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Deadly sophistication seen in trout-eel hunting partnership

Sept. 8, 2014
Courtesy of the University of Cambridge
and World Science staff

Cor­al trout and mor­ay eel col­la­bo­rate with deadly ef­fec­tive­ness in hunt­ing. Now, re­search­ers have stud­ied their tech­nique and con­clud­ed that the trout’s col­la­bo­ra­tive skills ri­val those of the much bigger-brained chimp, our clos­est ev­o­lu­tion­ary rel­a­tive.

The trout, they say, join the chimps as one of the few non-hu­man spe­cies known to choose the right situa­t­ion and part­ner to suc­ceed in a col­la­bo­ra­tive task, learn­ing as it goes.

Coral trout and moray eel. (Image © Alex Vail)


Cor­al trout can give quick chase in open wa­ter, but not if their prey es­capes in­to a hard-to-reach cor­al reef crev­ice. In that case, the trout will team up with a snake-like mor­ay eel to flush out the un­for­tu­nate vic­tim. The trout sig­nals the eel to slip in­to the reef af­ter the tar­get. The eel then ei­ther gets the prey or scares it back in­to the open so the trout can pounce.

The trout use ges­tures and sig­nals to flag the prey’s loca­t­ion to an eel, in­clud­ing head­shakes and head­stands that ac­tu­ally point the eel in the right di­rec­tion, ac­cord­ing to the stu­dy. The trout have a startling abil­ity to as­sess when a situa­t­ion needs a col­la­bo­ra­tor and to pick the right part­ner near­by, the sci­en­tists say. A close rel­a­tive of the cor­al trout, the rov­ing cor­al group­er, shows si­m­i­lar be­hav­ior.

The re­search­ers, at the Uni­vers­ity of Cam­bridge, com­pared these ca­pa­ci­ties of the trout with those of highly-intelligent chim­pan­zee us­ing si­m­i­lar ex­pe­ri­ments, and found the fish pe­r­form as well if not bet­ter. The find­ings are pub­lished Sept. 8 in the jour­nal Cur­rent Bi­ol­o­gy.

The re­search­ers caught wild cor­al trout and recre­ated hunt­ing sce­nar­i­os in set-ups that mir­rored their nat­u­ral en­vi­ron­ment. The aim was to cre­ate ex­pe­ri­ments anal­o­gous to old­er stud­ies with chim­pan­zees, called rope-pull ex­pe­ri­ments, but rel­e­vant to the trout’s hab­i­tat.

In the 2006 rope-pull ex­pe­ri­ments, chimps were shown fruit placed on a plank out of reach. A rope was at­tached to the end of each plank, and two chimps would have to pull the rope at the same time to get the fruit. Sim­i­lar­ly, the trout were pre­sented with out-of-reach food in the form of prey in a crev­ice, and the pos­si­bil­ity of a col­la­bo­ra­tor that took the shape of a fake mor­ay eel as fash­ioned by the re­search­ers.

The trout un­der­took the same num­ber of tri­als as the chimps over a si­m­i­lar time frame. When con­di­tions re­quired col­labora­t­ion, the trout were at least as good as chimps at de­ter­min­ing when they needed to re­cruit a col­la­bo­ra­tor, do­ing so in 83 per­cent of cases, and learn­ed more ef­fec­tively than chimps when the col­la­bo­ra­tor was un­nec­es­sary, the study found.

When the trout were giv­en a choice be­tween two mod­el mor­ay eel­s—one a suc­cess­ful col­la­bo­ra­tor, and an­oth­er that swam away in­stead of help­ing—the trout’s abil­ity to pick the suc­cess­ful part­ner was meas­ured as iden­ti­cal to that of the chimps. Both spe­cies learn­ed by the sec­ond day of the study to choose the right part­ner three times more of­ten than the wrong one. For both trout and chimps, six sub­jects par­ti­ci­pated in six tri­als per day for two days.

A 2009 study also found that hyenas per­form well at the rope-pull task, although the fish’s col­la­bor­ation across spe­cies may take things to a diff­e­rent lev­el.

“Our re­sults show that, like chim­pan­zees, trout can de­ter­mine when a situa­t­ion re­quires a col­la­bo­ra­tor and quickly learn to choose the most ef­fec­tive one,” said Cam­bridge re­search­er Al­ex­an­der Vail, who led the stu­dy. “This study strength­ens the case that a rel­a­tively small brain—com­pared to warm-blood­ed spe­cies—does not stop at least some fish spe­cies from pos­sess­ing cog­ni­tive abil­i­ties that com­pare to or even sur­pass those of apes.”

The au­thors cau­tion that the pro­cesses un­der­ly­ing such “supe­rficially sim­i­lar” cog­ni­tive be­hav­ior aren’t known—com­plex be­hav­ior does­n’t al­ways re­flect a com­plex mind.

“Per­haps the big­gest ques­tion is wheth­er the pro­cesses un­der­ly­ing col­la­bo­ra­tive part­ner choice in hu­mans, chim­pan­zees and trout are the re­sult of com­mon an­ces­try or an ev­o­lu­tion­ary con­ver­gence,” added Vail. “Con­ver­gence—where spe­cies of dif­fer­ent lin­eages evolve si­m­i­lar fea­tures—has been sug­gested as the rea­son for oth­er supe­rficially si­m­i­lar ape and hu­man abil­i­ties, and is the most likely rea­son why trout would seem to share this one too.”


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Coral trout and moray eel collaborate with deadly effectiveness in hunting. Now, researchers have studied their technique and concluded that the trout’s collaborative skills rival those of the much bigger-brained chimp, our closest evolutionary relative. The trout, they say, join the chimps as the only non-human species known to choose the right situation and partner to succeed in a collaborative task, learning as it goes. Coral trout can give quick chase in open water, but not if their prey escapes into a hard-to-reach coral reef crevice. In that case, the trout will team up with a snake-like moray eel to flush out the unfortunate victim. The trout signals the eel to slip into the reef after the target. The eel then either gets the prey or scares it back into the open so the trout can pounce. The trout use gestures and signals to flag the prey’s location to an eel, including headshakes and headstands that actually point the eel in the right direction, according to the study. The trout have a startling ability to assess when a situation needs a collaborator and to pick the right partner nearby, the scientists say. A close relative of the coral trout, the roving coral grouper, shows similar behavior. The researchers, at the University of Cambridge, compared these capacities of the trout with those of highly-intelligent chimpanzee using comparably similar experiments, and found the fish perform as well if not better. The findings are published Sept. 8 in the journal Current Biology. The researchers caught wild coral trout and recreated hunting scenarios in set-ups that mirrored their natural environment. The aim was to create experiments analogous to older studies with chimpanzees, called rope-pull experiments, but relevant to the trout’s habitat. In the 2006 rope-pull experiments, chimps were shown fruit placed on a plank out of reach. A rope was attached to the end of each plank, and two chimps would have to pull the rope at the same time to get the fruit. Similarly, the trout were presented with out-of-reach food in the form of prey in a crevice, and the possibility of a collaborator that took the shape of a fake moray eel as fashioned by the researchers. The trout undertook the same number of trials as the chimps over a similar time frame. When conditions required collaboration, the trout were at least as good as chimps at determining when they needed to recruit a collaborator, doing so in 83% of cases, and learned more effectively than chimps when the collaborator was unnecessary, the study found. When the trout were given a choice between two model moray eels—one a successful collaborator, and another that swam away instead of helping—the trout’s ability to pick the successful partner was measured as identical to that of the chimps. Both species learned by the second day of the study to choose the right partner three times more often than the wrong one. For both trout and chimps, six subjects participated in six trials per day for two days. “Our results show that, like chimpanzees, trout can determine when a situation requires a collaborator and quickly learn to choose the most effective one,” said Cambridge researcher Alexander Vail, who led the study. “This study strengthens the case that a relatively small brain—compared to warm-blooded species—does not stop at least some fish species from possessing cognitive abilities that compare to or even surpass those of apes.” The authors caution that the processes underlying such “superficially similar” cognitive behavior aren’t known—complex behavior doesn’t always reflect a complex mind. “Perhaps the biggest question is whether the processes underlying collaborative partner choice in humans, chimpanzees and trout are the result of common ancestry or an evolutionary convergence,” added Vail. “Convergence—where species of different lineages evolve similar features—has been suggested as the reason for other superficially similar ape and human abilities, and is the most likely reason why trout would seem to share this one too.”