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Fish logic surprises researchers

Jan. 24, 2007
Courtesy Stanford University
and World Science staff

A male fish can size up po­ten­tial ri­vals, rank­ing them from strongest to weak­est, just by watch­ing how they per­form in fights with oth­er ma­les, ac­cord­ing to a new stu­dy.

The re­search­ers say their finding pro­vides the first di­rect ev­i­dence that fish, like peo­ple, can use log­ic to learn their place in a peck­ing or­der. The stu­dy, pub­lished in the Jan. 25 edi­tion of the re­search jour­nal Na­ture, in­volved ci­ch­lids (SIK-lids), small ter­ri­to­rial fish from Af­ri­ca. 

A dominant male A. bur­toni cichlid. (Courtesy Logan Grosenick)


“Male ci­ch­lids are con­stant­ly try­ing to as­cend so­cially by beat­ing each oth­er up,” said study co-author Rus­sell D. Fer­nald of Stan­ford Uni­ver­si­ty in Stan­ford, Ca­lif. “It would be real­ly val­u­a­ble for them to know in ad­vance who to pick a fight with.”

The sci­en­tists aimed to find out wheth­er ter­ri­to­rial fish use a basic type of rea­son­ing, tran­si­tive in­fer­ence, in which known re­la­tion­ships are used to fi­g­ure out un­fa­mil­iar ones. 

“It’s some­thing that kids gen­er­al­ly fig­ure out by age four or five: Mary is taller than Fred, Fred is taller than Pete, there­fore Mary is taller than Pete. It’s been dem­on­strat­ed in pri­ma­tes, rats and some bird spe­cies, but how and why it evolved in an­i­mals is a mat­ter of de­bate.” 

The researchers used a pop­u­lar lab­o­ra­to­ry fish called As­ta­toti­lapia bur­toni. The ma­les, ex­treme­ly ter­ri­to­rial, reg­u­lar­ly en­ter ag­gres­sive jousts whose out­come de­ter­mines ac­cess to food and mates. Males that re­peat­ed­ly lose, can’t hold ter­ri­to­ries and thus drop in sta­tus, the au­thors wrote.

When they fight, it’s easy to spot the win­ner. Ma­ture ma­les have a men­ac­ing black stripe, or eye­bar, on their face. Af­ter a bout, the win­ner re­tains his showy ap­pear­ance, but the los­er’s eye­bar tem­po­rar­i­ly fades away as he tries to flee.

The re­search­ers staged a se­ries of one-on-one com­bats be­tween ma­les of equal size. Fish that lost their eye­bar were de­clared the los­er, sep­a­rat­ed from their op­po­nent and put back in their orig­i­nal tank. With­in min­utes, the los­er’s eye­bar re­turned, and he looked like all the oth­er dom­i­nant ma­les again. 

The fights were staged in a tank di­vid­ed in­to com­part­ments. A cu­bi­cle in the cen­ter con­tained lone male “by­stander.” Around him were five smaller com­part­ments, each with one male ri­val iden­ti­fied as A, B, C, D and E. Re­search­ers made sure that the by­stand­er had nev­er met any of these po­ten­tial ri­vals.

The by­stand­er was al­lowed to watch a se­ries of fights be­tween ri­val pairs: A vs. B, B vs. C, C vs. D, and D vs. E. Re­search­ers ma­nip­u­lat­ed the fights so that A would dom­i­nate B, B would dom­i­nate C, and so on. 

Tak­en to­geth­er, the fights im­ply a dom­i­nance hi­er­ar­chy with A on top, fol­lowed by B, C, D and E in that or­der. Did the by­stand­er grasp this peck­ing or­der? And could he use that knowl­edge to make log­ical de­ci­sions about the same fish paired in new re­la­tion­ships?

To find out, eight dif­fer­ent by­stand­ers were tested in the fa­mil­iar square tank and in a new set­ting: a rec­tan­gu­lar aquar­i­um with three ad­ja­cent com­part­ments. In each test, a by­stand­er was placed in the mid­dle com­part­ment be­tween two sets of ri­vals that he had nev­er seen to­geth­er—A and E on the one hand, and B and D on the oth­er. All ri­vals had re­cov­ered from ear­li­er losses, so their phys­i­cal ap­pear­ance was sim­i­lar. 

Us­ing a vid­e­o cam­era, re­search­ers recorded which ri­val the by­stand­er ap­proached first, and the over­all time he spent next to each of them. Pre­vi­ous tests had shown that by­stand­ers pre­fer to spend more time near the ri­val they per­ceive as weaker, the au­thors ex­plained.

The re­sults were dra­mat­ic, ac­cord­ing to the re­search­ers: vir­tu­al­ly all by­stand­ers swam to the weaker ri­val first and stayed near him for sig­nif­i­cantly long­er. In the A-E tests, by­stand­ers pre­ferred E, the wimp­i­est fish. In the more sub­tle B-D tests, most by­stand­ers chose D. So “fish do, in fact, use tran­si­tive in­fer­ence to fig­ure out where they rank,” Fer­nald said. “I was amazed that they could do this through vi­car­i­ous ex­pe­ri­ence, just by watch­ing oth­er ma­les fight.”

In Lake Tangan­yika in east­ern Af­ri­ca, the ci­ch­lids’ hab­i­tat, con­di­tions change con­stant­ly and “it would be ad­van­ta­geous for a male to know who the new boss is go­ing to be and who his weak­est ri­vals are,” he added. “Our ex­per­i­ment shows that male ci­ch­lids can ac­tu­al­ly fig­ure out their odds of suc­cess by ob­ser­va­tion alone. From an ev­o­lu­tion­ary stand­point, tran­si­tive in­fer­ence saves them val­u­a­ble time and en­er­gy.”

Fish might have the ru­di­men­ta­ry brain cir­cuit­ry for tran­si­tive in­fer­ence that ap­peared lat­er in birds and mam­mals, he con­tin­ued. “Any an­i­mal that has evolved a so­cial sys­tem that re­quires com­bat among ma­les will have some kind of eaves­drop­ping ca­pa­bil­i­ty al­low­ing them to sur­rep­ti­tiously draw in­fer­ences about their so­cial rank,” Fer­nald said. “Ca­pac­i­ties that evolved in fish may con­trib­ute to hu­man tran­si­tive in­fer­ence, or per­haps this ca­pac­i­ty evolved in­de­pend­ent­ly. The ques­tion re­mains un­re­solved.”


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A male fish can size up potential rivals, and even rank them from strongest to weakest, simply by watching how they perform in territorial fights with other males, according to a new study. The researchers say their discovery provides the first direct evidence that fish, like people, can use logic to figure out their place in a pecking order. The study, published in the Jan. 25 edition of the research journal Nature, involved cichlids (SIK-lids), small territorial fish from Africa. “In their natural habitat, male cichlids are constantly trying to ascend socially by beating each other up,” said study co-author Russell D. Fernald of Stanford University in Stanford, Calif. “It would be really valuable for them to know in advance who to pick a fight with.” The scientists aimed to determine whether territorial fish use a type of reasoning called “transitive inference,” in which known relationships serve as the basis for understanding unfamiliar ones. “Transitive inference is essential to logical reasoning,” Fernald explained. “It’s something that kids generally figure out by age four or five: Mary is taller than Fred, Fred is taller than Pete, therefore Mary is taller than Pete. It’s been demonstrated in primates, rats and some bird species, but how and why it evolved in animals is a matter of debate.” The scientists used a popular laboratory fish called Astatotilapia burtoni. The males are extremely territorial, regularly entering aggressive jousts whose outcome determines access to food and mates. Males that repeatedly lose can’t hold territories “and consequently descend in social status,” the authors wrote. When the males fight, it’s easy to spot the winner. Mature males have a menacing black stripe, or eyebar, on their face. After a bout, the winner retains his showy appearance, but the loser’s eyebar temporarily disappears as he tries to flee. The researchers staged a series of short combats between males of equal size. Fish that lost their eyebar were declared the loser, separated from his opponent and put back in their original tank. Within minutes, the loser’s eyebar returned, and he looked like all the other dominant males again. The fights were staged in a tank divided into compartments. A lone male “bystander” was placed in a cubicle in the center of the tank. Surrounding him were five smaller compartments, each with a solitary male rival identified as A, B, C, D and E. Researchers made sure that the bystander had never met any of his five potential rivals. The bystander was allowed to watch a series of fights between rival pairs: A vs. B, B vs. C, C vs. D, and D vs. E. Researchers manipulated the fights so that A would dominate B, B would dominate C, and so on. These fights, taken together, imply a dominance hierarchy with A on top, followed by B, C, D and E in that order, the authors noted. But did the bystander grasp this pecking order? And could he use that knowledge to make logical decisions about the same fish paired in new relationships? To find out, eight different bystanders were tested in the familiar square tank and in a new setting: a rectangular aquarium with three adjacent compartments. In each test, a bystander was placed in the middle compartment between two sets of rivals that he had never seen together—A and E on the one hand, and B and D on the other. All rivals had recovered from earlier losses, so their physical appearance was similar. Using a video camera, researchers recorded which rival the bystander approached first, and the overall time he spent next to each of them. Previous tests had shown that bystanders prefer to spend more time near the rival they perceive as weaker, the authors explained. The results were dramatic, according to the researchers: virtually all bystanders swam to the weaker rival first and stayed near him for significantly longer. In the A-E tests, bystanders preferred E, the wimpiest fish. In the more subtle B-D tests, most bystanders chose D. “These results show that fish do, in fact, use transitive inference to figure out where they rank in the social order,” Fernald said. “I was amazed that they could do this through vicarious experience, just by watching other males fight.” In Lake Tanganyika in eastern Africa, the cichlids’ habitat, conditions change constantly and “it would be advantageous for a male to know who the new boss is going to be and who his weakest rivals are,” he added. “Our experiment shows that male cichlids can actually figure out their odds of success by observation alone. From an evolution ary standpoint, transitive inference saves them valuable time and energy.” The results raise the possibility that fish brains might contain the rudimentary brain circuitry for transitive inference that appeared later in birds and mammals, he continued. “Any animal that has evolved a social system that requires combat among males will have some kind of eavesdropping capability allowing them to surreptitiously draw inferences about their social rank,” Fernald said. “Cognitive capacities that evolved in fish may contribute to human transitive inference, or perhaps this capacity evolved independently. The question remains unresolved.”