You Dirty Vole

No one knew that voles were monogamous until Lowell Getz, a now-retired professor of ecology, ethology, and evolution at the University of Illinois, began studying them in 1972. At the time, Getz wanted to figure out why the vole population would boom during certain years and then slowly go bust. He set traps in the grassy plains of Illinois and checked them a few times a day, tagging the voles he caught. What surprised him was how often he'd find the same male and female sitting in a trap together.


Voles build soft nests about 8 inches below ground. A female comes of age when she is about 30 days old: Her need to mate is then switched on as soon as she encounters an unpartnered male and sniffs his urine. About 24 hours later, she's ready to breed-with the male she just met or another unattached one if he's gone. Then, hooked, the pair will stick together through thick and thin, mating and raising young.


Getz found vole mating behavior so curious that he wanted to bring the animals into the lab to study them more carefully. But he was a field biologist, not a lab scientist, so he called Sue Carter, a colleague and neuroendocrinologist. Carter had been studying how sex hormones influence behavior, and investigating monogamy in voles dovetailed nicely with her own research. The animals were small: They made the perfect lab rats.


The scientific literature was already rich with studies on a hormone called oxytocin that is made in mammalian brains and that in some species promotes bonding between males and females and between mothers and offspring. Might oxytocin, swirling around in tiny vole brains, be the catalyst for turning them into the lifelong partners that they are?


Sure enough, when Carter injected female voles with oxytocin, they were less finicky in choosing mates and practically glued themselves to their partners once they had paired. The oxytocin-dosed animals tended to lick and cuddle more than untreated animals, and they avoided strangers. What's more, when Carter injected females with oxytocin-blocking chemicals, the animals deserted their partners.


In people, not only is the hormone secreted by lactating women but studies have shown that oxytocin levels also increase during sexual arousal-and skyrocket during orgasm. In fact, the higher the level of oxytocin circulating in the blood during intercourse, the more intense the orgasm.


But there's more to vole mating than love; there's war too. Male voles are territorial. Once they bond with a female, they spend lots of time guarding her from other suitors, often sitting near the entrance of their burrow and aggressively baring their beaver-like teeth. Carter reasoned that other biochemicals must kick in after mating, chemicals that turn a once laid-back male into a territorial terror. Oxytocin, it turns out, is only part of the story. A related chemical, vasopressin, also occurs in both sexes. Males, however, have much more of it.