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Double-sided. When a male cuttlefish (right) is courting a female (left), he displays two different sides: a male pattern facing the female, and a female pattern facing away, to trick other males.

Culum Brown

Two-Faced Fish Tricks Competitors

Talk about showing your feminine side. On one flank, a courting male cuttlefish looks like a normal male of his species, with tigerlike stripes extending horizontally down his skin. But on the other, he resembles a female, displaying marbled browns and whites. He needs the male pattern to attract the female, while the female motif keeps competing males from fighting him. That’s scientists’ best guess for now, at least, to explain the devious cuttlefish behavior that they’ve observed and reported for the first time.

“Cuttlefish are a very smart group of fish,” says lead researcher and ecologist Culum Brown of Macquarie University in Sydney, Australia. “And it’s pretty obvious that they are specifically using this display in a tactical way.”

Researchers knew that cuttlefish (Sepia plangon) could camouflage their skin to match their surroundings, and that they could show different patterns on each side. Their skin contains a highly concentrated layer of chromatophores—various colored pigment-containing cells—that can be moved closer or further from the surface to change the pattern on the fish. But scientists had never seen a male fish mimicking a female on only one side as a trick of courtship. Brown and his colleagues first observed the behavior in a large aquarium in their lab. They wondered whether males in the wild did the same thing, and if so, when and why. So they combed through photos of 108 distinct groups of cuttlefish taken on previous dives of Sydney Harbour. They found that when a male was in a group with one female and one other male, he displayed the dual patterns—a male side facing the female and a female side facing the male—39% of the time. In other situations, such as an all-male group or a male matched with two females, the dual display was never seen.

“They’re only using this in a particular context,” says Brown. “It implies that the male’s best strategy if he finds an attractive female is to be sneaky about it and disguise the point that he’s found a potentially interesting mate.”

While the behavior—reported online today in Biology Letters—could be learned, Brown believes it is an example of the complex social intelligence possessed by the fish. Cuttlefish have one of the largest brain-to-body size ratios of any invertebrate, he notes, and research on their deception skills—and how they know when to employ the tactic—may help explain how brain size contributes to complexity of behavior. “We’ve only just started to scratch the surface in terms of understanding their intelligence."

Behavioral ecologist Shelley Adamo of Dalhousie University in Halifax, Canada, says the finding, while novel, isn’t surprising based on what’s already been seen. “Cuttlefish are great deceivers,” she says. “That’s what they do to hide from predators; they pretend to be coral, they pretend to be eel grass, they always deceive.”

The new observation needs further work to be verified, she adds, such as diving expeditions to observe the behavior in the wild rather than relying on past photos. “This research was well-done in that they did both field work and observed animals in an enclosure,” Adamo says. “And if you see it at least a few times, it’s probably playing at least some role in the animal’s natural behavior. But more observations are always better.”