Like humans, octopuses become more sociable and engaged after a dose of the party drug MDMA.

In the human brain, MDMA, commonly known as ecstasy, triggers the release of the neurotransmitter serotonin, causing feelings of happiness and closeness to others. The California two-spot octopus (Octopus bimaculoides) has a serotonin transport system similar to that of humans. To determine whether this system serves the same function in octopuses and humans, Eric Edsinger at the Marine Biological Laboratory in Woods Hole, Massachusetts, and Gül Dölen at Johns Hopkins University in Baltimore, Maryland, submerged five octopuses in MDMA-laced water and tested their behaviour around others of their kind.

After absorbing the drug, the animals ignored toys, such as Star Wars figurines, that would normally have intrigued them. Instead, the octopuses socialized and spent more time touching one another with their arms than these creatures usually do.

The findings suggest that serotonin played an important part in social behaviour in the common ancestor of octopuses and vertebrates, whose branches on the family tree separated more than 500 million years ago.