Right Finned ... or Left?
Scientists are discovering that, like humans, animals from whales and walruses to frogs, parrots and octopuses favor one side over the other
- David Brian Butvill
- May 01, 2007
WHEN GEORGE HARRISON of the Beatles made his 12-string guitar gently weep, he strummed with his right hand and balanced the instrument's neck in his left, like most guitarists. Band mate Paul McCartney, on the other hand, is an odd duck who drives a bass groove using his left fingers. Like McCartney, drummer Ringo Starr also favors his left side.
Handedness among humans is well-recognized, of course, but it turns out we're not the only creatures that prefer using one limb over the other. Many frogs are right-pawed as was Harrison, while parrots tend to be left-clawed like Paul. Lizards often lean left, the way Ringo does. And new research suggests that many walruses--like John Lennon, in fact--are righties. Imagine that.
Indeed, the more scientists observe nonhuman animals, the clearer it becomes that the very nature of nature is to favor one side over another--a phenomenon that goes beyond hands, paws, claws and other limbs. Humpback whales, for example, for the most part bank right. To dine on mud-dwelling critters, a whale dives down to the sea bottom, turns to one side while holding its beaklike mouth agape, and skims the top of the nutritious muck. Researchers have found that humpbacks have abrasions from this bottom-feeding maneuver on either their right or left cheek, never both. Gray whales and bottlenose dolphins have also been shown to be "right-jawed."
"Sidedness has been found in all vertebrate groups in which it's been studied--horses, cats, rodents, reptiles, different fish species, to name a few--just to different extents," says Ruth Byrne, a biologist from the University of Vienna, Austria. And recently, Byrne discovered that an invertebrate, the common octopus, also appears to use a favorite tentacle for inspecting or handling objects, even though all eight arms are equally talented.
While scientists have not agreed on a single theory to explain sidedness, clues are emerging. As early as the mid-1800s, in fact, scientists studying stroke victims discovered that just one of the two hemispheres of the human brain controls abilities such as speech and language comprehension. More recently, biologists have found that frog vocalizations and birdsong are also driven by a single brain hemisphere.
According to Lesley Rogers of Australia's University of New England, who has studied brain hemisphere specialization in chickens, "The left hemisphere seems to control responses that have to be considered, whereas emotional reactions--something that you must do quickly, like a strike at another animal--that's the right hemisphere." Given that each hemisphere tends to operate the opposite side of the body, such specialization has given rise to interesting behaviors. Crafty crows on the island of New Caledonia, which sculpt twigs into hooks they employ to fish out insects from tree crevices, use predominantly their right eyes while whittling. But "a toad is more likely to tongue-strike another that is on its left side," says Rogers, who has found similar tendencies among chickens and baboons.
Split-brain thinking might also lead to the tentacle bias in Byrne's octopuses. She discovered that 92 percent of the animals have a preferred eye for inspecting objects--and that their favorite feeler almost always lies directly between that eye and the object, which "suggests that octopus arm choice is driven by lateralized eye use," says Byrne.
Some partialities have less heady explanations. Many frogs, after swallowing something disagreeable, don't just blow chow but hurl their stomachs out their mouths like airbags. Then they empty the organ--curiously, always with the right foot. It turns out the membranes that anchor the stomach inside the body cavity are shorter on the right. Upon upchuck, the stunted tether inevitably swings the stomach so that it dangles out the right side of the mouth; it's handy to wipe it with the closest foot.
Skeletal differences can also throw an animal off course. Pacific tree frogs tend to veer left when they leap because their right legs are longer than their left. And biologist Nette Levermann of the University of Copenhagen recently drew a link between bone development and right-flipperedness in North Atlantic walruses. These ivory-tusked, one-ton beasts feast on shellfish, which they usually expose from the seabed by waving a flipper to wash away silt. Levermann found that feeding walruses in his study fanned with right flippers about 90 percent of the time. He also noted that walrus limb bones on that side are longer, just as they normally are in the dominant forearms of humans. Given that exercising a limb strengthens its skeleton, which in turn may lead to more use, the finding begs the question: Which came first--the behavior or the walrus's reinforced flipper?
This conundrum lies at the heart of John Pepper's studies of glossy black cockatoos on Australia's Kangaroo Island. The University of Arizona evolutionary biologist discovered that the birds used their left talons to handle food. More importantly, every single bird husked a seed by securing it with the top mandible and passing the lower one across it, always to the right. Pepper argues that bill-shifting is a unique directional prejudice that cannot be explained by current theories. Unlike walrus flippers and frog legs, the beak is an unpaired, central appendage--there is no left and right member.
It may not be lopsided limbs or mental programming that sway behavior, he says, but the other way around. "Once an animal begins performing a task in a randomly chosen way, it's better to repeat it than to learn it over again another way," Pepper explains. "Try brushing your teeth with your 'other' hand. You'll quickly realize that training both hands is equivalent to learning two separate skills and takes twice as long. For something as critical to survival as food-handling for cockatoos, doubling the learning period would be highly disadvantageous." Habits passed on through generations over evolutionary time could lead to, say, fortified flippers or the specialization of brain hemispheres that Rogers and so many others have worked out.
Whatever its causes, one thing is certain: Sidedness is ancient history, even among humans. Prehistoric artwork in caves in western Europe indicates the proportion of lefties and righties there hasn't changed in more than 10,000 years. Cave-dwelling Homo sapiens held a tube in one hand and blew dye over the other to stencil prints onto cave walls. Assuming these early artists painted with their dominant hand, the impressions reveal that 77 percent of the population was right-handed and 23 percent left-handed--the exact same distribution that exists in the region today.
Costa Rica-based writer David Brian Butvill reported on vultures using cosmetics in the August/September 2006 issue.
Like populations of humans--which tend to be 70 to 90 percent right-handed--animal populations seem to be split between "righties" and "lefties."