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If you watch prime-time television, or read celebrity gossip magazines, you might come to the conclusion that monogamy is an unnatural condition. And in many animals, in fact, it is: the male sticks around just long enough to mate with a female and then leaves her to take care of the kids. But other animals are genetically programmed to settle down with a long-term partner. In this Science Update, you’ll hear about two different species of rodents, called voles, which have two very different strategies in this department. What sets them apart?
A rodent's cheating heart. I'm Bob Hirshon and this is Science Update.
Why is one male monogamous, while another plays the field? In humans, that's still difficult to answer. But in little mouse-like rodents called voles, scientists are getting a pretty good idea.
Larry Young is a researcher at Emory University and the center for behavioral neuroscience. He says one vole species, called the Montane vole, is promiscuous -- the male skips out on his mate as soon as she's pregnant. But his closely related cousin, the prairie vole, is monogamous.
They form these long lasting social attachments with each other. And the males and females nest together, they have their babies, and the males spend just as much time taking care of those babies as the females do.
So why are they different? Young says a prairie vole gets a natural high from being with its mate, while the Montane vole doesn't.
When a monogamous prairie vole mates, vasopressin is released and it activates receptors in certain areas of the brain that are involved in pleasure and reward. And they're actually the same areas of the brain that amphetamines and cocaine act on to produce addiction.
Young says understanding how such behaviors are controlled in simpler creatures could someday help us understand our own.
For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
Scientists have long been interested in the biological roots of mating behavior, since animals have so many different strategies and social norms. In this case, remember that we're not talking about apples and oranges: these are two different kinds of voles, with pretty subtle differences between them.
The difference Young is interested in has to do with the way the voles' brains respond to vasopressin. That's a brain hormone that helps male animals form social and, if you will, romantic attachments. The main difference between the monogamous prairie voles and the love 'em-and-leave 'em Montane voles isn't in how much vasopressin they have, but in the exact location of the cells that respond to vasopressin in the brain. In the prairie voles, they're concentrated in areas that produce feelings of pleasure and reward. So these are the kind of voles that might write mushy songs about how wonderful it feels to be in love, if in fact voles wrote songs. They more or less get "addicted" to mating with a particular female. For the Montane voles, on the other hand, the prospect of settling down just isn't so thrilling. That's because the cells that respond to vasopressin in their brains don't produce the same feelings of pleasure.
So in other words, it looks like something as simple as the distribution of a few very specific cells in the brain makes the difference between a playboy and a family guy. At least in voles. In humans, the picture is probably a lot more complicated. But humans have vasopressin too, and it's possible that individual differences in the way our brains react to it could help shape our attitudes toward marriage, monogamy, and commitment.
Now try and answer these questions:
- How are monogamous prairie voles different from promiscuous Montane voles?
- Suppose you put a male prairie vole in a cage with a female, allowed them to mate, but injected the male with a drug that blocked the activity of the vasopressin hormone. What do you think would happen? (Scientists have actually done this experiment. (Once you've guessed, click here to find out what they learned.))
- What if you gave a male Montane vole an extra injection of vasopressin before mating? Would he form an attachment to the female? Why or why not?
- In the animal world, what are some of the advantages of monogamy? What are the drawbacks? What do animals gain by avoiding long-term attachments?
- Some scientists think an animal's environment can influence the type of mating strategy it uses. The monogamous prairie voles live in open, flat grasslands. The promiscuous Montane voles live in the Rocky Mountains. How do you think their mating strategies fit into their environments? (Remember that voles are small and easy targets for predators).
- If humans could be given a drug, or some kind of medical treatment, that made them more able to commit to their mates, would you approve of it? What if it made them more social and loving in general?
Do you want to see what a vole looks like? Visit the National Museum of Natural History for a picture of the Montane vole; go to Howard White's homepage at Murray State University to see what a Prairie vole looks like. Then, check out the differences in the brains of a male prairie vole and a male Montane vole at enature.com.
Love and Sex: The Vole Story describes the work of researcher Tom Insell, who has studied voles to understand such behaviors as monogamy and parenting.