When you think about human social behaviour, what animals do you immediately think of as most similar to us? Apes, dolphins, wolves? Sure, these animals display incredibly complex social interactions, just like us. But Eric Edsinger from the Marine Biological Laboratory, USA, and Gül Dölen from Johns Hopkins University, USA, teach us in their latest study that we aren't actually all that different from our more distant cousin: the octopus. While octopuses typically hang out by themselves and fight when they come across each other, they let bygones be bygones during the mating season. Until now, we had no idea why octopuses suddenly set aside their aggressive tendencies during this ‘special’ time. Using a unique combination of molecular and behavioural studies, Edsinger and Dölen delved into the brain of the octopus to uncover the neurological mechanisms that regulate their social behaviour.

The duo suspected that the neurotransmitter serotonin may be involved. A neurotransmitter is a chemical in animal brains that helps to communicate signals between neurons. In many animal species, from insects to humans, serotonin is known to play a key role in regulating social behaviour by inhibiting aggression and spontaneous activity. To investigate whether octopuses use this chemical messenger, Edsinger and Dölen constructed an evolutionary tree for key genes involved in serotonergic signalling among 21 species, including their study animal, the California two-spot octopus (Octopus bimaculoides). They uncovered incontrovertible evidence that the octopus carries a gene for a key protein involved in transporting serotonin between neurons, with amazing similarity to the human gene for this protein. The fruit fly, the roundworm and a number of vertebrate species also share these gene sequences, probably as a result of evolution from common ancestors. Interestingly, social insects like the honeybee lack these genes, illustrating the complex progression of sociality across evolutionary history.

Following on from these molecular studies, Edsinger and Dölen performed behavioural tests to better understand the role of serotonin in octopus sociality. Using methodology designed for rats, the scientists gave the octopuses a choice between a ‘novel object’ (which, in this study, were Star Wars figurines of Chewbacca or a Stormtrooper) and an unfamiliar octopus (either a male or a female) of the same species and recorded which ‘object’ the octopus preferred to hang out with. They found that both males and females preferentially associated with the female octopus. But if presented with a male, both sexes preferred to hang out with the Star Wars figurine.

Then, they used MDMA, the main ingredient in the drug ecstasy, to ramp up the octopus's serotonin, to see whether they could stimulate the octopus to associate with the male that they had previously dodged. Amazingly, this avoidance of males was reversed with a healthy dose of serotonin-boosting MDMA. Following treatment with the drug, all of the octopuses (both male and female) interacted enthusiastically with the male octopus, which they had avoided previously.

Excitingly, these findings support the scientists’ theory that octopuses become lovey-dovey during the mating season as a result of a boost in serotonergic activity. Edsinger and Dölen speculate that while octopuses maintain all of the necessary machinery to express serotonin, they suppress signalling by this neurotransmitter outside of the mating season. So, potentially, serotonin is just the right love potion to kindle octopus romance.

A conserved role for serotonergic neurotransmission in mediating social behavior in octopus
Curr. Biol.