There's not much of a betting market for octopus fights. But if you wanted to wager on the outcome of a face-off between octopuses, you could get some insider information by looking at their colors. Octopuses, like their relatives the squid and cuttlefish, are famously adept at changing the colors and patterns on their skin. Most of the time, researchers have interpreted octopus color-shifting as a way to hide, says Alaska Pacific University marine biologist David Scheel. By adjusting their colors to their backgrounds, octopuses can stay camouflaged and live to jet another day. Yet by poring over 53 hours of video footage of octopuses off the coast of Australia, Scheel and his colleagues found something quite different. The researchers were studying Octopus tetricus, a shallow-water species also called the common Sydney octopus (or, more poetically, the gloomy octopus). The octopuses are territorial and like to sit alone in their dens. Sometimes, though, one octopus approaches another—and a battle may ensue. At their most extreme, these fights are brutal. The octopuses grapple with their many arms, and one octopus tries to engulf the other entirely, Scheel says. But more often, the disputes just involve posturing and stretching out of arms. Then one octopus retreats to safety. The researchers found that the outcomes of these face-offs were closely tied to how the two octopuses looked when they approached each other. If both animals wore dark skin, things were likely to escalate. The more closely matched their colors were, the more likely a fight was. But if one animal looked pale, it was likely to retreat. "I was quite surprised to see on the videos how obvious the interactions and signals were," Scheel says. And color changes weren't the only way that two octopuses signaled to each other. Darker octopuses were more likely to move to high ground, prop themselves up tall on their legs, and hold their mantles vertically like a top hat. In the photo above, the dark-colored octopus is using this aggressive stance. On the right-hand side of the photo, a pale octopus is fleeing backward. A close look reveals that this octopus has actually split its coloration half-and-half. The side of its body facing away from the aggressive octopus is neutrally colored. But the side facing the other octopus has a bold pattern of white and dark splotches. This coloring seems to be some kind of signal from the retreating octopus to the winning one. (Here's a video with more examples of octopus interactions.) The patterns that Scheel and his coauthors found are similar to those in some other animals. In cuttlefish, for example, males often fight when they're both colored dark. But if one cuttlefish turns pale, it's more likely to flee. Scheel thinks there's more to the signaling language of these octopuses waiting to be discovered. "There seem to be other color patterns going on as well, hinting that the full story is more complicated," he says. It seems clear that octopuses communicate with each other more than scientists had previously thought—even if humans never figure out exactly what they're saying. Image: Scheel et al.
Scheel, D., Godfrey-Smith, P., & Lawrence, M. (2016). Signal Use by Octopuses in Agonistic Interactions Current Biology, 26 (3), 377-382 DOI: 10.1016/j.cub.2015.12.033