Magnetoreception is Not a Party For a Supervillain

The majority of the more than 650 species of North American birds migrate. In search of food and nesting sites, some birds travel short distances and others (like Arctic terns) travel up to 12,000 miles each way. But all of them have to figure out where they are going, and much of how they do this is still unknown.

This is a magnetite rock. Scientists think
many animals have magnetite in their brains
to detect magnetic fields! For real?! Photo by
Rob Lavinsky at irocks.com and Wikimedia.

Last week, we learned that many birds get disoriented if the magnetic field around them is messed up by sunspots, magnetic rocks, or researchers gluing magnets to their heads. So they must sense magnetic fields, but how?

Sensation, whether by vision, touch, hearing, smell, taste or even magnetoreception (the sensation of magnetic fields) requires a stimulus to be transformed into an electrical signal that then must reach the brain. In the first five senses I listed, which we know considerably more about, this process occurs when sensory neurons (a type of cell in the brain and nervous system) convert the chemical or physical energy of the stimulus into an electrical signal. The sensory neurons then transmit the electrical signal to the brain, where it is processed and interpreted. So it is reasonable to think that magnetoreception works the same way too, right?

Where might such magnetoreception sensory neurons be? Bob Beason at State University of New York and Peter Semm at the Goethe University Frankfurt in Germany conducted a series of experiments to test whether the trigeminal nerve, a major nerve that provides sensation to the face, might play a role in magnetoreception. The trigeminal nerve has three major branches: the ophthalmic nerve, the maxillary nerve and the mandibular nerve. These nerves are each a bundle of sensory neuron fibers that send electrical signals to communicate to the brain what the head and face is sensing… a good place to look.

This illustration, titled Bubbling Bob the Bobolink,
was created by Louis Agassiz Fuentes in 1919.
Image at Wikimedia.

Bob and Peter exposed bobolinks, migratory birds that use magnetic fields to navigate, to a set of coils that could produce both vertical and horizontal magnetic fields. They then recorded the electrical activity of individual sensory neurons in the trigeminal nerve while exposing the birds to different magnetic fields. They found that many of these trigeminal sensory neurons, especially in the ophthalmic nerve branch, responded to the magnetic fields either by increasing or decreasing their electrical activity. Horizontal and vertical magnetic fields elicited different responses from different sensory neurons. Also, some sensory neurons responded to increases in magnetic strength and others responded to decreases in magnetic strength. The pattern of neuron activity could be a way that the nervous system could communicate the direction, and change in direction, of the magnetic field to the brain during navigation.

In order to test whether the ophthalmic nerve branch carries magnetic information to the brain, Bob and Peter tested another group of bobolinks that were preparing for migration. For each bird, they first tested what direction it preferred to go (for a control). Then, they magnetized the birds such that if their beak were iron, the tip of it would attract the south end of a compass. They figured that this process would send a confusing magnetic signal to the birds’ brains. Then they tested the birds’ preferred flying directions again (as expected, they got confused and went the wrong way). Finally, they numbed the ophthalmic nerve by putting a drop of Lidocaine on it and tested their preferred direction again. They found that although magnetizing the birds made them go the wrong direction, when their ophthalmic nerve was numbed, they ignored this incorrect magnetic information and went the right way again. Clearly, the ophthalmic nerve is sending magnetic information to the brain.

But how does a sensory neuron in the ophthalmic nerve respond to a magnetic force? For magnetoreception to work, magnetic forces need to affect receptors of some kind. Gerta Fleissner, Branko Stahl, Peter Thalau, Gerald Falkenberg, and Günther Fleissner at the Goethe University Frankfurt were the first scientists to systematically seek out such magnetoreceptors. They examined the skin lining the upper beak of homing pigeons with fancy microscope and X-ray techniques that could identify iron compounds in the skin.

These researchers found two different types of iron, magnetite and maghemite, in dendrites, the receiving ends of sensory neurons. Not only do these dendrites have both metals, but the metals are arranged in a particular way. This particular arrangement could cause the dendrite to physically respond to a magnetic field that is oriented in a particular way, perhaps by changing shape and stretching the membrane of the cell (remember, a sensory neuron is a cell). This physical pull on the membrane could cause the neuron to send an electric pulse, in much the same way as a hearing cell does.


Researchers discovered two magnetic metals, magnetite and maghemite, in the receiving
ends of sensory neurons. These two metals were arranged in such a way that if a magnetic
force were to align with the neuron in a particular direction, the metals would likely move and
stretch the membrane, which could activate the neuron, sending a signal to the brain. Figure
from Fleissner, Stahl, Thalau, Falkenberg, and Fleissner's 2007 paper in Naturwissenschaften.

Furthermore, the skin lining the pigeon beak has six separate iron-containing patches. In each of these patches, there is a different prevailing direction of how the iron-containing dendrites are aligned. This means that different nerve endings could be activated by different directions of magnetic field, potentially providing the bird with a complex perception of the magnetic field as it turns its head.

Magnetic dendrites (receiving ends of sensory neurons) were aligned in one of three
directions depending on where they were in the beak. This arrangement could allow birds
to know the direction of a magnetic field based on which neurons are activated. Figure from
Fleissner, Stahl, Thalau, Falkenberg, and Fleissner's 2007 paper in Naturwissenschaften.

Now, before you declare, “These birds have iron in them? What, are they some kind of superhero?” remember, we all (pretty much everyone except for our arthropod and mollusc friends) have iron in us… in our respiratory pigments (like hemoglobin). But this (new to us) use of iron does seem to give these birds super-human abilities.

Want to know more? Check these out:

1. Beason RC, & Semm P (1987). Magnetic responses of the trigeminal nerve system of the bobolink (Dolichonyx oryzivorus). Neuroscience letters, 80 (2), 229-34 PMID: 3683981

2. Semm P, & Beason RC (1990). Responses to small magnetic variations by the trigeminal system of the bobolink. Brain research bulletin, 25 (5), 735-40 PMID: 2289162

3. Beason R, & Semm P (1996). Does the avian ophthalmic nerve carry magnetic navigational information? The Journal of experimental biology, 199 (Pt 5), 1241-4 PMID: 9319100

4. Beason R, Dussourd N, & Deutschlander M (1995). Behavioural evidence for the use of magnetic material in magnetoreception by a migratory bird The Journal of experimental biology, 198 (Pt 1), 141-6 PMID: 9317510

5. Fleissner G, Stahl B, Thalau P, Falkenberg G, & Fleissner G (2007). A novel concept of Fe-mineral-based magnetoreception: histological and physicochemical data from the upper beak of homing pigeons. Die Naturwissenschaften, 94 (8), 631-42 PMID: 17361399

6. Cadiou H, & McNaughton PA (2010). Avian magnetite-based magnetoreception: a physiologist's perspective. Journal of the Royal Society, Interface / the Royal Society, 7 Suppl 2 PMID: 20106875

A Sixth Sense

Birds have long been known for their incredible navigational abilities. More than 4000 years ago, ancient Egyptians used carrier pigeons, the domesticated descendants of wild rock doves, to carry urgent messages to distant lands. They proved to be cheaper, faster and more efficient than human messengers and their use spread throughout the Mediterranean, central and northern Europe, and then throughout the world. Yet it wasn’t until the mid-1800s that scientists began to ask how they do it. To this day, how animals accurately navigate on long migrations is still one of biology’s great mysteries.

A modern day rock dove. Photo by Ingrid Taylar at Wikimedia.

That’s not to say science hasn’t made a lot of headway on this investigation. Scientists have found that some animals learn landmarks when they travel in one direction, and use those landmarks to find their way back. Some animals follow odor cues. But one of the more intriguing theories is that animals have an internal map and compass… but not a literal map and compass. The “map” is how the brain knows where things are in relation to each other and the “compass” is how the animal knows what direction it is facing with respect to where it wants to be.

How might such a compass work? One internal compass is a sun compass, in which an animal can use the position of the sun and the time of day to determine what direction it is facing. Some of the most convincing evidence supporting the existence of such a compass is that pigeons that are kept in a room with a time-shifted light cycle will fly in a predictably wrong direction on a sunny day. They will fly this wrong direction for long distances and even when they can see known landmarks. So pigeons clearly rely on the sun to achieve their great navigational feats… But what do they do on cloudy days… or at night?

A cartoon of the Earth's magnetic field by Zureks at
Wikimedia. In reality, the directions of magnetic pull
are not this straight and uniform, but you get the idea.

Maybe pigeons have another compass based on the Earth’s magnetic field. Over 30 years ago, researchers found that racing pigeons (the new profession of decedents of carrier pigeons after mailmen in trucks and airplanes took their previous jobs) arrive at their destinations a little bit later when there have been recent magnetic storms due to sunspots. Pigeons also get disoriented in places with magnetic anomalies, such as areas with lots of iron ore. But experiments in which researchers have placed magnets or magnetic coils on the backs, wings, necks, heads or legs of pigeons have not had consistent effects, particularly on sunny days (when the sun compass likely comes into play).

Enter Cordula Mora and Michael Walker from the University of Aukland, New Zealand. Cordula and Michael reasoned that because the magnetic field gets weaker with distance and because we think that magnetoreception (the ability to perceive magnetic fields) occurs in or around the head, maybe these previous studies had inconsistent results because the magnets used were too far away and/or too weak to affect the receptors in a consistent way. So they did their own study with smaller, stronger magnets applied to pigeon beaks.

Cordula and Michael glued magnets to the cere of pigeons. The diagram on the left shows how they did it and the photo on the right is a pigeon showing off his new nosepiece. (Check out the rock dove image above to see what a naked cere looks like). Diagram and image from Mora and Walker 2012 Animal Behaviour paper.

Cordula and Michael glued either a magnet or a brass weight (as a control) to the cere (the fleshy upper-part of the beak that contains the nostrils) of experienced racing pigeons right before a flight. In one experiment, researchers released the pigeons 11 consecutive times from the same place (called Gernsheim), and alternated whether they had a magnet or a brass weight glued to their cere. In a second experiment, the researchers released every bird once from each of 25 different places, each time with either a magnet or a brass weight glued to their cere. The birds were always flying to the same place (their loft), but the direction and distance they needed to fly was different for each of the release sites. Thus, the first experiment provides the birds with an opportunity to learn and compensate for any effect of the magnet, while the second experiment does not. All of the flights were done on sunny days.

The places the researchers released the pigeons from were all
different directions and distances from their home loft (at the center).
Diagram from Mora and Walker 2012 Animal Behaviour paper.

For each flight, the researchers watched the bird through binoculars until it vanished from view, at which point they recorded the vanishing bearing (the direction the pigeon was flying before it vanished from view). They also timed how long it took the bird to return to the loft and recorded any instances in which the bird did not return to the loft.

The pigeons with magnets consistently flew just a little to the right of pigeons with brass weights. The effect was very small (ranging from 11° to 22°), but it almost always happened, regardless of the bird or the release site. The effect was also consistent over consecutive years, even in birds tested repeatedly from the same release site. However, although the vanishing bearing of birds with magnets was regularly to the right of the birds with brass weights, the magnets did not prevent the pigeons from finding their loft and did not even cause them to take longer to get home. This shows that some time after the vanishing distance, the pigeons with magnets compensated for their originally slightly-off bearing.

The fact that the pigeons with magnets almost always started off flying too-far right suggests that they do have and use a magnetic compass, or perhaps even a magnetic map. But the fact that they always got back to the loft just as fast as their brass weight carrying counterparts shows that they also rely on other mechanisms, like a sun compass and landmarks. Perhaps magnetoreception is only important to determine take-off direction. Or alternatively, maybe the birds learn to ignore the confusing signals of the magnets after awhile.

We still have a lot to learn about how animals use magnetic fields. And how does magnetoreception even work? How animals navigate over long distances is still a great mystery, but scientists are on the case.

Want to know more? Check these out:

1. Mora, C.V., & Walker, M.M. (2012). Consistent effect of an attached magnet on the initial orientation of homing pigeons, Columbia livia. Animal Behaviour, 84, 377-383 DOI: 10.1016/j.anbehav.2012.05.005

2. Wiltschko, R., & Wiltschko, W. (2003). Avian navigation: from historical to modern concepts. Animal Behaviour, 65, 257-272 DOI: 10.1006/anbe.2003.2054

3. Bingman, V. P., & Cheng, K. (2005). Mechanisms of animal global navigation: comparative perspectives and enduring challenges. Ethology Ecology & Evolution, 17, 295-318 DOI: 10.1080/08927014.2005.9522584

4. Winged Migration, a fantastic movie by Jacques Perrin

Cooperating For Selfish Reasons

If you were a young adult Ethiopian wolf, you would have a choice to make: Should you be a member of a monogamous breeding pair or a helper to an already established breeding pair (who are probably your parents)? The choice seems obvious, right? I mean, who wants to be a helper? Why should you forgo all the glory and status of being part of the breeding pair to be a babysitter? 

The Governess painted by Rebecca Solomon in 1851 shows a modestly-dressed
Victorian era governess (in black) who diligently cares for the education needs of
her employer's young children, while the well-dressed employer is free to flirt.
Image provided by Wikimedia.

But Ethiopian wolves often do make that choice. These wolves are territorial rodent hunters and their survival and success depends on how many giant mole rats (their favorite food) and Murinae rats (a second-choice food-option) are available in the territory. In territories with fewer rodents, Ethiopian wolf families are likely to consist of a mother, a father, and their pup born that season. However, in territories with lots of rodents available, wolf families also include some of the older siblings from previous years. Why do they stick around?

An Ethiopian Wolf photographed
by Gert Vankrunkelsven.
Image available at Wikimedia.

Jorgelina Marino, Claudio Sillero-Zubiri, Paul Johnson, and David Macdonald from the University of Oxford in the U.K., set out to ask this question. They collected data on 17 wolf packs in the Bale Mountains of southern Ethiopia for 13 years. They did this by following the packs on foot or on horseback and watching them with binoculars. The researchers also mapped the quality of the habitats to estimate the number of giant mole rats and Murinae rats available.

These wolf packs included 13 wolf packs with territories in optimal rodent-hunting areas (high-quality habitat in the Web Valley-Sanetti area) and 4 packs with territories with very few rodents (poor-quality habitat in the Tullu Deemtu area). The packs in the high-quality habitat had from 3-13 wolves, usually including the breeding pair, their pup, their adult sons from previous years and some of their adult daughters from previous years (Adult daughters were more likely to set out on their own than the sons). The packs in the poor-quality habitat only had 2-3 wolves, including the breeding pair and maybe their pup.

The researchers discovered that the small packs generally had large but poor-quality territories. The wolf packs in high-quality habitats had smaller habitats, but the bigger the pack, the bigger their territory and the more high-quality habitat they had on their territory. This may be because for each additional wolf in the pack, the more hunting territory is needed to support it. But the researchers discovered that these large wolf packs had more high-quality territory per wolf than the smaller packs had. So if you were a young adult Ethiopian wolf, you would have more high-quality hunting territory for you if you were to choose to stay home with mom and dad and your other siblings than if you were to seek a mate of your own.

Wolves that lived in the Tullu Deemtu area had small groups and large territories, but the
territories did not have a lot of access to food. Wolves that lived in the Web Valley-Sanetti
area had more access to food and could live in larger groups on smaller territories. The more
wolves in the pack in the Web Valley-Sanetti area, the more territory they could defend
per wolf.  Figure from Marino, Sillero-Zubiri, Johnson, and Macdonald's 2012 Behavioral
Ecology and Sociobiology paper.

The researchers also explored other possible advantages of group living, but didn’t find much. These animals hunt alone, so larger groups do not hunt more effectively than smaller groups. And the helpers were not all that helpful as babysitters either: The breeding pair did not have more pups, and pups were not more likely to survive, in families that had more helpers.

So the main advantage for a young adult Ethiopian wolf to stay home with mom and dad a bit longer seems to be more access to better hunting grounds. Why would this be? Ethiopian wolves patrol the boundaries of their territories and pee on them to mark their territory. More wolves in the pack means more patrols and more pee. In this way, larger packs are more able to defend more and better-quality territory. This benefits each of the young adults that stay with the family, and even mom, dad and pup too… to a point. Once the pack reaches a size of 8 adults, the benefits per wolf decline. Packs larger than 8 are more likely to split into multiple smaller packs, each with its own breeding pair. One more benefit of being in a larger group: When young adults split off from the family pack to establish their own breeding pair, they often get to inherit some of their natal territory.

If you find yourself living with mom and dad later than you may have anticipated, it may just be worth it as long as the refrigerator stays stocked and the diggs are comfortable. And if you find yourself a mom or dad with an adult child living with you later than you may have anticipated, it may just be worth it as long as they help stock the fridge and keep the place clean. But as soon as the arrangement stops being beneficial for everyone, it is time to strike out on your own.

Want to know more? Check this out:

Marino, J., Sillero-Zubiri, C., Johnson, P.J., & Macdonald, D.W. (2012). Ecological bases of philopatry and cooperation in Ethiopian wolves Behavioral Ecology and Sociobiology, 66, 1005-1015 DOI: 10.1007/s00265-012-1348-x

Baby, You Light Up My World Like Nobody Else: A Guest Post

One Direction was inspired by the brightly
shining love of the bioluminescent ostracod.
Photo by Fiona McKinlay at Wikimedia.

by Rachel Wang

You might not have guessed that the song lyrics of the band One Direction could apply to the courtship of bioluminescent marine animals, but the female ostracod crustacean (relatives of crabs and shrimp) might want to sing her heart out when she finds a bright guy to light up her world. 

This month's cover of the Journal of
Experimental Biology features a picture
of one of Trevor and Jim's ostracods!

Bioluminescent ostracods, also called “marine firefleas,” are tiny creatures (at most just 2 mm!) that live at the bottom of the ocean and have the awesome ability to light up the water in the western Caribbean with their natural bioluminescence. Males wait until it’s completely dark to put on a light show for the ladies. They secrete molecules that react with the water and produce pulses of bright blue light. To court females, males flash brightly 3-4 times in the same spot before swimming up in a spiral pattern and producing up to 16 flashes. Then they do this over and over for an hour! Talk about a bright time!

Males have three different types of courtship tactics as they compete for a mate. Males who initiate a display are described as “leading,” males who synchronize their flashes with another male are “following,” and males who stay close to another guy without doing any flashing are “sneaking.” Males aren’t just capable of all three tactics – they can also switch between tactics within seconds, even multiple times within a single 12-second-long display!

You can see an example of a light display here:
Video provided by Trevor Rivers

Jim Morin at Cornell University and Trevor Rivers previously at Cornell (now at Bowdoin College) provided an illuminating look at what affects a male’s decision to lead, follow, or sneak. Since the males only show off in the dark, Trevor and Jim used infrared video to shed some light on the situation and track each male’s movement, speed, and distance to other males. They randomly picked five males (a normal group size) and observed their behavior for 30 minutes. They looked at how much time each male spent leading, following, or sneaking, as well as the distance and angle to other males.

Pay close attention to see some followers in this infrared video:
Video provided by Trevor Rivers

The researchers found that once one male decided to be the leader and initiate a display, the other males’ choice between following and sneaking was strongly predicted by their distance to the leader – mainly vertical distance! The figure below illustrates what they found. The leader is represented by the point where the vertical and horizontal lines cross. At 8 cm above the leader, males were equally likely to follow or sneak (green area). Those more than 8 cm above the leader (the blue area) were more likely to follow, while males who were less than 8 cm above (pink area) were more likely to sneak! 

This figure shows that males way above the leader (at the center of the circle) choose to
follow (i.e. "entrain"), whereas males near the leader choose to sneak. The graph on the
left (a) shows the starting position and eventual tactic of each male. The graph on the
right (b) shows where each male was when he started using his chosen tactic. Figure
from Rivers, T.J., & Morin, J.G. (2009). Plasticity of male mating behavior in a marine
bioluminescent ostracod in both time and space. Animal Behavior, 78(3): 723-734.

So why would vertical distance make a difference in picking a tactic? Trevor and Jim offer some possible explanations. Males who are closer to the leader tend choose sneakiness. Rather than wasting energy on a flashy show, they focus all their attention on snatching a female. Males who are farther away from the leader tend to follow. They decide to go for it, and synchronize their flashes with the leader to compete for the Best & Brightest Award, with the winner receiving the prize of a lovely female mate. Followers also swim farther out, which could help them intercept females approaching from the side and increase their chances of scoring a touchdown. The bottom line is that these male bioluminescent ostracods are always checking out their competition and quickly deciding on the best move!

So what can we learn from this enlightening tale? Guys, keep your friends close and your competition closer. Imitating or sticking close to the guy who’s got it all could pay off in the end. Just remember: stalking your competition? That might work for bioluminescent ostracods, but it’s not sexy by human standards.

If you’re interested in more info, check out:

1. University of Wisconsin-Lacrosse’s great, informative website on bioluminescent ostracods

2. Rivers, T.J., & Morin, J.G. (2009). Plasticity of male mating behavior in a marine bioluminescent ostracod in both time and space Animal Behavior, 78 (3), 723-734 DOI: 10.1016/j.anbehav.2009.06.020

Uncontrollable Love: A Guest Post

By Yunhan Zhao

Image from Freedigitalphotos.net

What is love? Under Shakespeare’s leather pen, love is the tragedy of Romeo and Juliet. In the poet’s eyes, love is the courage of Paris to elope with Helen and stand against the world. In Pretty Woman love doesn’t care about social status or wealth. When people fall in love the whole world feels brighter and more vibrant. Orpheus and Eurydice, Jane Eyre and Rochester, Darcy and Elizabeth…thousands of romantic stories, poems and songs illustrate the countless versions of love tales. Now scientists have revealed their scientific version of the love tale. They say love is comprised of magic chemicals interacting with our brains.

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

In 1999, researchers Mary M. Cho, Courtney De Vries, Jessie Williams and Sue Carter at the University of Maryland conducted NIMH funded research aimed to unlock the secret behind ‘love.’ By using the monogamous prairie vole (Microtus ochrogaster) as subjects, they investigated the important neurophysiological events that contribute to the development and maintenance of pair bonding between heterosexual adults. Here they designed an experiment to test the hypothesis that oxytocin and vasopressin (chemicals that are released in the brain during sexual behavior) facilitate the development of partner preference.

In their experiments, the researchers randomly assigned prairie voles to three different dose groups of oxytocin, three different dose groups of vasopressin and a control group which used cerebrospinal fluid. After receiving injections directly to their brains, each prairie vole was housed with an opposite sex partner for one hour (referred as cohabitation below). Then partner preference was assessed immediately by measuring the time spent in physical contact with either the familiar partner or a stranger vole.

Researchers found that the test subjects made stronger partner preferences depending on which kind of injection they received. As expected, results showed that in male prairie voles, almost all the vasopressin and oxytocin treatments (except the lowest dose of oxytocin) led to a loyal choice for the cohabiting partners over an unacquainted female. In females, however, only the highest dose of vasopressin and oxytocin treatments resulted in a loyal choice. In contrast, with the cerebrospinal fluid control treatment in both genders of voles, the test voles were always equally attracted to their previous partners and the unacquainted strangers.

Partner preferences of male (left) and female (right) after receiving different injections.
OT = oxytocin, AVP = vasopressin, CTL = cerebrospinal fluid. Figure from Cho, M.,
DeVries, A., Williams, J., & Carter, C. (1999). The effects of oxytocin and vasopressin
on partner preferences in male and female prairie voles (Microtus ochrogaster).
Behavioral Neuroscience, 113(5), 1071-1079.

This experiment found that high doses of both vasopressin and oxytocin may indeed associate with high partner fidelity in both gender of prairie voles. However, are these two chemicals the love elements in humans as well? Does it mean in the near future people can prevent divorces with only a few chemical injections to their spouses? The answer is still not clear. Especially since scientists have recently revealed that vasopressin and oxytocin may relate to distress in pair-bonded human relationships.

Shelly Taylor, Shimon Saphire-Bernstein and Teresa Seeman from the University of California, Los Angeles assessed 85 young adults in committed relationships. They asked participants to complete a series of self-reported questionnaires which included items like “how often they argue with you,” “how often they criticize you,” and “how often they get on your nerves”. Additionally, they also had the participants’ blood drawn within a week of finishing the assessments.

Image from Freedigitalphotos.net

Analysis showed that the elevated plasma oxytocin levels were associated with relationship distress in women while the increased plasma vasopressin levels were associated with relationship distress in men. But what should be noticed is that the direction of such effects remains unknown. Although no evidence revealed the exact relationship between humans’ plasma vasopressin and oxytocin levels and relationship distress, it appears that these two chemicals may be associated with very different emotional effects in voles and humans. However, an alternative explanation could also be that oxytocin and vasopressin contribute differently in different stages or contexts of romantic relationships.

Love seems uncontrollable even in the scientific world. Prairie voles crave emotional and sexual union after being injected with a high level of oxytocin and vasopressin into their brains, but people with the high presence of the same chemicals become vulnerable to relationship problems. So what is the key here?

References

1. Cho, M., DeVries, A., Williams, J., & Carter, C. (1999). The effects of oxytocin and vasopressin on partner preferences in male and female prairie voles (Microtus ochrogaster) Behavioral Neuroscience, 113 (5), 1071-1079 DOI: 10.1037//0735-7044.113.5.1071

2. Taylor, S.E., Saphire-Bernstein, S., & Seeman, T.E. (2009). Are Plasma Oxytocin in Women and Plasma Vasopressin in Men Biomarkers of Distressed Pair-Bond Relationships? Psychological Science, 2010 (21), 3-7 DOI: 10.1177/0956797609356507