Let me tell ya 'bout the birds and the bees
And the flowers and the trees
And the moon up above
And a thing called "Love"
—“The Birds and the Bees,” recorded by Jewel Akens, 1965
What biological processes stimulate animals to mate and humans to enter into romances?
New research presented at the Society for Neuroscience meeting reveals aspects of what happens in the brain of someone feeling intense love, as well as the sensory and molecular processes involved in love and mating.
Secrets of long-term relationships
Can romantic love last? Anthropologist Helen Fisher, an investigator with the Center for Human Evolutionary Studies at Rutgers University, said physical evidence suggests so.
In one of Fisher’s many studies about romantic love, she used functional magnetic resonance imaging (fMRI) to observe the brain activity of 17 volunteers (ten women and seven men) who said they were still intensely in love after having been married an average of 21 years.
When looking at photos of their loved ones, the study participants’ ventral tegmental area, located in the midbrain, was activated. Among long-term lovers, the highest level of activity occurred in areas of the brain associated with calmness, attachment and pain management.
Short-term lovers also showed activity in the midbrain when looking at images of their lost loves, but in specific areas associated with obsession. Two months after being rejected by their lover, participants’ brains showed signs similar to addiction. “It looked similar to a cocaine rush,” said Fisher.
Biological factors along with cultural evolution have moved humans beyond simple reproduction to romantic love, Fisher said. Oxytocin, a hormone released by touch and involved in the ability to bond, trust, and learn, may have played a role, she said.
Genetics influence human odor perception
Along with visual-based attraction, researchers are learning more about the importance of smell and taste in the mating game. Humans have 387 pheromones—chemicals that trigger instinctive behaviors. Androstenone, the first one discovered, is present in human sweat, saliva and urine.
Exposure to androstenone can cause physiological responses in both men and women, but only when one’s own androstenone has been activated, according to Hiroaki Matsunami, a researcher at Duke University in Durham, N.C.
In a study of 400 volunteers, Matsunami found three genetic variations that affect olfactory perception. Depending on their genes, some people sensed androstenone and considered it pleasant, while about 30 percent who had a different gene combination found it to be a “sickening,” noxious odor. A third group could not detect the pheromone at all.
Pheromones affect animals’ behavior
Since discovering the first pheromone receptor and many since, Catherine Dulac, a molecular biologist at Harvard University, has researched how the chemicals influence the way rodents and birds react to potential rivals and mates.
Dulac discovered that pheromones (released from the vomeronasal organ, or VNO, an olfactory organ located at the base of the nasal cavity) relay their signals through a common gene called TRPC2. In repeated experiments, male mice bred without the TRPC2 gene did not act with typical aggression toward other males. In fact, they initiated sexual and courtship behaviors toward both males and females.
Dulac’s team also found that female mice without TRPC2 displayed fewer female-specific behaviors and instead adopted male sexual behaviors. Those developments suggest that genes determine the ability to differentiate between sexes. Instead of triggering mating behavior, the VNO modulates the sex specificity of mating, Dulac said.
“If you put a VNO deficient male in a cage [with newborn mice] it can’t differentiate between male and female,” said Dulac. A video showed males without TRPC2 genes making nests and performing other nurturing actions. Dulac said the research shows that the brain is not encoded as all male or all female.
"The concept of a 'male brain' and a 'female brain' went out the window with that study," Dulac said.
It’s not expected that the VNO has such a singular role in detecting gender in the human brain, but Dulac’s work has generated interest from investigators in other fields, including those studying gene expression in transgender people.