In 2015, Peter Godfrey-Smith at the University of Sydney and his colleagues filmed several common Sydney octopuses (Octopus tetricus) interacting at a site in Jervis Bay dubbed “Octopolis”. It is one of the few places in the otherwise sandy sea bottom where octopuses can make dens, so there are an unusual number of the animals in a small area.
The cameras captured fights, matings and an extraordinary behaviour that the team calls throwing. “It’s hard to know how best to describe it,” says Godfrey-Smith.
The octopuses hold silt, algae or objects such as shells under their bodies in their tentacles, then angle their siphons and shoot a jet of water at the projectiles, propelling them up to several body lengths.
This throwing behaviour was known to be used for discarding the remains of meals or for excavating dens, but the videos also revealed many instances where octopuses hit other individuals with thrown objects.
When Godfrey-Smith described this behaviour in a 2015 talk, he wasn’t sure whether they were intentionally targeting the other octopuses or just accidentally hitting them.
Now the team has more footage, and detailed analysis has also revealed differences between the throws targeting others and those used for den clearing, suggesting the octopuses are indeed deliberately targeting others.
In 2016, for instance, one female octopus threw silt 10 times at a male from a nearby den who was attempting to mate with her. She hit him on five occasions. “That sequence was one of the ones that convinced me [it was intentional],” says Godfrey-Smith.
On four of these occasions, the male tried to “duck”, though he didn’t always succeed. In two cases, he anticipated the throws from the female’s movements and started dodging before the silt was propelled at him.
When targeting others, the octopuses were more likely to throw silt than shells and the throws were also more vigorous.
In addition, the throws used during den-building were almost always shot between the front two tentacles. When throwing at others, however, the octopuses sometime angled the throw between the first and second tentacles on the left or right. “That suggests a kind of targeting,” says Godfrey-Smith.
On one occasion, the researchers did see an octopus throw a shell at – and hit – another octopus by flinging it with a tentacle like a frisbee, rather than by propelling material with its siphon.
While a number of wild animals throw or propel things at other animals, only a handful, including chimpanzees, are known to target members of their own species. “It’s pretty rare. Especially rare is throwing of objects at other members of the same population,” says Godfrey-Smith.
On two occasions, an octopus hit a fish, though one of these collisions appeared to have been accidental. The animals also seemed to target the camera on occasion, hitting the tripod twice.
While the throwing appears to be used as a form of attack, the team hasn’t seen any targeted octopus respond by attacking or throwing things back. What’s more, some throws that happen after intense social interactions aren’t directed at another octopus but into empty space, suggesting the animals might be venting their frustration.
In one case, after a male’s advances to a female were rejected, he threw a shell in a random direction and changed colour.
The team decided to release the findings as a preprint after a paper was released suggesting that polar bears sometimes use rocks or lumps of ice as weapons during hunting, for instance throwing rocks off cliffs at walruses.
Reference: bioRxiv, DOI: 10.1101/2021.08.18.456805