The African cichlid fish (Astatotilapia burtoni) can make inferences about its place in a social hierarchy by watching other fish fight, according to a study published in Nature this week.

Humans use transitive inference all the time. Logical inference appears on logic tests, for example, as: if A > B and B > C then A > C. (By kindergarten, most kids can make inferences like this.) But in animals, the ability to infer indirectly—to make a calculation based solely on the behavior others—has never been shown in the lab, says Logan Grosenick, a graduate student at Stanford and co-author of the study.

Grosenick and his colleagues partitioned a fish tank and put a bystander cichlid (pronounced sik-lid) in the middle. The bystander watched a series of staged fish fights, in which cichlid A beat B, B beat C and so on. The loser is easy to spot: African cichlids have a dark stripe across the face, an eyebar, that fades from a fish's brow for a few minutes if it loses a fight.

Then the bystander was placed with two fish that had never gone head to head. If you can infer, you’d know that cichlid A should be stronger than E. Cichlids like to situate themselves near weaker fish, so the test was to see where the bystander spent most of its time. Sure enough, bystanders seemed to be able to infer the hierarchy—they nearly always gravitated towards weaker fish.

For the bystander to gravitate towards the weaker fish, it had to be able to 1) tell the winners and losers apart; 2) remember which fishes won and lost; 3) infer the hierarchy of fish that he had never seen fight before based on other fights; and 4) make a choice using that information.

This seems like a lot of thinking for an organism with a brain about the size of a raisin. Grosenick admits that the finding “goes against a lot of previous thought on the matter. People were unwilling to grant animals this kind of cognitive ability.”

But this fish’s ability to infer perhaps shouldn't be so surprising: winning fights is closely tied with the ability to reproduce. If a fish loses enough fights, its gonads shrink (seriously), its bright coloration fades, and it stops reproducing. Fish in the wild, therefore, are at an advantage if they can infer which fish are the strongest and steer clear of them. Plus, not having to fight every fish to know one's rank, saves that fish a lot of energy.

Russell Fernald, a biology professor at Stanford and co-author on the paper, says: "The big picture here is that animals have sophisticated skills that closely match their environmental needs. Just as their morphology evolved in response to needs, so too did their mental or reasoning skills."

Grosenick adds, “If we put on our evolutionary goggles and look at things with that kind of logic, it is entirely sensical that these fish would devote brain resources to a skill that greatly enhances their ability to survive and reproduce.”

-Flora Lichtman

 

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