NEW YORK – Many of the animal kingdom’s most fashionable decorations are the charm attacks put on by creatures trying to mate. Some of these ornaments are obvious to humans, such as the tail feathers of a peacock or the antlers of an elk, but others can only be perceived with sensory faculties that humans do not have.

A new study published Monday in the Proceedings of the National Academy of Sciences provides the first evidence of such a display. Cuttlefish create patterns on their skin based on the direction of light waves that are invisible to the human eye, dazzling potential sexual partners.

When humans (and most other mammals) see light, we cannot distinguish between light waves that sway vertically up and down as they travel, and light waves that weave horizontally from side to side. However, many animals, including some fish, insects, and soft-bodied marine organisms such as squid, can perceive the direction of light waves’ vibrations as they travel through air and water. If light passes through a filter that blocks some of these directions, it is said to be “polarized.”

Because we can’t tell the direction of light waves, “it’s very difficult to know exactly what polarized light looks like to animals that can see polarized light,” said study author Zhongshan Hsin, a researcher at National Taiwan Normal University. As far as researchers can tell, polarized light adds contrast and texture to what animals see, helping to make objects stand out more clearly against their surroundings.

Scientists have known for 30 years that squid, a member of the squid and octopus family, can sense the direction of light waves and that parts of their bodies can reflect polarized light. For his PhD at the University of Tokyo, Nakayama studied whether polarized light is incorporated into squid mating displays.

Male cuttlefish have an extra long pair of arms that they stretch out in front of them during courtship. Mr. Nakayama observed the squid in a tank in the basement of Asamushi Aquarium, hoping to capture the mating process on camera. After more than a month of “sitting in front of the aquarium all day, every day,” Nakayama finally captured a clear photo of the male cuttlefish’s outstretched arms.

To the naked eye, light and dark stripes were visible on both outstretched arms, but a specialized camera revealed another “hidden” pattern of vertically polarized light that oscillated up and down, and horizontally polarized light that oscillated from side to side.

“When I saw the polarization pattern on my laptop, I was incredibly excited, but also confused, because it didn’t look like any polarization pattern previously reported in animals,” Nakayama said. “Honestly, that was one of the most unforgettable moments of my life.”

Further observations of the cuttlefish and examination of its arm tissue helped explain how cuttlefish create this strange pattern. During courtship displays, male squid shrink the pigment cells in the skin of their arms, exposing the reflex cells underneath. These cells polarize the light horizontally.

However, from the perspective of the female, who is typically positioned several inches below the male during display, some of this horizontally oriented light passes through the transparent muscles of the outstretched arm, rotating the orientation vertically.

This polarized pattern is only seen in flirtatious males, so it may have evolved as an eye-catching mating display.

“This just opens the door to a very novel and very different way of manipulating light for animal signals,” said Roger Hanlon, chief scientist at the Marine Biological Laboratory in Woods Hole, Massachusetts. He was not involved in this study.

Hanlon said the paper could “inspire bio-inspired engineering of new types of materials” and could have military applications in underwater signaling in ships and submarines.

For Nakayama, the importance of this project lies in providing insight into how animals’ sensory abilities work together with communication. “Different species stand out in different ways, and studying those differences helps us understand evolution itself,” he said.

This article was first published new york times.