Why Some Orangutans Never Want to Grow Up

Some males take decades to fully mature; this arrested development can improve their odds of mating success

Adult male orangutans
Adult male orangutans have large cheek pads and a big throat pouch, but it can take decades to develop such traits. Image courtesy of Flickr user nebarnix

In Interview with the Vampire, Claudia, portrayed by Kirsten Dunst in the movie version, becomes a vampire at age 6. Six decades later, she still has the body of a child but the thoughts and desires of a grown woman.

In this way, orangutans are kind of like vampires. They have their own form of arrested development.

When male orangutans hit puberty, they develop distinct traits known as secondary sex characteristics that separate them from females. In addition to being much bigger, males grow longer, shaggier hair on their arms and back and sport giant cheek pads. They also have throat pouches that resemble large double chins, allowing males to beckon females with loud long calls.

Some males are late bloomers, not acquiring these traits until as late as age 30. But looks can be deceiving. Even though these males appear to be youngsters, they are sexually mature and capable of siring offspring.

Scientists think the two different types of adult males—those with secondary sex characteristics and those without—are two alternative mating strategies that evolved in orangutans. A new study published online in the American Journal of Physical Anthropology tries to pinpoint the circumstances under which orangutan arrested development emerges.

To do this, Gauri Pradhan of the University of South Florida and Maria van Noordwijk and Carel van Schaik, both of the University of Zurich, considered the differences between orangutans living in Borneo and those in Sumatra. These Indonesian islands are the only two places in the world where orangutans are still found in the wild. But arrested development is largely limited to Sumatra.

Orangutans in both locations are mostly solitary. They roam the treetops alone, but they live in home ranges that overlap with those of other orangutans. In Sumatra, a female prefers to mate with the dominant male that lives in her neck of the woods. This male always has his full set of male features. A female finds the dominant male by following the sound of his long call, and when she’s ready to be pregnant, the two enjoy a sort of honeymoon—traveling and mating together for up to three weeks. Other adult-looking males may live in the same area, but females actively avoid their calls and stay hidden from them.

Because the dominant male is so popular, he can be choosy about mates. These males tend to pass over inexperienced females who haven’t yet had a baby. With younger adult females, it’s hard to tell if they are truly ready to become mothers, so it’s a better bet to stick with females who are already moms.

Yet some males are interested in these naïve females: the sexually mature males lacking adult traits. Unlike the other male orangutans, these guys don’t wait for females to come to them. They search the forest for receptive females, and Pradhan and his colleagues speculate that these males might father a lot of the children of first-time orangutan moms.

The sex lives of orangutans on Borneo are quite different. Here, no single adult-looking male is dominant. Many full-fledged males mate with an area’s females. Orangutan honeymoons are much shorter, and males may fight with each over a potential mate. Because the competition is so fierce, males aren’t choosy about who they mate with—and sometimes, even if a female’s not in the mood for mating, a male might force her to copulate.

Pradhan’s team incorporated these differences, as well as some assumptions about male growth, into a mathematical model. Their equations allowed them to determine which factors best explain the presence of immature-looking adult males in a population. The most important variable, they conclude, is the ability for one male to dominate an area. When this happens, as in Sumatra, it becomes beneficial for other males to have a covert mating strategy.

But if there is a lot of direct competition among males, as in Borneo, then it’s better to be a full-fledged male, who will always beat out immature males. No one male can monopolize females in Borneo because males tend to travel more on the ground there, the researchers say. That improves their mobility and makes it easier to quickly find females, even those who may not want to be found.

Thousands of years ago, orangutans once lived throughout much of Southeast Asia, even on the mainland. I wonder how pervasive arrested development was back then. Even if we had large bone samples, would anthropologists ever be able to detect such behavior in the fossil record?