National Geographic News
Night, or owl, monkeys in a tree in Yasuni National Park, Ecuador.

Owl monkeys peek out of a tree in Yasuni National Park, Ecuador.

Photograph by Tim Laman, National Geographic

Christine Dell'Amore

National Geographic News

Published February 13, 2013

It may not seem like monkey business, but emotional bonds in animals such as primates may have evolved into love as we know it.

Take owl monkeys, tiny tropical tree-dwellers that treat every day like it's Valentine's Day. A male and a female stick together as long as possible, never cheat, and never "divorce" their mates—extremely unusual behavior, even among people. (Also see "Male Monkeys Wash With Urine to Attract Females?")

Sometimes, though, young adult owl monkeys that can't find mates—monkeys that scientists call floaters—pick vicious fights with established pairs, eventually kicking one of them out.

Now, new research shows that the monkeys forced to take on new partners have fewer babies than owl monkeys that haven't been broken up, said Eduardo Fernandez-Duque, a biological anthropologist at the University of Pennsylvania in Philadelphia who led a new study on owl monkey relationships.

The results show how monogamy helps owl monkeys—and may even shed light on how human relationships evolved, said Fernandez-Duque, who has received funding for his work from National Geographic's Committee for Research and Exploration. (National Geographic News is part of the National Geographic Society.)

"Call it love, call it friendship, call it marriage—there is something in our biology that leads to this enduring, emotional bond between two individuals that is widespread among human societies," Fernandez-Duque said in a statement.

Trouble in Paradise

Only about 5 percent of mammals are monogamous, and the phenomenon most often arises when both parents are needed to raise offspring, as in the case of people.

With owl monkeys, fathers take on most of the childcare after a baby is born, relying on the mother only for milk. (See video: "Owl Monkey Fathers Know Best?")

But floaters—which Fernandez-Duque and colleagues first noticed in 2003 in Argentina's Chaco region (map)—can spell trouble in paradise.

Drawing on nearly two decades of observations of 18 owl monkey groups, the team discovered that pairs that stay intact produce 25 percent more babies than monkeys in severed pairs.

The exiled animal from those broken relationships, meanwhile, is usually injured and often dies.

Since the team studied more than 150 animals, "I felt very confident that what he was telling us is a real phenomenon—it's not a flash in the pan," noted Patricia Wright, who was one of the first people to study owl monkeys in the 1980s.

"He had the goods on the animals. I was really excited about that," said Wright, an anthropologist at Stony Brook University in New York.

Wright said she was personally pleased that the study reinforced findings that owl monkeys stay true to one another unless forced to separate.

"I knew that these little monkeys didn't fool around," she said.

Chemistry of Love

Why monkeys that are broken up have fewer babies is unknown, though Fernandez-Duque suspects there's an emotional component. (See more pictures of all-star animal dads.)

Just as a man and a woman need time to get to know each other and form a deep connection, so do owl monkeys. So when a marauding monkey enters into a new relationship, there's a delay in mating—usually about a year, Fernandez-Duque  said.

In fact, pair bonding in monogamous animals, such as owl monkeys, may be "sort of evolutionary antecedent to love in humans," said Larry Young, a behavioral neuroscientist at Emory University in Atlanta and author of the new book The Chemistry Between Us: Love, Sex, and the Science of Attraction.

Young, who studies the brain chemistry of love and emotion, does most of his research on monogamous prairie voles.

Though human love is a rich emotion reflective of our advanced brains, he said, "the foundation of that emotion is very similar to the neuromechanisms that are causing the bond between these two prairie voles."

For instance, experiments have shown that if a vole loses its partner, the "widowed" animal shows depressive symptoms—measured by a lack of willingness to escape a dangerous situation.

According to Young, our brains are in the love seat, so to speak: The organs "have evolved the mechanism to produce an emotional attachment," he said.

That attachment is spurred by oxytocin—produced during intimate contact in both people and animals—and dopamine, which is responsible for feelings of exhilaration and happiness.

So, many splendored as it is, love, he said, "is really the result of a cocktail of chemicals."

The owl monkey study was published January 23 in the journal PLoS ONE.

4 comments
Papa Foote
Papa Foote

"..Take owl monkeys, tiny tropical tree-dwellers that treat every day like it's Valentine's Day. A male and a female stick together as long as possible, never cheat, and never "divorce" their mates—extremely unusual behavior, even among people..."

FYI - from The Old Mountain Goat!

Kathleen Hillman
Kathleen Hillman

No offense, James Kohl, but it sounds like the owl monkey of Ecuador has evolved beyond the human mammal when it comes to monogamy and "love". I enjoyed the article and your comments, as well.

James Kohl
James Kohl

Excerpt: "According to Young, our brains are in the love seat, so to speak: The organs "have evolved the mechanism to produce an emotional attachment," he said."

Organs don't evolve mechanisms. The molecular mechanisms of evolved brain development and behavior associated with emotional attachment are nutrient-dependent and pheromone-controlled via the well-detailed gene-cell-tissue-organ-organ system pathway that links sensory input directly to gene activation in hormone-secreting nerve cells of the mammalian brain, and to behavior and back to gene activation -- as is required for species diversification.

"It is now clearer how an environmental drive probably evolved from that of food ingestion in unicellular organisms to that of socialization in insects. It is also clear that, in mammals, food odors and pheromones cause changes in hormones such as LH, which has developmental affects on sexual behavior in nutrient-dependent, reproductively fit individuals across species of vertebrates." -- Kohl (2012)


Kohl, J.V. (2012) Human pheromones and food odors: epigenetic influences on the socioaffective nature of evolved behaviors. Socioaffective Neuroscience & Psychology, 2: 17338.

Johnathan Fitch
Johnathan Fitch

@James Kohl

You say "organs don't evolve mechanisms" but then you link "sensory input" to "nerve cells of the mammalian brain." Seems contradictory since the brain is an organ. 


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