Brittle stars put their best foot forward

Brittle stars are sea creatures with five limbs and no brain. Found on the seafloor across the world, they have no obvious front, unlike humans and most other animals. Now, a new study reports that the brainless creatures are nonetheless able to move in a coordinated way, by designating one limb as the “front-facing” limb, and using two others to propel forward.

“They are pushing forward with the front two limbs, like a turtle,” said Henry Astley, the study’s author and an evolutionary biologist at Brown University. “The back limbs aren’t highly involved.” Astley reports his findings in The Journal of Experimental Biology.

Most animals, including humans, are bilaterally symmetrical. In other words, drawing a line down the centre results in symmetrical halves. A few animals, brittle stars included, are radially symmetrical: They can be sliced in many ways and still be symmetrical, giving them no clear “front.” The brittle star is also able to switch its front-facing limb as needed, and this enables it to swiftly change direction, Astley said.

“They just decide another direction is front, and they’re off.” Scientists have long thought that bilateral symmetry confers an evolutionary advantage, because it allows for directed movement when searching for food or avoiding predators.

“You can get the benefits of bilateral symmetry without being bilaterally symmetrical,” he said. “You can become behaviourally bilaterally symmetrical.”

Chimp table manners vary by group

Depending on where a meal is served, a person might tuck in with a fork and knife, with chopsticks or with bare hands. Chimpanzees, it turns out, have a similar kind of cultural variation: Neighbouring groups of the animals have unique nut-cracking styles, a new study in the journal Current Biology reports. Researchers noted that one group of wild chimps in Tai National Park in Ivory Coast preferred stone tools to hammer open coula nuts.

Two other groups of chimps used stone tools early in the season, when the nuts were harder, but then switched to wood tools as the nuts grew softer. The chimp groups also had preferences for different sizes of wood, said Lydia Luncz, the study’s first author and a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. The chimpanzees are displaying a sort of cultural preference with their tool choice, said Luncz, a graduate student.

“It’s just a preference they have, because they grew up that way,” she said.
On occasion, when there were not enough stones to be found, the chimpanzees that preferred them would resort to using wood. “They know how to do it,” Luncz said.

Although the chimpanzee groups neighbour one another and interact often, their interactions are never friendly and they don’t learn from one another. “It’s always war,” Luncz said. “They don’t interact in a way where they could watch each other nut-cracking.”

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