Bales, and Sue Carter at the University of Illinois at Chicago recently explored what happens to male prairie voles when they are exposed to unfamiliar vole pups. Male voles without any experience with females or pups were placed in a new clean cage. Then the researchers put either a pup (that was not related to the male), a dowel rod (an unfamiliar object), or nothing into the cage with them for 10 minutes. Afterwards, they measured oxytocin (a hormone associated with bonding between mothers and their offspring) and corticosterone (a stress hormone) in the males’ blood at different time points. In another study, they also looked at the activity of brain neurons associated with the production of these hormones.

A male prairie vole is startled to find a baby in his cage...

But then he takes care of it. Video by Will Kenkel.

Both adult and juvenile males exposed to a pup for 10 minutes had higher oxytocin and lower corticosterone compared to the males not exposed to a pup. But this effect was short-lived, as male hormone levels quickly evened out again. Most of these males that were exposed to a pup showed alloparental care (care of a baby that is not their own), like approaching the pup, cuddling with it and grooming it. Males with higher oxytocin and lower corticosterone levels were more attentive towards the pups. Additionally, alloparental males exposed to pups had more activity of oxytocin-producing neurons and less activity of neurons associated with corticosterone-production in a specific brain region called the paraventricular nucleus (or PVN for short).

Oxytocin is strongly associated with pair bonding in prairie voles, particularly in females, and corticosterone affects pair bonding too (generally increasing pair bonding in males and preventing it in females). If exposure to a pup affects these hormones, maybe it affects how the male would interact with adult females. To test this, the researchers put male voles in a new clean cage and put a pup, a dowel rod, or nothing into the cage with them for 20 minutes. Then they put the males with an unfamiliar adult female for 30 minutes. After getting acquainted with the female, the males were put in a “partner preference apparatus”, which has three connected chambers: a neutral center chamber, a connected chamber with the familiar female tethered into it, and a connected chamber with an unfamiliar female tethered into it. The researchers measured how much time the males spent in each of the three chambers and with each of the two females over the next 3 hours.



A prairie vole pair snuggles. Photo from Young,
Gobrogge, Liu and Wang paper in
Frontiers in Neuroendocrinology (2011)

Males that were exposed to a dowel rod or to nothing before they were introduced to a female spent equal amounts of time with each of the two females. But males that were exposed to a pup before they were introduced to a female spent nearly 4 times as much time with that female than with the unfamiliar one. In other words, hanging out with a random pup acted like Love Potion #9 on these bachelor males and made them fall for the next female they encountered! Interestingly, this effect was true not only for the males that acted in an alloparental way towards the pups, but it was also true of males that attacked the pups (The researchers quickly rescued the pups if this occurred). Perhaps, males that were alloparental with the pups had increased oxytocin and males that were aggressive with the pups had increased corticosterone, either of which would make it more likely for them to form a preference for the female they were with.

Hmm… Got your eye on a special someone? Try volunteering him to babysit before your next date.

Want to know more? Check this out:

Kenkel, W., Paredes, J., Yee, J., Pournajafi-Nazarloo, H., Bales, K., & Carter, C. (2012). Neuroendocrine and Behavioural Responses to Exposure to an Infant in Male Prairie Voles Journal of Neuroendocrinology, 24 (6), 874-886 DOI: 10.1111/j.1365-2826.2012.02301.x

The Love Chemical of 2018

Hello and welcome to the Love Chemical Pageant Results Show! The voting results are in, and today we get to crown the Love Chemical of 2018… Vasopressin! Now let’s get to know Vasopressin a little bit better.

Vasopressin (also known as Antidiuretic Hormone) is a molecule that is widely involved in the balance of water and ions (such as salts) in mammals. (Other taxonomic groups have variations of it as well). But this chemical has gone to our heads, influencing behavior as well.

In the brain, vasopressin acts on a specific receptor type, called vasopressin 1a receptor (V1aR). There are lots of V1aR receptors in brain areas that regulate social and emotional behaviors. When vasopressin binds to many of these receptors, it can result in aggression, territoriality, and fight-or-flight responses. It is also involved in the formation of memories that are necessary to avoid danger. Interestingly, males and females usually have different patterns of where in the brain these V1aR receptors are.

Although we often think of love and aggression as opposites, the life-preserving roles of vasopressin have made it well-suited to become an important chemical of love. In animals, pair bonding (the formation of a strong and unique connection between mates of a socially monogamous species) is often accompanied by an increase in aggression towards non-mates. This aggression can serve to protect the mate and family, but also to reject competitive suitors towards either partner.

Photo of a prairie vole pair from Young, Gobrogge, Liu and Wang paper
in Frontiers in Neuroendocrinology (2011)

Researchers often use several closely-related vole species to study how the brain regulates pair bonding; While prairie voles and pine voles are monogamous, raise their offspring with their partners, and defend their homes and families, montane voles and meadow voles are promiscuous and females raise their young by themselves. Oddly, giving monogamous vole species vasopressin increases their preference for spending time with their mate, their parental behaviors, and their selective aggression against outsiders, but giving promiscuous vole species vasopressin does not. Vasopressin is also more likely to increase these monogamous behaviors in males more than in females. Both males and females respond differently to vasopressin depending on their reproductive status.

It turns out, the pattern of V1aR receptors in the brain is similar between the monogamous prairie and pine voles, but different from the promiscuous montane and meadow voles. Genetic factors drive this difference, and if you alter the gene for the V1aR of a promiscuous species to be more like the prairie vole’s version of the gene, the previously promiscuous species behaves in a monogamous way! The reason promiscuous vole species don’t behave in a monogamous way when given vasopressin is because they don’t naturally have the V1aR receptors in certain brain regions to respond to it that way.

We are still learning about the role of vasopressin in pair bonding behaviors. Much of what we know has focused on these vole species, and we know much less about vasopressin’s involvement in pair bonding in other species. We also don’t know as much about the role of vasopressin in females across different reproductive stages. But one thing is for sure: Love wouldn’t be the same without Vasopressin!


Want to know more? Check these out:
Carter, C.S. (2017). The Oxytocin–vasopressin Pathway in the Context of Love and Fear. Frontiers in Endocrinology, 8(356): 1-12.

Phelps, S.M., Okhovat, M. and Berrio, A. (2017). Individual Differences in Social Behavior and Cortical Vasopressin Receptor: Genetics, Epigenetics, and Evolution. Frontiers in Endocrinology, 8(537): 1-12.

Tickerhoof, M.C. and Smith, A.S. (2017). Vasopressinergic Neurocircuitry Regulating Social Attachment in a Monogamous Species. Frontiers in Endocrinology, 8(265): 1-10.

The Love Chemical Pageant of 2018

A modified repost of an original article from February 15, 2012.

Hello and welcome to the Love Chemical Pageant of 2018! I’m your host, Miss Behavior, and YOU are the judges.

Since the beginning of…well, social animals, many hormones and neurotransmitters have been quietly working in their own ways to fill our world with love. Lately (over the last few decades), some of them have been brought out of the background and into the limelight, credited with every crush, passionate longing, parental hug, embrace among friends, and cuddle between spouses. But who truly deserves the title of The Love Chemical?

Let’s meet our contestants!

Let’s first meet our reining title-holder, Dopamine! Dopamine is a neurotransmitter produced in the brain. Sex increases dopamine levels in both males and females and blocking its effects during sex can prevent prairie voles (a monogamous species often used to test questions on pair bonding) from forming preferences for their own partner. Dopamine also plays a role in maternal and paternal behaviors.

But dopamine is not just involved in love. It has a wide range of known functions in the brain, involved in everything from voluntary movement, mood, motivation, punishment and reward, cognition, memory, learning, aggression, pain perception and sleep. Abnormally high levels of dopamine have been linked to schizophrenia and psychosis. And dopamine is especially well-known for its role in addiction... in fact, many researchers believe that we may even be addicted to our own romantic partners.

Now let’s meet Dopamine’s partner, Opioids! When natural opioids are released in the brain, they can cause a rewarding feeling that often cause us to seek more of it. When prairie voles are given drugs that prevent opioids from acting on a particular opioid receptor type (mu-opioid receptors) in a particular brain region (the caudate-putamen), they do not form pair bonds with sexual partners. Interestingly, people that see the faces of their loved ones experience lots of activity in the caudate-putamen region of the brain, especially if they rate their relationship with that person as very romantic and passionate. The caudate-putamen region of the brain also uses dopamine, so the two chemicals appear to work together there to promote the feelings of romantic love.

Please welcome Oxytocin! Oxytocin is a peptide hormone, most of which is made in the brain. Some of this oxytocin is released into the blood and affects body organs, such as the uterus and cervix during child birth and the mammary glands during breast feeding. But some of it stays in the brain and spinal cord, acting on neurons with oxytocin receptors to affect a number of behaviors. Released during child birth and nursing, oxytocin is important for helping mammalian mothers behave like moms and in species in which both parents raise young, it helps fathers behave like dads. Also released during sex, oxytocin plays an important role in pair bonding in prairie voles (particularly in the female of the pair). In humans, people given oxytocin nasal sprays have been reported to have less fear, more financial trust in strangers, increased generosity, improved memory for faces, improved recognition of social cues, and increased empathy.

But before you fall head-over-heels for oxytocin, you should know a few more things. For one thing, oxytocin isn’t exclusively linked with feel-good emotions; It has also been associated with territoriality, aggressive defense of offspring, and forming racist associations. Also, oxytocin doesn’t work alone. It has been shown to interact with vasopressin, dopamine, adrenaline and corticosterone and all these interactions affect pair bonding.

Next up is Vasopressin! Vasopressin is closely related to oxytocin. Like oxytocin receptors, vasopressin receptors are expressed in different patterns in the brains of monogamous vole species compared to promiscuous vole species. Released during sex, vasopressin plays an important role in pair bonding in monogamous prairie voles (particularly in the male of the pair). If you block vasopressin in the brain of a paired male prairie vole, he will be more likely to prefer spending time around a new female rather than his mate. On the flip side, if you increase vasopressin activity in specific brain regions of an unpaired male prairie vole or even a promiscuous male meadow vole and introduce him to a female, he will prefer spending time with her than other females. Vasopressin may also make male prairie voles more paternal.

But vasopressin does a lot of things. In the body, its primary function is to regulate water retention. In the brain, it plays a role in memory formation and territorial aggression. And even its role in monogamy is not exclusive: Vasopressin interacts with oxytocin and testosterone when working to regulate pair bonding and parental behavior.

Look out for Cortisol! Cortisol is produced by the adrenal glands (on top of the kidneys) and is involved in stress responses in humans and primates. Both men and women have increased cortisol levels when they report that they have recently fallen in love. Many studies have also found relationships between cortisol and maternal behavior in primates, but sometimes they show that cortisol increases maternal behavior and sometimes it prevents it. In rodents, where corticosterone is similar to cortisol, the story is also not very clear. Corticosterone appears to be necessary for male prairie voles to form pair bonds and it plays a role in maintaining pair bonds and promoting paternal behavior. But in female prairie voles, the opposite seems to be true! Corticosterone in females appears to prevent preference for spending time with their partner and pair bond formation.

Put your hands together for Testosterone! Testosterone is a steroid hormone and is primarily secreted from the gonads (testes in males and ovaries in females). Frequently referred to as “the male hormone”, both males and females have it and use it, although maybe a little differently. Testosterone is associated with sex drive in both men and women. But men who have recently fallen in love have lower testosterone levels than do single males, whereas women who have recently fallen in love have higher testosterone than single gals.

This is Estrogen! Estrogen is another steroid hormone, frequently referred to as “the female hormone”, although again, both males and females have it. Estrogen also seems to play a role in sex drive in both men and women. The combination of high estrogen levels and dropping progesterone levels (another steroid hormone) is critical for the development of maternal behavior in primates, sheep and rodents. But look closer and you will find that the activation of estrogen receptors in particular brain regions is associated with less sexual receptivity, parental behavior, and the preference for spending time with the mate.

So let’s have a round of applause for this year’s contenders in The Love Chemical Pageant! Now it is your turn to voice your opinion in the comments section below. Vote for the neurochemical you believe deserves the title The Love Chemical. Or suggest an alternative pageant result!


Want to know more? Check these out:

Burkett, J.P. and Young, L.J. (2012). The behavioral, anatomical and pharmacological parallels between social attachment, love and addiction. Psychopharmacology, 224:1-26.

Fisher, H.E. (1998). Lust, attraction, and attachment in mammalian reproduction. Human Nature, 9(1) 23-52.

Marazziti, D. and Canale, D. (2004). Hormonal changes when falling in love. Psychoneuroendocrinology, 29, 931-936.

Van Anders, S.M. and Watson, N.V. (2006). Social neuroendocrinology: Effects of social contexts and behaviors on sex steroids in humans. Human Nature, 17(2), 212-237.

Young, K.A., Gobrogge, K.L., Liu, Y. and Wang, Z. (2011). The neurobiology of pair bonding: Insights from a socially monogamous rodent. Frontiers in Neuroendocrinology, 32(2011), 53-69.

Addicted to Love

Image from imagerymajestic at FreeDigitalPhotos.net.

Early exposure creates a sense of euphoria, a heightening of senses, a rush of pleasure. In order to recreate and further heighten the experience, more is sought, but the euphoric effect eventually starts to wear off. Craving and palpable longing intensifies in its absence, but the effect of exposure is now a calm relief rather than a euphoric high. And once cut off abruptly and completely, desperation, grief, pain and depression set in. The pull to return to it is (almost) insurmountable.

This describes the phases of substance addiction, listed by the DSM-5, the latest version of the Diagnostic and Statistical Manual of Mental Disorders, published by the American Psychiatric Association. These phases include consumption (taking the substance), reinforcement learning (intense pleasure associated with consuming the substance), seeking more of the substance, developing a tolerance (intense pleasure is replaced with avoidance of discomfort), withdrawal (psychological and physical discomfort associated with not consuming the substance), and relapse (returning to consume the substance, even in the face of large costs of doing so).

Now re-read the first paragraph, but instead of imagining the development of a substance addiction, imagine the process of falling in love.

It sounds the same, doesn’t it? According to one theory, it sounds the same because it is the same. In essence, falling in love is the process of becoming addicted to another individual.

There are undeniable similarities between how the brain responds to substance addiction and how the brain responds to falling in love. Both substances of addiction and individuals we are attracted to cause the brain to release dopamine, a neurotransmitter, into a brain region called the nucleus accumbens. Dopamine acting in this region helps us learn to associate cues with rewarding feelings. However, dopamine acts on two different types of receptors, called D1-receptors and D2-receptors, in complex ways. Activation of D2-receptors promotes bonding with a partner; it also promotes the reward value of a substance. Activation of D1-receptors reduces bonding with a partner; it also reduces the reward value of a substance. During this time early on in a romantic relationship or early exposure to an addictive substance, dopamine is primarily acting on D2 receptors, heightening our senses and focusing our attention on the cues of our next encounter… developing our craving, our longing, our drive for the next meeting.

When we are in the early obsessive stages of love, every encounter (and especially sexual encounters) cause a pleasurable release of not just dopamine, but also natural opioids. These two brain chemicals work together in the brain to continually strengthen the association of the stimulus (the one you are falling in love with) with intense positive feelings. This will cause you to seek more and more of these interactions, craving them intensely in the times in between. These same chemicals act on the same receptors in the same way during the process of forming an addiction to a substance, causing the person to seek more and more of it.

With time, the brain adapts. Repeated encounters no longer cause the same euphoria they once did, but rather, a sense of calm contentment. The dopamine that is released before and during these encounters is now activating more of the D1-receptors, which result in less of a feeling of pleasure, and more agitation and aggression. In terms of relationships, it is thought that this transition actually helps maintain a pair bond with one individual, because in this stage you are less driven to seek a competing pair bond and you are more likely to aggressively defend the pair bond you have already established. In terms of substance abuse, this phase is called tolerance. (I know, this perspective really takes the romance out of long-term marriages, but...)

During this tolerance phase, lack of exposure to the object of your addiction (whether it is a person or a substance) results in a lack of dopamine and opioid release and an increase in stress hormone release. If we are talking about addiction to a substance, we call this withdrawal. If we are talking about a relationship, we call this separation anxiety or even heartbreak. To avoid these horrible feelings, we often relapse… right back into the arms of our addiction.

Love is not listed as a psychological disorder in the DSM-5, nor do we think of it as one. But in a true physiological sense, we may actually be addicted to the ones we love.


Want to know more? Check these out:

Burkett, J.P. and Young, L.J. (2012). The behavioral, anatomical and pharmacological parallels between social attachment, love and addiction. Psychopharmacology, 224:1-26.

Potenza, M.N. (2014). Non-substance addictive behaviors in the context of DSM-5. Addictive Behaviors, 39(1): 1-2.

Body Clocks: What They Are and How They Work

Lately, with the new year, the #TimesUp movement, awaiting Disney’s movie A Wrinkle in Time based on one of my favorite childhood book series, and just watching my children grow faster than I thought possible, I have been thinking a lot about time. The continuous march forward, the constant rotation of the planet, and the revolution of the Earth around a distant star in its determined path all have far-reaching effects on our physiology and behavior. Our biological clocks affect everything from our sleep-wake cycles to our fertility to our mental and physical health. And it’s not just us that have them: Every living thing on Earth, including bacteria, protists, fungi, plants and animals, has them. But what do they do and how do they work?

A sleepy ferret minds his biological rhythms. Photo by Kimberly Tamkun at Wikimedia Commons.

Generally speaking, a biological clock is an organism’s inborn way of regulating its functions with respect to time. Many of these biological clocks follow circadian rhythms (changes that follow a 24-hour cycle). Vast portions of our planet have been exposed to dramatic but mostly predictable environmental changes on a 24-hour cycle since long before life existed, so it makes sense that us lifeforms have developed a means to make the best of those changes: sleeping when food is less available, having higher metabolisms when we are active, being more alert during times we are most likely to be interacting with the world.

Diagram of a human circadian rhythm by YassineMrabet at Wikimedia Commons.

Melatonin is a hormone widely known to synchronize circadian rhythms in vertebrates (animals with backbones) to the light-dark cycle of the day and night (or to an indoor room with a light timer). Melatonin is produced in response to darkness, and the longer the night, the more melatonin is produced. Rising and falling melatonin levels help determine sleep-wake cycles in animals. In animals that breed seasonally, the changing peaks of melatonin levels that correspond with dark nights getting longer or shorter stimulate the reproductive system to help synchronize breeding physiology and behavior with the seasons. Although we have known about melatonin and its effects for nearly a hundred years, we are now learning that it seems that all organisms, including bacteria, protists, fungi, plants and animals, make it. Whether it has the same effect in all organisms is yet to be determined.

In vertebrates, melatonin is produced by the pineal gland, a small structure in the center of the brain. In birds, reptiles, amphibians and fish, the pineal gland has light-sensitive cells that receive light as it passes directly through the skull and the brain! In mammals, the pineal gland receives a light signal through a more complicated pathway: Light is detected by light sensitive cells in the retinas of the eyes. They send this signal to the suprachiasmatic nucleus (SCN) in the brain, which relays it to other brain areas and then to the pineal gland. The SCN in mammals is commonly called “the master clock” due to its important role in synchronizing body rhythms with light cues.

Diagram of the human brain and the SCN by the
National Institute of General Medical Sciences at Wikimedia Commons.

Body rhythms are determined at the cellular level through the interaction of a small number of genes called clock genes. Clock genes have been found in every animal, plant and fungus studied so far. Originally, it was thought that in mammals, clock genes would only be found in the SCN. However, it now looks like clock genes are active in all cells and the SCN functions more like an orchestra conductor synchronizing the rhythms of the organs throughout the body.

Many clock genes have been discovered, and they all seem to work based on similar processes. Just last year, scientists Jeffrey Hall, Michael Rosbash, and Michael Young, were awarded the 2017 Nobel Prize in medicine for their research on clock genes in fruitflies. They found that biological clocks are self-regulated within the cell: Morning sunlight turns on a gene called the PERIOD gene, which starts to produce a protein called the Period protein. As long as there is light, Period protein accumulates to higher and higher levels. Another protein, named Timeless, shuttles Period proteins into the nucleus, where the DNA lives. The Period proteins shut down the activity of the PERIOD gene, while a third protein, called Doubletime, regulates the destruction of the excess Period proteins. The result of this process is that by nightfall, Period proteins have disappeared and sunlight is needed to start the cycle anew. This work by Hall, Rosbash and Young inspired a whole new field of molecular biology of circadian rhythms.

We have a lot more to learn about biological clocks and circadian rhythms, but what we do know is that their effects are wide-ranging. Whacky circadian rhythms have been implicated in sleep disorders, depression, bipolar disorder, cancer, obesity, and diabetes. And what else will we learn about them? Only time will tell.

Reindeer Games: 8 Surprising Facts About Reindeer

A Swedish reindeer watches you. Photo by Alexandre Buisse at Wikimedia Commons.

1. Reindeer are caribou (kinda): Reindeer are the same species as caribou (with the scientific name Rangifer tarandus), but the terms are not completely interchangeable. Rangifer tarandus is a species of deer that is native to Northern regions of Europe, Siberia and North America, which includes many different habitat types, like arctic, subarctic, tundra, snow forest and mountains. These variations in harsh environments have led to variations among populations, resulting in multiple subspecies. The Rangifer tarandus subspecies that live in North America are commonly called caribou and the subspecies that live in Europe and Siberia are commonly called reindeer. We also often refer to domesticated populations as reindeer, regardless of where they are.

A map of reindeer and caribou distributions. Image by TBjornstad at Wikimedia Commons.

2. Rudolf’s red nose was an adaptation: Technically, reindeer don’t have red noses, but they do have lots extra blood flow in them. The inside of their noses are twisted and vascularized so the warm blood can heat up the frigid Arctic air before it gets into the lungs.

3. Santa’s reindeer were probably girls: Not only do reindeer have the biggest antlers of all deer species (relative to body size), but they are the only deer species in which both males and females grow antlers. Both males and females use their antlers to scrape through the snow and look for food, but males also use their antlers to compete with one another and impress the ladies during the breeding season. Unlike horns, antlers shed and regrow every year, and this process is regulated by sex hormones. When the new antlers grow in spring, they are made up of cartilage and lots of blood vessels and are covered in a furry skin called velvet. The blood carries lots of calcium into the antlers, which helps them to grow and harden into bone. When testosterone levels drop in males at the end of their breeding season in early December, their antlers fall off. Females, however, generally keep their antlers until March or April. So, if Santa’s reindeer had antlers at the end of December, they were probably female!

4. If you’re going to pick an animal to travel the world in one night, reindeer are a good choice: Some North American caribou migrate over 3,000 miles a year (more than any other land mammal). They can run up to 50 miles per hour and swim over 6 miles per hour. Migration herds can be up to 500,000 animals and baby reindeer learn to run within two hours of birth!

A swimming caribou herd. Photo by Lestar Kovac at Wikimedia Commons.

5. Reindeer eat weird stuff: Like cows, reindeer are ruminants, which means their stomachs have multiple compartments, some of which specialize in maintaining microbial communities to help them digest. Unlike cows, reindeer predominantly eat lichen, which are combinations of algae and fungi that are typically high in carbohydrates and low in proteins. To make up for this low amount of protein in their diet, reindeer may occasionally eat rodents and bird eggs.

6. They have the coolest feet: Their hooves have four toes: two that splay out like snow shoes and two dew claws. Their hooves have sharp edges to dig for food and are paddle-shaped for swimming. Their hooves even change with the seasons to provide the best traction, being softer in the summer when the ground is soft and hard in the winter to walk on slippery snow and ice.

7. Some reindeer use clicking knees to communicate: Some subspecies have knees that click when tendons slip over bone extensions in their feet. They use this sound to stay with their herd, even when weather conditions limit visibility. But because larger reindeer have larger legs and therefore make louder knee-clicks, they also use these sounds to establish dominance.

8. Reindeer are the only mammals that can see UV light: They have a reflective layer in the back of their eyes that is golden in summer and blue in winter. When it is blue, this allows reindeer to see contrasts in UV light, such as lichen (which absorbs UV) versus snow (which reflects UV).

A New Key to the Story of How the Sexes Have Come to Be

In the beginning, we are all male and female… More specifically, we are all in between male and female. So what makes our embryonic selves choose and follow a developmental path to becoming the sex that we are today? New research has dramatically changed our understanding of this process.

During early embryonic development, all mammals develop a single pair of gonads that are neither testes nor ovaries, but have the potential to become either. Likewise, the external genitalia at this early stage has the potential to become either a penis and scrotum or a clitoris, vagina, and labia. Two pairs of ducts develop to connect the gonads to the undifferentiated external genitalia: One set of ducts, the Wolffian ducts, would become the epididymis, vas deferens, and seminal vesicles if this animal becomes male. The other set of ducts, the Müllerian ducts, would become the oviducts, uterus and innermost part of the vagina if this animal becomes female. So what determines if a given animal will develop male or female reproductive anatomy?

Early in development, mammalian embryos have one set of gonads that has the potential to become either testes or ovaries (here labeled as "bipotential gonad"). These gonads are connected the the developing external genitalia by two sets of tubes: The Wolffian ducts become the reproductive tracts in males and the Müllerian ducts become the reproductive tracts in females. The ducts that do not become reproductive tracts typically disintegrate. However, XX female embryos that lack the COUP-TFII protein do not dismantle their male-like reproductive tracts. Figure from Swain, 2017.

The sex of a mammal is determined by the combination of sex chromosomes it has. If the mammal has two X chromosomes, it will likely become female, and if it has an X chromosome and a Y chromosome, it will likely become male. The story physiologists have been telling for decades is that there is a single gene located on the Y chromosome, called the SRY gene, that single-handedly makes an embryo become a male. When expressed, the SRY gene produces a protein, called testes-determining factor, which interacts with the cells of the undifferentiated gonads to turn them into testicular cells. These newly formed testicular cells produce two key hormones: testosterone, which causes the Wolffian ducts to become the epididymis, vas deferens, and seminal vesicles, and anti-Müllerian hormone (AMH), which causes the Müllerian ducts to degenerate. In other words, if an animal has a Y chromosome, it will typically have an SRY gene that will trigger the sequence of events that causes the animal to develop into a male. If the animal does not have the Y chromosome, it will typically become female. However, it is not just the lack of a Y chromosome that can make a female; Any disruption of this pathway (such as an SRY gene that is not expressed, or the lack of testicular hormones) typically causes the animal to develop into a female. For this reason, females in mammals have been called the default sex. The scientific understanding since the 1950s has been that, in mammals, the development of a male reproductive system is an active process and the development of a female reproductive system is a passive process. However, a new study reveals that the process of becoming female mammal is not as passive as we have thought.

Fei Zhao, Humphrey Yao and their research team at the National Institute of Environmental Health Sciences and Baylor College of Medicine discovered a critical role for a specific protein, called COUP-TFII, in the active process of becoming a mammalian female. The research team examined female mouse embryos (which lack a Y chromosome, and hence an SRY gene) that had been genetically modified to lack a particular protein called the COUP-TFII protein. They compared these genetically modified XX embryos to genetically typical XX mouse embryos. When the unmodified XX embryos had developed to have only Müllerian ducts (the “typical” female reproductive pathway), the XX embryos without COUP-TFII protein retained both Müllerian and Wolffian ducts! Unfortunately, these XX mice that lacked the COUP-TFII protein died shortly after birth, so it was difficult to tell if this developmental process would have continued. The research team cultured reproductive organs of XX mice with and without COUP-TFII protein and found that this developmental trajectory likely would have continued after birth.

Images A and B show the reproductive tract from the side (A) and as a cross-section (B) in a "typical" XX female mouse embryo. Images D and E show that XX females that lack the COUP-TFII protein retain both Müllerian (pink arrows) and Wolffian (blue arrows) ducts. Figure from Zhao et al., 2017.

We know that testosterone helps promote the development of Wolffian ducts in XY males, so the most likely explanation of what they witnessed is that the lack of COUP-TFII protein somehow increased action of testosterone in these genetically modified XX embryos. The researchers ran a number of tests to explore this possibility. Testosterone is mostly produced by the gonads, so they compared the gene expression and enzymes of ovaries of unmodified XX mice with the ovaries of XX mice that lacked the COUP-TFII protein, and they found no differences that pointed to differences in testosterone production. They then considered the possibility that testosterone was produced somewhere else in the body, but the XX mice that lacked the COUP-TFII protein did not have more masculine body features compared to the unmodified XX mice. Finally, the researchers gave extra testosterone to the mother mice that were pregnant with unmodified XX mice and XX mice that lacked the COUP-TFII protein. The extra testosterone did not affect any of the mouse pups; it did not cause the Wolffian ducts of the XX mice that lacked the COUP-TFII protein to regress. Together, the researchers found that no, XX embryos that lack COUP-TFII protein do not have any more testosterone-like activity than their non-genetically modified XX sisters. This means that testosterone alone is not enough to keep Wolffian ducts.

This research has shown us that for the Wolfian ducts to go away during the reproductive development of a mammalian female, they need to be actively dismantled using a biochemical process (similar to how AMH dismantles Müllerian ducts during male reproductive development). COUP-TFII protein appears to be the chemical in charge of triggering this process. Female mammals are not the passive result of simply not becoming male, as has been taught in physiology classes for decades. Becoming a female mammal requires a process all its own, and we are only now starting to learn what that is.


Want to know more? Check these out:

F. Zhao et al. Elimination of the male reproductive tract in the female embryo is promoted by COUP-TFII in mice. Science. Vol. 357, August 18, 2017, p. 717. doi: 10.1126/science.aai9136

A. Swain. Ductal sex determination. Science. Vol. 357, August 18, 2017, p. 648. doi: 10.1126/science.aao2630

The Complexities of “The Love Hormone”

New York street art. Photo in
Wikimedia Commons posted by Pedroalmovar.

Oxytocin, commonly known as “the love hormone”, is a small chemical that is produced in the brain of mammals, but can both act as a neurotransmitter and enter the blood stream and act as a hormone. It has long been heralded for its role in both maternal and romantic love, but more recent research is showing us just how complicated the physiology of love can be.

Oxytocin is released in mammalian mothers after birth. It promotes nursing and bonding between a mother and her young. As children grow, oxytocin is involved in how both mothers and fathers “baby-talk” and mirror their children. It is involved in pro-social behaviors in both young and adults: trust, generosity, cooperation, hugging, and empathy. And of course, oxytocin promotes positive communication and pair bonding in romantic couples. Countless studies have found these relationships between affiliation and oxytocin in many mammalian species, giving oxytocin its commonly used nickname “the love hormone”.

But more recent studies show that it’s not so simple.

In a number of recent studies, people have been given oxytocin nasal sprays and tested for various behavioral effects in different contexts… and the context really seems to matter. Oxytocin increases trust, generosity, cooperation, and empathy towards people we already know and like. But it decreases trust, generosity, cooperation, and empathy towards strangers. When we play games with strangers, oxytocin makes us more jealous when we lose and it makes us gloat more when we win. It also seems to enhance many attributes relating to ethnocentrism: It increases our ability to read facially-expressed emotions in people of our own race while making it harder to read facial expressions of people of a different race. When forced to choose between being nice to a stranger of our own race versus a stranger of another race, oxytocin makes us more likely to choose the person of our own race. In studies of both people and rodents, oxytocin decreases aggression towards our families and friends, but increases aggression towards strangers.

Oxytocin is not the universal love hormone we once understood it to be. It helps us direct our positive support towards our “in-groups” (our family and friends) and defend them from our “out-groups” (individuals we don’t know). It is a delicate balance: Too little of it can cause social impairment and make it difficult to connect with loved-ones; Too much of it can increase our anxiety towards strangers and racist tendencies. And to make things more complicated, each of us has a slightly different oxytocin system: sex, gender, social history, history of childhood trauma or neglect, psychiatric illnesses and genetic variations all have profound effects on the oxytocin system.

There is much we don’t know about the role of oxytocin and love. But they are a good fit, because both, it seems, are complicated.


Want to know more? Check these out:

Shamay-Tsoory SG, & Abu-Akel A (2016). The Social Salience Hypothesis of Oxytocin. Biological psychiatry, 79 (3), 194-202 PMID: 26321019

Zik JB, & Roberts DL (2015). The many faces of oxytocin: implications for psychiatry. Psychiatry research, 226 (1), 31-7 PMID: 25619431