There’s a wrinkle — or many — in the story behind an elephant’s trunk

Why is the elephant trunk so wrinkly?

It sounds like the start of one of Aesop’s fables. But in a new study in the journal Royal Society Open Science, researchers offer up some answers.

This all started when Michael Brecht, a neurobiologist at Humboldt University in Berlin, and his colleagues were studying the brains of African savannah elephants and Asian elephants when they noticed a bump on each of the brain stems the size of a fava bean.

“It’s very unusual. It looks very different from all other mammals,” Brecht says. “It has this striped appearance.”

It soon became clear to the researchers that the number of stripes — each one a bundle of nerve fibers — corresponded nearly exactly to the number of wrinkles in the elephant’s trunk. “You see every wrinkle in the brain stem,” he says.

How a trunk grows — and crinkles

This discovery motivated him to take a closer look.

First, Brecht and his team tracked down as many photographs of elephant fetuses as they could to examine how the trunk develops.

“It’s an incredibly fast growing body part, more than any other body part,” Brecht says. “So that’s how they come out to be an elephant instead of a normal mammal with a little nose like us.”

A couple months into development, the fetal trunk has about four wrinkles — at which point there’s sudden exponential growth.

“Every 20 days, the number of wrinkles doubles,” he says.

After a few months, things slow down.

“African [savannah] elephants only add relatively few wrinkles in adulthood,” Brecht says. “In Asian elephants, there are noticeably more wrinkles added.”

Brecht suspects this may be due to differences in how the two groups of elephants use their trunks for grasping. African savannah elephants use the tips of their trunks to pinch objects, pick them up and manipulate them.

Asian elephants can do some pinching. “But when you give them a big fruit like a melon or a mango,” says Brecht, “they wrap their trunk around the object. So our thinking is that the wrinkles serve for making the trunk flexible.”

In other words, the more flexible a particular section of the trunk needs to be, the more wrinkles there are to provide the extra skin that’s needed.

For instance, Brecht and his colleagues observed differences between elephants that are left-trunkers (meaning they grasp to the left) and right-trunkers (meaning they grasp to the right). This suggests that the way the elephants use their trunks might influence the arrangement of wrinkles, given that baby elephants are born with symmetrical trunks.

Trunks may hold lessons for the rest of us

Brecht also used a micro-CT scanner to look at newborn trunks in detail. He found that the skin is super thin in the troughs of the wrinkles, allowing them essentially to behave like joints.

Maëlle Lefeuvre, a behavioral ecology doctoral student at Jagiellonian University in Kraków who wasn’t involved in the study, thinks there may be engineering lessons to learn from these findings.

“The trunk of the elephant has been studied a lot,” she says, “because we try to understand how we can create things that are strong without any hard support.”

Lafeuvre says understanding wrinkle formation and function could help inform the construction of a flexible and powerful trunk-like machine or robotic arm, for example.

“This study represents a significant step towards the understanding of this important structure in these animals, showing this connection between form and function,” says Kamilla Souza, a neuroscientist at the Federal University of Rio de Janeiro who didn’t contribute to the research.

While a study like this isn’t easy to pull off, Souza points out there are a few shortcomings.

First, the African forest elephant, a third species, wasn’t included in the analysis. She says those animals would have provided another point of reference for the development and use of the trunk.

Second, the research is based entirely on zoo animals. “Probably they don’t exhibit the whole range of possible behaviors that could be seen in the wild population,” she says.

For Brecht, the findings reinforce how these body parts are exquisitely honed to help giant creatures feed, wherever they may live.

“You cannot be such a big animal without being very heavily specialized,” he says.