How can a mother--brain-dead from lack of sleep--still respond lovingly to a howling infant?
What makes lovers stay in relationships, even those doomed to failure?
And why is a rodent called the prairie vole such a paragon of monogamy and parental devotion?
Scientists think part of the explanation lies in a biochemical dance that orchestrates the interdependence and survival of a species. Researchers have isolated the hormone oxytocin, which may be responsible for social bonding in humans and some other mammals.
Sometimes called the love hormone or cuddle hormone, it not only stimulates uterine contractions in childbirth and generates milk production, it's the chemical that fuels a mother's love and affection for her baby.
Oxytocin is also a primary sexual arousal hormone for both sexes, a sort of internally produced Love Potion No. 9. It makes nerve endings highly sensitive to touch and pleasure, triggering muscle contractions in orgasm and encouraging lovers to emotionally bond.
"Oxytocin is the attachment hormone," says Charlotte Modahl, formerly a staff psychologist at Boston City Hospital who has investigated oxytocin in humans since the 1970s. "Breast-feeding mothers get a real jag from the hormonal stimulation. They cuddle, talk and coo to the baby more than when they bottle feed. Impotent men given oxytocin were more sexually aroused . . . talked [with] and cuddled their wives more. Multi-orgasmic women have higher levels than single-orgasmic women."
Great, you say, ship me a 55-gallon drum of the stuff.
Sorry, it's not quite that simple, Modahl says. Scientists are still exploring whether and how we might boost our supply.
While oxytocin affects the sexes in some of the same ways, Modahl says women appear to be more sensitive to its emotional effects. Women not only have oxytocin levels 2 1/2 times higher than men's, they also have much higher levels during sex, according to a Stanford University study.
This may explain, Modahl says, why women prefer to cuddle after sex while men typically take the roll-over-and-snore approach.
Some of the most compelling clues about oxytocin are found in the prairie vole, one of only 3% of mammals that are monogamous.
Thomas R. Insel, director of the Yerkes Primate Research Center at Emory University, and colleagues found that oxytocin and the hormone vasopressin, which in males stimulates monogamous, parental and territorial behavior, drive the animal's unusual pair-bonding habits.
Once voles forge an attachment through a marathon sex session, (talk about stamina . . . every 45 minutes for more than 24 hours!) they remain monogamous for life.
The hormones also seem to transform males into doting Mr. Moms, master snugglers in the nest and fierce defenders of the family against intruders.
Research on humans offers its own hints. Modahl and psychologist Deborah Fein, a professor at the University of Connecticut at Storrs, thinks a lack of the same hormones may help explain autistic children's aversion to touch and inability to form attachments.
In a study the researchers presented at the International Neuropsychological Society meeting in February, Modahl, Fein and colleagues found that oxytocin levels of 30 autistic boys were half that of average children. Even among average children, the researchers found gregarious and highly social children had higher oxytocin levels than introverted children.
Such research may help explain what has gone awry in the brains of people who have difficulty forming emotional attachments, including those with autism, Alzheimer's and learning disabilities.