Loneliness isn't good for you. Sarah Dalesman from Hotchkiss Brain Institute, The University of Calgary, Canada, says, ‘Isolation has been shown in the past to act as a stress that often has negative effects on the ability of mammals to learn and form memories.’ But how does isolation affect the memory of an animal with a much simpler brain: the great pond snail, Lymnaea stagnalis? According to Dalesman, isolation does affect the mollusc's behaviour. She explains that the snails – which are hermaphrodites, having both male and female sexual organs – usually engage in reciprocal sex, taking it in turns to adopt the role of the male. However, after 8 days of isolation, the snails only take on the male role. Given the dramatic effect of isolation on the snail's simple behaviour, Dalesman and her principal investigator, Ken Lukowiak, wondered how loneliness might affect the mollusc's ability to from long-term memories (p. 4179).

However, when Dalesman tested the snails' long-term memories after 8 days of isolation, they seemed unaffected. According to Dalesman, pond snails can be trained to keep their breathing tubes closed when they are in oxygen-starved water by giving them a gentle tap when ever they try to extend it to breathe. With the correct training regime, the snails can remember the lesson for over a day. Having expected that 8 days in solitary confinement would prevent the snails from laying down the long-term memory, the duo were surprised when the molluscs successfully remembered to keep their breathing tubes firmly shut 24 h after training. ‘We almost stopped there,’ admitted Dalesman, adding, ‘sbut we were fairly sure the snails were aware of lack of contact with other individuals, so we thought that changing the context in which they experience isolation might alter our results.’

Knowing that low calcium water concentrations and predator odours are stressful for the snails – low calcium concentrations prevent the snails from forming long-term memories, a whiff of predator odour improves their memory, and a combination of the two leads to normal memory formation as if there was no stress – the duo decided to find out how combinations of the stresses in association with isolation affected the molluscs' memories.

Amazingly, isolation improved the memories of the molluscs in stressful low calcium conditions. Meanwhile, 8 days of isolation did not affect the memories of snails experiencing the scent of a predator. However, when the duo combined all three stresses – solitary confinement, low calcium and the scent of a predator – the snails were unable to from the long-term memory after training. ‘This is the first time that we have found that kairomones [predator odour] don't enhance memory formation; instead their presence seems to result in memory being blocked’, says Dalesman.

So isolation did affect the ability of the snails to form long-term memories, but it was entirely context dependent, with isolation improving memory in some situations while abolishing it in others.

Dalesman and Lukowiak suspect that the snails' behavioural response to isolation – adopting the male role during mating – may make the low calcium situation less stressful because they require less calcium when reproducing as males than they do when producing eggs as females, freeing the isolated snails to form better memories. However, they suspect that a combination of all three stressors might push the snails over the edge. ‘Several different environmental stressors may just be too much to cope with and they are no longer able to pay enough attention to training to form long-term memory’, suggests Dalesman.

Social snails: the effect of social isolation on cognition is dependent on environmental context
J. Exp. Biol.