"Do I dare challenge him - or not? Maybe I should do some observation first and see which opponent gives me the best chance of winning." - Too calculating for a fish? No way!
The black-throated mouthbrooder Astatotilapia burtoni is under a lot of pressure. Its territory in Lake Tanganyika at home is repeatedly destroyed, for example when hippos trample through it. Then it's time for the Pisces to redefine the boundaries - and that means a fight, and a fight with enormous pressure to succeed. Because the males of this species of cichlid all too often lose to their opponents, they lose their territory, move down the social ladder and thus also lose any hope of fatherhood.
If A. burtoni wants to become a proud dad, he is well advised to choose his opponents carefully, after all, a weak opponent increases the chances of victory. The more successes, the higher the male is in the hierarchy - the best guarantee for a lot of children. So the clever fishman is on the lookout for losers - but how do you spot them?
It's simple, thought Logan Grosenick of Stanford University in California: The fish watches other males fight and deduces which males are the weakest - these would then be the most promising opponents.
But is a fish actually capable of such cognitive achievements as to draw conclusions about the relationship of other matings through so-called transitive inference based on the information it can gain from the fight of two conspecifics? Children are only capable of such logical conclusions at the age of about four to five years, primates can do it too and even some birds - but fish?
Grosenick and his colleagues therefore placed one male black-throated mouthbrooder in an aquarium from which he could watch two other males fighting. The scientists then first let fish A win against fish B, then fish B against fish C, fish C against fish D, and finally fish D defeated opponent E. This resulted in a clear ranking of A over B over C over D over E. The loser is always easy to spot: he temporarily loses his black stripe between his eyes, but the winner stays there.
After this learning phase, it was tested whether the observer could derive the ranking from what he saw. To do this, they first put the viewer in a tank together with fish A (the highest ranked) and fish E (the lowest ranked). In such a situation, a black-throated mouthbrooder stays closer to the weaker fish, as the researchers knew from previous studies. In fact, in this situation, the test fish always sought the vicinity of fish E - it had clearly recognized that A was above E. This wasn't too difficult a task, however, since the viewer had previously observed that A always won (against Fish B) and E always lost (against Fish D). Logically, then, as the eternal winner, A would also win against the eternal loser E.
These results show that fish do indeed use transitive inference to purposefully integrate themselves into the social fabric
(Russell Fernald) Now a much more difficult test followed: The observer ended up in a tank together with fish B and fish D. Would he know who was the weaker one, even though he had never seen these two compete against each other? In fact, the formerly bystander sought the vicinity of fish D and avoided the higher-ranking rival B. Black-throated mouthbrooders can indeed infer the overall ranking of the observed animals from the individual fights in which they passively attended.
"These results show that fish do indeed use transitive inference to purposefully integrate themselves into the social fabric," says study leader Russell Fernald. In their natural habitat, this ability saves them a great deal of time and effort: instead of haphazardly challenging the nearest male to a fight, the clever black-throated mouthbrooder carefully observes its potential opponents and then simply fights against those it finds promises the greatest chance of victory - and thus saves nerves and energy!