Science

An octopus can voluntarily throw projectiles at its relatives

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So far, apart from humans, only a few social mammals have been observed throwing objects at individuals of the same species. Scientists have found that the wild octopus (Octopus tetricus) is also able to project various objects (vases, algae, shells, etc.) in various contexts, including while interacting with its relatives.

Throwing objects is an unusual behavior among animals. Target throwing is even rarer. Although considered exclusively human, it has been observed in some non-human primates, elephants, mongooses, and birds. These targeted throws are usually aimed at hard-shelled prey and food, or at threatening animals in a defensive context. On the other hand, throws directed at individuals of the same species are very rare. Dolphins have been observed to throw objects, sometimes between humans, but only in the context of play.

Octopuses commonly display antisocial behavior. They hunt alone, can fight in skirmishes, and are sometimes cannibalistic. Recent studies, however, have shown that they can also show a certain amount of tolerance and send simple deterrent signals to their fellows. Marine biologists closely study the octopus population in Jervis Bay, Australia. It turns out that this population is very social and organizes itself into small communities in which octopuses often interact with each other … and not always in a friendly way.

Especially frequent interactions

Food is plentiful in Jervis Bay, but habitat is scarce, so octopuses live close together in a very limited area. This forced aggregation causes certain behaviors, including garbage scatter during interaction and in other contexts, as shown in the following video:

A team of researchers looked closely at these throws, trying to determine whether those that reach other octopuses are intentional or accidental. The images were captured using fixed video cameras installed throughout the site from 2011 to 2018, as well as during dives. The dataset collected in 2015 was exceptionally good, so the new study is based on these videos.

The researchers studied the images in detail, noting for each throwing event all elements of the context, the individual’s gender, body image, the nature of the materials thrown, and the strength of the throw. “We assigned sex to octopuses using behavioral criteria because rigorous anatomical identification would require significant intervention in octopus behavior,” the researchers say.

The data included over 21 hours of video. These images showed four to eight octopuses at a time; the group probably consisted of about ten people. Interactions were frequent (1543 total), ranging from 11 to 234 per hour (mean 73 per hour). “There were fights of varying intensity, fights, and one octopus approaching or attacking another, which then reacted (by changing color or posture, reaching for another, dodging, retreating, etc.),” details the team.

Disposal of Octopus tetricus in the wild. A) An octopus throws silt and algae into the water. B) An octopus was hit by a cloud of silt dropped by another. C, D) Mechanics of throwing behavior. © Godfrey-Smith et al.

The remarkable behavior seen in the video data is due to the coordinated use of the arms, body, and siphon, a tubular structure that can eject water at high speeds. The octopus starts by picking up the material with its hands (two most back hands), then uses a siphon to push it out under pressure (in most cases between the two most front hands).

Aimed throws that differ on several points

“These jets can be strong enough to throw the material several body lengths away from the animal in calm water, or so weak that the material falls almost directly in front of the animal,” the researchers note. A total of 102 castings were observed in 2015. The two females in the group made up the majority (66%) of the casts, but researchers believe that half (or more) of the individuals present cast occasionally. Females were more “followers” of this practice than males.

More than half of all presentations (53%) took place in an interactive context; 32% happened during den cleaning, 8% after eating, and 8% with no visible context. Interactive throws differed from other contexts in the materials they threw: octopuses threw silt more often, and shells more often threw in the context of den cleaning.

The projectile hit another octopus on 17 occasions, or 33% of interaction rolls. In two other cases, the throw hit the fish. Throws that hit another octopus stood out from the rest in several ways. Let’s start with the fact that the position of the hands was atypical. These throws were also more energetic and were usually performed by octopuses with a uniform body pattern (especially the dark pattern commonly associated with aggression). Octopuses often changed their behavior by dodging a shot or raising their arms in the direction of the thrower.

The researchers note that it is still difficult to determine the true intentions of octopuses, but taken together, these results show that these animals are indeed capable of making targeted throws at other people in certain contexts.

P. Godfrey-Smith et al., PLOS One.

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