Getting picked on can have a dramatic effect on an animal's confidence. Vulnerable creatures shrink in on themselves, back down from confrontations and lose the will to explore after suffering continual defeat by more dominant animals. Yet, resilient creatures don't succumb to the effects of bullying. ‘Most studies on chronic social defeat are performed in mammals, such as mice, rats and hamsters’, says Rose Wayne from Louisiana State University, USA. But how does constantly being picked on affect social fish such as cichlids and how does repeatedly encountering defeat mess with their brains? Curious to find out how bullied male Burton's mouthbrooder cichlids (Astatotilapia burtoni) react when forced into a confrontation with a larger male, Ava Karam, Alora McInnis and Catherine Arms from Karen Maruska's lab at Louisiana State University, USA, monitored how the fish responded to constantly losing out and what set resilient males apart from more sensitive souls.
Quickly transferring a small male from his home territory into the tank of a larger male, Karam, McInnis and Arms allowed the fish to stand off, nip at each other, shake their bodies aggressively and pursue one another until the smaller male gave up, touring around the tank after fleeing from the victor and intermittently freezing with fear. ‘This part of the experiment closely resembles the natural ecology of the Lake Tanganyikan fish, in which males can be exposed and suppressed by the same individuals on a regular basis’, says Wayne. Once the confrontation was over, the team returned the vanquished fish to its home tank to recover from the skirmish, before reintroducing the smaller intruder to the larger bully three more times over consecutive days. On each subsequent occasion, the defending bully quickly closed down the confrontation, with the defeated male losing interest in exploring the bully's home, freezing in terror more often.
Once the smaller males were thoroughly intimidated into submission, the team gave the bullied fish a 4 day break from the harassment, to allow them to rebuild their confidence, before installing a new resident in the original bully's tank and staging a final confrontation between the two. This time, the team watched to see whether the bullied fish showed any signs of resilience by boldly exploring the tank, as they had after the first altercation, or were vulnerable and became quickly overwhelmed, freezing frequently with fear having lost again.
After identifying 16 resilient and 12 vulnerable fish, the team checked for signs of the hormones produced by stressed animals – which could be causing the vulnerable fish to succumb to the bullying – but found little difference between the bold and timid cichlids. Increased stress was not causing the fish to become more susceptible to bullying.
Karam, McInnis and Arms along with Michael Kaller (Louisiana State University) then scrutinised the fish's brains to see whether they could identify differences between the two groups of fish and discovered several brain regions – involved in making social decisions – that were activated differently in the confident and bullied males. These included a region that processes sensory information – which could help a male to assess the social circumstances – and triggers a second brain area, a part of the social decision-making network, which controls the fish's behaviour. Wayne says, ‘The unique pattern of brain activation in the social decision-making network of susceptible males suggests an important role in regulating vulnerability to repeated social defeat stress’. The team adds that understanding the simpler brain circuits that underpin how fish struggle with difficult social situations could also help us to unravel the mysteries of some human mental health conditions, which share many of the same basic brain circuits.