Tarantulas are more like us than we thought: Many of them are "right-handed."

While biologists used to believe that showing a preference for the right or left side is unique to humans, more and more studies are showing that other animals, like kangaroos, do it too.

For the first time, scientists have discovered handedness in tarantulas.

In a recent study published in the Journal of Zoology, scientists put captive male Honduran curly hair tarantulas into a T-shaped maze with two possible choices at either end.

The team put the grapefruit-size arachnids through a variety of tests. In a set of odor-based experiments, the tarantulas could smell live cockroaches—a favorite prey—at the end of both tunnels; in another, the male tarantulas could detect the smell of female tarantulas. In both cases, the males chose the right path more often. (See 10 beautiful photographs that will make you love spiders.)

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The team also evaluated what time of day and under what light conditions the males were most active, and found them to move around most during the night and early morning. The spiders moved toward light while in the tunnels, and when exposed to an identical level of light at the end of both tunnels, the male tarantulas, again, more often opted for the right.

Furthermore, the team observed that the male spiders prefer to use their right eyes and feet while moving.

Potential Flaws

Stuart Longhorn, an honorary research associate at the Oxford University Museum of Natural History in the United Kingdom, says the study is generally good, but he sees a few flaws.

He says that many of the variables are well controlled, but the researchers don't seem to have accounted for potential subtle differences in temperature, humidity, or air pressure in the laboratory—changes to which spiders are particularly sensitive, Longhorn says.

The team, led by Marie Trabalon from University of Rennes 1 in France, tried to minimize factors that may have influenced the outcome. For instance, they rotated the whole maze to reduce the possible impact of vibrations that might come from one side or the other of the maze. (Read how tarantulas are often misunderstood.)

The researchers also made sure to place two sisters from the same egg clutch—but from a different clutch from the male—at the ends of both passages, so the males didn't have a choice between moving toward their sister or an unrelated spider. Trabalon and co-authors did not respond to requests for an interview.

Spider Symmetry

Bor-Kai Hsiung, a Ph.D. candidate at Ohio's University of Akron who studies spiders, says the study's main conclusion is “pretty solid,” but he would have conducted some of the methods differently.

Instead of changing the spider subjects under each set of conditions—such as prey or light—he would have used the same group of spiders every time to see if those individuals consistently chose the right or left side.

And while the study says that the spiders’ bodies did not show differences in physical features on the right or left sides, Hsiung says the arachnids could still be asymmetrical.

“I don’t think that’s surprising. Usually the asymmetry is [reflected] in the neuron system,” he says, adding that humans, for the most part, also look symmetrical on the outside despite being right- or left-handed. (See "Science Still Can't Explain Why These Tarantulas Are Blue.")

Longhorn adds that the evolutionary significance of these preferences for one side or the other remains unclear. “They don’t really get to the core reasons behind why the spiders are showing this.”

It's also unknown whether female Honduran curly hair tarantulas or other spider species show such a preference.

Arachnophobes, Take Note

Meanwhile, people who like to reduce close encounters of the eight-legged kind may want to consider staying to the left of any arachnids, according to Hsiung.

“If there’s only one human, there’s only one stimulus,” Hsiung joked. “They will move toward the prey very fast and very precisely.”

Jokes about spider attacks aside, Honduran curly hairs likely won’t cause many problems.

“In general this species is really quite docile,” Longhorn says.