Science Watch

Side preferences reign in the animal kingdom--that is, species from mice to monkeys, humans to crows, all tend to use one of their hands, paws or claws more often than the other. A phenomenon this pervasive must have some survival benefit, say scientists, but what that might be is not immediately obvious.

"There should be no advantage at all for preferring one hand or one foot because our world demands from us that both sides should be able to perform manipulations equally well," says Matthias Konstantin Laska, PhD, a biologist at the University of Munich who studies side preferences in new world monkeys. For example, a monkey grabbing for a banana would benefit from being able to choose the hand closest to the food, he notes. Additionally, localization of an ability to a particular area of the brain means an animal is more vulnerable to having that skill knocked out by a stroke or brain damage, notes Franziska Martin, PhD, a biologist at the Free University in Berlin.

Such apparent disadvantages to side preferences, notwithstanding, new research on elephants by Martin suggests there are also some benefits. She finds that limiting precise movements to one side of the body--and subsequently one-half of the brain--may lead to better muscular control, leaving the ambidextrous with less dexterity than animals who specialize.

This research, in addition to other studies on brain lateralization, may eventually help scientists understand the factors--evolutionary, genetic and environmental--that control what skills get localized to each side of the brain. And that could lead to progress in human medicine, including brain-injury rehabilitation, says Martin.

"Research on side preferences leads to a better understanding of how the brain works, and thus can be applied in medicine," Martin notes.

New findings

With precisely timed contractions of roughly 150,000 muscle fibers, an elephant's trunk can perform tasks as delicate as picking up a single peanut and as powerful as transporting a tree trunk. Controlling this remarkable appendage takes a lot of elephant brainpower, says Martin, and, consequently, its operation may take up significant brain space.

This may be why elephants show side specialization in an appendage that doesn't seem, at first glance, to be capable of it, she notes. And because each half of the elephant brain primarily controls the opposite half of its body, like humans, a side preference in the trunk implies that control of that appendage is mostly located in one side of the brain.

"If they specialize movement to one side of the brain, time might be saved in [cross-hemispheric] communication, and it might reduce the neural substrate devoted to this motor control in the neocortex," says Martin.

Some of the finest movements occur in the elephant's finger-like trunk tip, which can grasp and manipulate small objects. For example, a feeding elephant will wrap that finger around a tuft of grass and pull it from the ground. It can then cradle the grass in the trunk's middle while uprooting more with the trunk's end. Once the animal has collected enough roughage, it straightens its trunk to let the grass slide down to the finger. The finger then pinches the entire bunch and places it into the animal's mouth.

To uproot grass tufts, elephants must curl their trunk tips around them either clockwise or counter-clockwise, says Martin. And more than 90 percent of the time, they will choose to grasp the grass in the same way. In her view, "this is comparable to the strength of handedness in human--it shows how fundamental side preferences are for motor control."

Martin first shared her observations of this phenomenon in a study published in the December issue of the Journal of Comparative Psychology (Vol. 117, No. 4).

In that study, Martin and co-author Carsten Niemitz, PhD, professor of biology and anthropology at Berlin's Free University, videotaped a group of 41 wild Asian elephants feeding at the Uda Walawa National Park in Sri Lanka. The researchers captured 3,048 incidents of object contact--that is, wrapping the trunk finger around a tuft of grass. They also observed the side of the mouth the animal placed the grass into, a movement termed retrieval, and the side of the mouth the elephant then extended its trunk from--known as reaching.

In a subsequent analysis, Martin and Niemitz timed the elephants' feeding and found that those who preferred to reach and retrieve from one side of their mouths were up to 0.7 seconds faster at eating than more ambidextrous animals. A similar comparison could not be made for object contact because none of the animals demonstrated a weak side preference when moving the trunk finger, says Martin.

This finding suggests that localizing the fine motor control of the trunk to one-half of the brain may lead to more skillful and quicker movements, Martin says, and may explain the evolutionary advantage to having such a side preference.

Due to their large frames and inefficient digestive systems, "elephants need to eat a lot of food, so being able to do it quickly is important to their survival," notes Martin.

The only other study from the literature of comparative psychology or biology looking at an unpaired grasping organ, says Martin, reported an even stronger side preference in a very different animal: spider monkeys. The research, conducted by Laska and published in Cortex (Vol. 34, No. 1), found that when captive adult monkeys reach for raisins with their tails, they curl their tails around the food in the same direction 100 percent of the time.

"We found some of the strongest lateralization ever recorded" in monkeys, Laska notes.

Future applications

Martin is now beginning work on a project to determine whether these side preferences in elephants are learned from family members or determined primarily through genetics. Unlike humans, who are about 89 percent right-handed, elephants tend to be left- or right-trunked in equal measure. This suggests that elephant side preferences may be less genetically determined and possibly more plastic than human side preferences.

Fueling side-preferences research on elephants is the hope that it may provide an instructive contrast to lateral brain development in humans, researchers say.

Additionally, the neurological underpinnings of hemispheric dominance in elephants as well as humans and other animals are largely unknown, says Martin. But future research on this aspect of brain organization may eventually shed light on the impact of strokes and help develop therapies to hasten recovery.

Edward Taub, PhD, a psychology professor at the University of Alabama at Birmingham who develops physical therapies for stroke patients, cautiously agrees.

"Basic research like this often leads to important clinical advances," says Taub. "And sometimes the most unexpected areas and the most unexpected of findings gives someone an idea they can try out in human beings."

But first, says Martin, more research on the lateralized brain function of nonhuman animals must be done.

"We are still at the beginning," she explains. "Application in human medicine will come later on, if the chapter about laterality in unpaired grasping organs is enlarged to further examples."

Further Reading

  • Laska, M. (1998). Laterality in the use of the prehensile tail in the spider monkey (Ateles geoffroyi). Cortex, 34(1), 123-130.

  • Laska, M., & Tutsch, M. (2000). Laterality of tail resting posture in three species of New World primates. Neuropsychologia, 38, 1040-1046.

  • Martin, F., & Niemitz, C. (2003). "Right-trunkers" and "left-trunkers": Side preferences of trunk movements in wild Asian elephants (Elephas maximus). Journal of Comparative Psychology, 117(4), 371-379.