Current Biology Vol 19 No 3R124

Animal Behavior: The Right Toolfor the Job

A recent study has shown that wild capuchins choose a functionallyappropriate tool from a set of apparently similar tools, suggesting a surprisinglevel of understanding of the contingencies of tool use.

Sarah F. Brosnan

Tool use has long been considereda hallmark of cognitively advancedspecies. Successfully using a toolrequires not only understanding therelationship between the tool and thegoal, but the ability to locate a toolwhich is appropriate for the task.Existing data suggest that severalspecies have the capacity to choosea tool on the basis of functionalcharacteristics necessary for thetask. But these data are typicallycorrelational and rely on immediatelyobvious characteristics of the tools:the animals’ actions could be dueeither to understanding the necessaryparameters of the tool, or to practicewith existing tools. In this issue ofCurrent Biology, Visalberghi et al. [1]report strong evidence that capuchinmonkeys take the functionalcharacteristics of a tool into accountwhen choosing a hammer stone tocrack a nut (Figure 1), even when thetools are visually identical. Ironically,these monkeys have previouslyshown only limited abilities to performthis task in experimental settings [2].The new observations indicate thatthese monkeys may be far morediscerning than previously assumed,and that a true understanding of thecontingencies of a tool use task may bewidely present among animals.

Previous tool-use studies innon-human primates have indicatedsensitivity to the parameters of thetask. For instance, chimpanzeeschoose nut-cracking tools on thebasis of the hardness of the nut tocrack, and transport tools over longdistances to obtain the appropriateone [3]. Apes also use tools in a widevariety of other situations [4], planahead [5,6], and make tools bymodifying objects available in theirenvironment [7]. Moreover, many ofthese tool use tasks appear to bepassed on socially, for instance frommother to offspring [8]. Amongmonkeys, several species use toolsduring foraging behaviors [9,10], and

capuchins select tools of theappropriate weight when nutcracking [11].

Of course, primates are not theonly taxa that use tools, nor are theythe only animals which are selectivein their choice of tools [12–15]. Thenon-primate champions of tool useand manufacture are New Caledoniancrows, a corvid species which showsselectivity based on tool length [16]and diameter [17], and can select theproper tool even when it has been‘disguised’ by bundling with anothertool [17]. Moreover, these crowsmanufacture and modify tools [18,19].Thus, it is clear that a wide variety ofspecies can discriminate appropriatetools, even in novel situations.

In none of these cases, however, isthere clear evidence that the animalsfully understand the task parameters.Often data are gathered in the wild,making it unclear whether they trulyunderstand the parameters of the task,or have simply learned throughtrial-and-error which tools are the mosteffective. This latter possibility doesnot require any deeper understandingof the functional characteristics of thetools. Even experimental tasks haverelied upon functional characteristicswhich were correlated with animmediately obvious, but potentiallyirrelevant, characteristic, such as sizebeing correlated with weight. Thus,individuals could solve these tasks bymatching the current tool to onesused previously, without a trueunderstanding of the relationshipbetween the task and the toolcharacteristics (for instance, thatweight is the relevant feature).

What is different about the new studyby Visalberghi et al. [1] — in which theweight of the stone was the criticalfeature, with size as the potentiallymisleading characteristic — is that thecapuchin monkeys search for thecritical functional feature (weight) evenwhen other potential cues (size) areidentical, or, even more impressively,when they contradict the criticalfeature. In these cases, the monkeys

resort to techniques which can providethe appropriate weight informationirrespective of the object’s size. Thisrequires more than matching basedupon previous experiences, andimplies an understanding that not alltools which look appropriatenecessarily are so.

In this study [1], capuchins were ableto choose the appropriate tool froma range of options. Choices were madebefore any attempts to crack the nut inquestion, and among novel hammerstones, ruling out trial-and-errorlearning. Unlike many otherexperimental tool use studies, thisone involved a group of wild capuchinswho were already engaged in nutcracking, using stones that wereavailable naturally [20]. All naturalstones were removed and replacedwith experimental tools. In the firstseries of studies, capuchins againdemonstrated that they would choosethe more appropriate of two tools whenvisual differences sufficed todiscriminate between them. First, themonkeys were presented with choicesbetween two stones made fromminerals they would encounter in theirnormal environment, sandstone andsiltstone. The monkeys reliably chosethe functional siltstone, which is lesslikely to splinter when used asa hammer. In the second test,capuchins reliably chose the heavierof two stones of the same material(quartzite) but different size and weight(heavier stones are required to crackthe nut).

The more difficult choices were thosein which experimenters createdartificial rocks of variable weight andsize, such that size no longer predictedweight. In the first experiment, theartificial stones were identical in sizebut varied in weight, yet all but onecapuchin continually chose the heavierof the stones, even though their initialinteractions were randomly distributedbetween the two stones. In the secondexperiment, the stones presentedprovided visual cues that conflictedwith their true properties; the smallerstone was the only one heavy enoughto crack a nut. Again, all subjects chosethe heavier stone, based on the weightcue rather than the more obvious sizecue (Figure 1). Finally, in the thirdexperiment, the subjects werepresented with two large (one heavy,one light) and one small (light) stones.Despite the information from the

Hedgehog Signaling: Is Smoa G Protein-Coupled Receptor?

The Hedgehog signal transducer Smoothened is structurally similar toG protein-coupled receptors. Now there is direct evidence that Smoothenedrelies on heterotrimeric G proteins in order to transduce the Hedgehog signal.

Melanie Philipp and Marc G. Caron

The Hedgehog (Hh) signaling pathwayis one of the most important andevolutionarily conserved pathwaysassociated with embryonicdevelopment, and cancer and isinvolved in the formation andhomeostasis of a multitude of tissuesand organ systems [1]. Smoothened

(Smo) is the transducing molecule ofthe extracellular signal Hh following itsinteraction with the receptor Patched.Topographically, Smo resemblesa seven transmembrane domainprotein with a high degree of similarityto the family of G protein-coupledreceptors.

Seven transmembrane receptors arecalled G protein-coupled receptors

DispatchR125

previous study — and the choicebetween two larger stones — allcapuchins again chose correctly.Critically, in all of these studies, thesubjects had to evaluate the propertiesof the stone by interacting with it,typically by moving it, lifting it, ortapping it, because visual cues wereno longer informative. Thus, thesecapuchins did not simply learn throughtrial-and-error to identify stones ofcertain mineral composition or size, butappeared to understand that the mostimportant characteristic of the hammerstone was weight, and evaluated theirchoices accordingly.

This paper [1] adds two interestingangles to the literature. First, this abilitywas demonstrated in a species whichwas initially believed not to regularlyuse tools, based on experimentalstudies [2]. This reiterates theimportance of investigating behaviorsacross multiple studies, as well as theimportance of providing the animalswith sufficient experience andenrichment for these sorts of abilities toemerge. Second, the authors providesound evidence that animals use morethan just past experience to evaluateobjects, and actually understand the

Figure 1. Tool selection by capuchinmonkeys.

Mansinho, an adult male bearded capuchin,cracks open a palm nut on a sandstone anvil.In this case, he was given a choice betweentwo artificial stones, one heavy and smalland the other light and big, and correctlyselected the smaller and heavier stone as hishammer. (Photo by Elisabetta Visalberghi.)

critical characteristics relating to thetask at hand. This implies that thesemonkeys, and quite possibly otherspecies, are far more discerning thanpreviously believed. It will beinteresting to see whether futurestudies find this same discrimination inother tasks and among other species.Such knowledge will help to clarify theconditions which lead to theemergence of an understanding ofcomplex tasks in animals.

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Spagnoletti, N., Ottoni, E., Izar, P., andFragaszy, D. (2009). Selection ofeffective stone tools by wild beardedcapuchin monkeys. Curr. Biol. 19,213–217.

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15. Thouless, C.R., Fanshawe, J.H., andBertram, B.C.R. (1989). Egyptian vulturesNeophron percnopterus and Ostrich Struthiocamelus eggs - the origins of stone-throwingbehavior. Ibis 13, 9–15.

16. Chappell, J., and Kacelnik, A. (2002). Toolselectivity in a non-primate, the NewCaledonian crow (Corvus moneduloides). Anim.Cogn. 5, 71–78.

17. Chappell, J., and Kacelnik, A. (2004). Selectionof tool diameter by New Caledonian crowsCorvus moneduloides. Anim. Cogn. 7, 121–127.

18. Weir, A.A.S., Chappell, J., and Kacelnik, A.(2002). Shaping of hooks in New Caledoniancrows. Science 297, 981.

19. Weir, A.A.S., and Kacelnik, A. (2006). A NewCaledonian crow (Corvus moneduloides)creatively re-designs tools by bending orunbending aluminium strips. Anim. Cogn. 9,317–334.

20. Fragaszy, D.M., Izar, P., Visalberghi, E.,Ottoni, E.B., and de Oliveira, M.G. (2004). Wildcapuchin monkeys (Cebus libidinosus) useanvils and stone pounding tools. Am. J.Primatol. 64, 359–366.

Department of Psychology & LanguageResearch Center, Georgia State University,PO Box 5010, Atlanta, GA 30302-5010, USA.E-mail: sbrosnan@gsu.edu

DOI: 10.1016/j.cub.2008.12.001