Adolescent Boys Roam Farther To Find Mates

Sex differences in human travel behavior may have evolutionary roots.
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Nala Rogers, Contributor

(Inside Science) -- From myths to cartoons, our stories are full of teenage boys going off on roaming, romantic adventures. These stories may have a basis in fact, according to a new study: Men in a South American society travel more than women, but only during adolescence, when they are most actively seeking romantic partners. This pattern is common among mammals, where males often roam farther in order to mate with multiple females. The study suggests that young males' travels are guided by the same evolutionary forces, regardless of whether the males are voles leaving the nest or young men seeking their fortune.

"We saw an upward bump in travel for one sex, males, during the mate formation period," said Steven Gaulin, an anthropologist at the University of California, Santa Barbara and senior author of the study. "It looks like at least this human population follows the general pattern that we see for mammals."

From an evolutionary standpoint, it makes sense for males to travel more. Female reproduction is usually limited by how many babies a female can nourish, whereas males can keep having offspring as long as they can find someone to impregnate. Often, the number of offspring varies more for males than females; some successful males father young with multiple mates, while other males have no offspring at all. This tends to be less true in the few mammal species that form life-long monogamous pair bonds, and more true in species where a single male can have many mates, according to Gaulin.

Mating systems vary widely in our species, but men vary more than women in how many mates and children they have over their lifetime, even in societies like the United States, according to Gaulin. This suggests that some human men seek multiple mates, and if they’re like other species, they will travel to find them. 

To understand how humans travel, the researchers interviewed 105 Tsimane adults. The Tsimane are an indigenous group who hunt, farm and forage in the Bolivian jungle. Most of their marriages are monogamous, but the number of children varies more for Tsimane men than women. They live in villages of about 100 to 200 people, and they generally travel to neighboring villages on foot or by canoe, according to Emily Miner, who gathered the study data while she was a Ph.D. student at UC Santa Barbara.

Miner asked participants which Tsimane villages they had visited during childhood, adolescence, and adulthood, with detailed questions about their travel in the past month. She interviewed adults because many of the adolescents were traveling at any given time. Most metrics revealed no sex difference in overall travel across the lifetime. However, during adolescence, boys traveled greater distances and visited more villages than girls. Women with young children reported less travel in the past month than those without young children, a pattern not found in the men. The study was published online October 15 in Proceedings of the Royal Society B.

While other researchers have observed that men travel more than women, few have studied the difference systematically, and even fewer have looked at changes across the lifetime. One team found a similar relationship between age, sex, and travel pattern among the Aka of Africa, but the age effects were hard to pin down because the Aka do not keep track of dates. The Tsimane study confirms the pattern with better age data, according to Barry Hewlett, an anthropologist at Washington State University in Vancouver and first author of the Aka study. Hewlett called the new study “excellent, very systematic and thorough.”

The age effects help explain why men travel, according to Gaulin. Other researchers have suggested that men travel more than women in order to hunt. However, among the Tsimane, adult men do most of the hunting, while teenage boys do most of the traveling. Gaulin sees this as evidence that Tsimane males travel in search of romance, not food.

Some people object to the idea that evolution has shaped human and other animal behavior in the same way, according to Gaulin. But biologists already understand why males travel more than females in non-monogamous species, and they don't need a new explanation for humans. To learn how we're special, said Gaulin, we first have to learn how we're like other animals.

"I want people to think about humans with the lenses of a Martian biologist," said Gaulin. "I want them to say, 'until proven otherwise, we're just another mammal. Just another somewhat hairless mammal.'"

Nala Rogers is a science writer based in Santa Cruz, California. She tweets at @Nala_Rogers.