What Prairie Voles Teach Us About Love

So brain evolution is a strange topic to fall in love with, I know, but I’ve been daydreaming about it since I was an undergraduate at the University of Illinois. I took freshman biology in an enormous lecture hall with 800 students and the room was so big that I could fit my entire hometown in it and still have empty seats, but I loved it. I learned for the first time about animal behavior as a scientific discipline. I learned about neuroscience and evolution and in my dorm room I remember daydreaming about what the brains of different species might have to teach us about emotion or memory. I remember thinking about the great crested grieve and their courtship, the friendship of young chimpanzees. And I wonder what it’s like to be in orangutan. There are great apes like us, but they’re solitary and they don’t form bonds. And so sex for them is just sex. I guess there are a lot of reasons to study the brains of other species to understand, for example, how social bonds can protect us against stress. But for me it’s kind of always really been about the draw of inhabiting another mind.
In 1994, I came to the University of Texas as a graduate student and for my dissertation I studied mate choice and I studied as evolution using simulations of kind of early versions of artificial neural network models. I went to the Smithsonian and chased frogs to test my models, and it was there that I learned about poison frogs. Some species are so toxic that a single frog could stop a hundred human hearts, but that’s not what interested me. What interest me is their behavior. So males and females lay eggs together on the forest floor, just a few, and the male will tend them. And when an egg hatches, the tadpole climbs onto his back and then he carries that tadpole from the forest floor up into the canopy of the rainforest, and he deposits the tadpole in a little pool of water at the base of a md and he comes and checks on the tadpole.
If the tadpole is hungry, he’ll wag his tail to beg and then the male will call his mate and the mother will come and lay unfertilized eggs for the tadpole to eat. And strangely though, they don’t recognize they’re young by markings or by scent, but by the place the brom is in the tree. Tree tops. After my PhD, I went to Atlanta to join a group studying a weird rodent. Most mammals are like orangutans. They don’t form bonds in response to mating, but prairie vs do. After a day of intense, a day of intense copulation, they form a bond that lasted lifetime. They raise young together, they share a nest, and so they become the dominant way that neuroscientists study the neurobiology of love. We find that hormone receptors are expressed in brain regions critical for reward and addiction. And species that don’t form bonds have these hormone receptors, but they’re lacking in these key regions.
Well, inspired by the vol works psychologists image the brains of undergraduates as they looked at photographs of someone they were in love with. And what they found is that these same brain regions lit up. Now it’s striking to see the similarity across species, but as a young scientist, I was really curious about differences within a species. And so I went to Illinois, back to the University of Illinois where the Prairie Vs are from, and I set traps baited with cracked corn in the little bits of field that surround the corn fields around the University of Illinois. And I caught vols and I took them back to the lab, and then I looked at their brains. I sliced them into very thin sections and incubated them with a radioactive hormone and exposed that to x-ray film to visualize where the receptors were. And I found enormous variation. The main place we saw differences was in a brain circuit, important for memory, navigating space use for recalling distant events. And some animals had lots of receptor in this brain region and others lacked it altogether. Neuroscientists often think about brains as being sort of characteristic of a species as though any differences were some sort of disorder. But I caught these animals in the wild and they were thriving. It was kind of a puzzle.
Well, I took a job at the University of Florida as a faculty member, and there I met somebody interesting. He was handsome and shy, and I remember that he was standing with his back to the wall and I was facing him and behind me, our boyfriends were dancing together on a crowded dance floor. And I remember how uncomfortable he was as I asked him question after question. He just answered yes or no. We didn’t see each other for a few years after that. And so it’d be a while before I got to know how different we were. I’m an extrovert. He’s an introvert. I love the outdoors. He finds it frankly terrifying. But today, 20 years later, we’re married and still I can picture that first meeting like it was yesterday. I was such a sucker.
Meanwhile, my lab was really interested in understanding these differences in bowls. So we outfitted them with little collars with radio transmitters, and we tracked them in the wild. And what we found is that animals with a lot of hormone receptor in this memory circuit use space differently. They tended to stay close to their nest and they kept track of their loved one and they never strayed. Whereas animals that lack the receptor tended tended to stray into others’ territories and often M it with other individuals. So we kept working on this and then we chased down this brain variation to do two different DNA variants. There was a high expressing variant and a low one. And then we asked, well, maybe we can detect natural selection operating on these different genotypes. And so we then looked at the fitness of animals in the wild, and we found that the high expressing genetic variant was more fit in the context of faithful pairings, but the other allele was more fit in the context of these unfaithful pairings.
And when we threw the data together, to our surprise, we found that both alleles were equally fit. They were perfectly balanced. And when we looked at them, the DNA sequences more closely, we saw that there was an excess of differences in their sequences. And that’s a signature of something we call balancing selection. It meant the natural selection had kept these two brain types around for a very, very long time because each strategy had its own benefits. So people often ask me what any of this has to do with human love? And I mean, we know that there are brain differences, that there’s brain activity patterns that draw on reward and pleasure more generally, that are used in love. And we know that, that it’s powerful and addictive, that it leaves an imprint on memory. And I encourage you to think about a time when you met somebody that you would later come to love. Do you remember where you were or what was said? Well, I don’t think differences between us come down to differences in memory, but I do think brains come in many forms and there are many ways to experience intimacy and getting to know another species is a little, getting to know another person. There are things we share and things that set us apart, but ultimately that understanding is its own reward. Thanks.