Octopuses, long considered solitary creatures, may have more complex social lives than previously thought — including signals to tell the competition whether they're picking a fight or backing down.

Researchers watched more than 50 hours of video of shallow-water octopuses in Jervis Bay, Australia, and found intriguing patterns, they report in a new paper published today in Current Biology.

The authors were tipped off by local divers in the area, who had seen the octopuses change colour and spar, said lead author David Scheel, a marine biologist at Alaska Pacific University in Anchorage.

"I thought, that's very interesting. Here's this supposedly very non-social animal that doesn't really interact a lot with others," said Scheel.

"And yet, they're signalling to each other."

Octopus signal example

An octopus (foreground) displays pale color and stretches out one arm before it withdraws from an approaching octopus (background). The approaching octopus displays dark color, a tall stance and spread web and arms. (David Scheel)

Watch the big dark one

Scheel recalls watching one minute of video — posted above — that told the tale.

"The octopus from the background approached in this menacing way. He stood up very tall and turned very dark, and the other one turned very pale and ducked down a little bit and withdrew offscreen." said Scheel.

Moments later, the first octopus approached another that didn't turn pale or run. It grew darker and stood its ground.

The other 52.8 hours of tape backed up the observation: The octopuses that turned pale swam away, while the ones that grew dark tried to look as big as possible and didn't back down.

Octopuses spar in Jervis Bay, Australia0:19

Avoiding a fight you can't win

The researchers don't know just what the octopuses are fighting over. It's not clear, for example, whether the big dark one in the video above wanted to evict the pale one from a strategic spot, or what.

But Scheel and his co-authors, who are philosophers, say the behaviour is consistent with other displays across the animal kingdom, where would-be competitors size each other up and signal their intention.

"When it's quite costly to fight, animals would be wise to signal whether they're about to run away or they're about to hold their ground," said Scheel.

"And by doing so, they avoid fights where they would have lost."

Turning dark and spreading itself out may then be this octopuses signal of aggression, just as turning pale might be a white flag.

Octopuses are known to eat each other, but that's not what these researchers saw.

"The octopuses are together day after day and as far as we've seen so far, they're not battling it out to the death."

Not just camouflage

Previously, biologists have mostly focused on octopuses' colour-changing abilities as camouflage tactics, said Scheel.

But these new observations of apparent signaling in one kind of octopus widen the understanding of these complex invertebrates, so far removed from our own vertebrate lineage.

"That can be useful in trying to tease apart evolutionary questions around signaling more broadly," said Scheel.

"It's kind of a fun time to be an octopus biologist."


Video credits, top: David Scheel, Alaska Pacific University; Peter Godfrey-Smith, City University of New York; Matthew Lawrence, University of Sydney, Australia.

Middle: Scheel, Godfrey-Smith, Lawrence and Stephan Linquist, University of Guelph.