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Animal Behaviour 80 (2010) 945–946

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Animal Behaviour

journal homepage: www.elsevier .com/locate/anbehav

In Focus

Featured Articles in This Month’s Animal Behaviour

Figure 1. (a) Nephila clavata spiders build complex web structures consisting of barrierwebs and prey carcass decorations that, together, help increase prey capture. (b) Cyclosamulmeinensis spiders also decorate their webs with prey carcasses, and aggregate theirwebs together to amplify the effect of their web decorations. Photos: I-Min Tso.

O, what a tangled web they weave.

There are some excellent architects in the animal kingdom.Many animals construct burrows, traps, webs, nests and retreatsthat improve shelter and thermoregulatory ability, while someuse such structures to help attract mates (the mole cricket, forexample, famously constructs a burrow that functions as a Klipschhorn to amplify its calls). Building structures is, of course, costly:there are time and energy costs, and animals may also increasetheir exposure to predators during the construction phase.

Spiders are particularly interesting as animal architects becausesome species add additional structures to their webs, decoratingthem with prey carcasses, constructing ‘barrier webs’ on eitherside of the main orbweb, and sometimes combining the two. Otherspecies aggregate their nests together and then decorate them simi-larly with partially consumed prey items.Why spiders shouldmakethe extra effort to augment theirwebs in this fashion is an intriguingquestion, and in this month’s issue (pp. 947–953) I-Min Tso and hiscolleagues, Sean Blamires, Yat-Hung Lee, Chia-Ming Chang, Ing-TingLin, Jou-AnChen and Tzu-Yi Lin fromTunghai University, Taiwan, setout to discover the benefits of web decoration in two species, Neph-ila clavata, which builds barrier webs that it decorates with preycarcasses, and Cyclosa mulmeinensis, which builds aggregate websthat they also decorate with prey (Fig. 1).

The researchers manipulated the webs of each species ina similar way to see whether barrier webs, aggregation and decora-tion functioned to increase prey capture independently of eachother, or whether they only enhanced prey capture when operatingin concert. For N. clavata, the researchers removed (1) both thebarrier web and decorations, (2) just the barrier web, leaving theprey decorations intact and (3) just the prey decorations, leavingthe barrier web intact. For C. mulmeinensis, the researchersremoved (1) decorations from solitary webs and (2) decorationsfrom aggregated webs. To create the solitary webs, all spiders andtheir webs except one, which acted as the focal spider, wereremoved from an aggregation. The researchers then set up videocameras for 15 days each to record all insect prey that were inter-cepted by the webs.

In the case of N. clavata, prey interception rates were actuallygreater after barrier webs and prey carcasses had been removed,although most of the prey captured in these ‘naked’ webs weresmall. The reduced capture rate of complex webs is probablybecause, with the barrier and decorations in place, the webs arehighly visible to insects, making it easy to avoid them. Barrierwebs alone also didn’t help increase prey capture rate; in fact, itwas significantly reduced under this condition. Adding prey deco-rations alone didn’t have the same kind of negative effect as

0003-3472/$38.00 � 2010 The Association for the Study of Animal Behaviour. Publisheddoi:10.1016/j.anbehav.2010.10.006

barriers, but nor did it have a particularly positive effect: preycapture was about the same as that of ‘naked’ webs, which meansthat, overall, decorated webs represent a net cost to spiders. Thecombination of barrier webs and prey decorations, however, did

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Figure 2. An adult capuchin monkey obtains juice from an apparatus while twoyounger monkeys look on. Photo: Katrina Landau.

In Focus / Animal Behaviour 80 (2010) 945–946946

significantly increase the likelihood that prey would be retained inthe web once caught. In addition, webs with barriers and decora-tions also trapped significantly larger insects. The authors suggestthat these larger insects were active scavengers attracted to theprey carcasses. Once in the vicinity, they were likely to collidewith the barrier web, bouncing off it in a manner that slowed theirspeed and so made it more likely that they would then becometrapped in the orb web: a kind of ‘ricochet effect’. As barrier websalso protect the spiders fromwasp attack and help retain moisture,while the prey carcasses also act as caches of food for the web-owner, the researchers suggest that, in combinationwith increasedretention of larger prey, there are sufficient benefits to outweighthe costs of construction of these complex webs.

For C. mulmeinensis, it was also the case that decorations on theirown proved costly: solitary webs with prey decorations caughtfewer prey than solitary, undecorated webs. When the webs wereaggregated, however, prey capture rates were enhanced, perhapsbecause aggregating decorations amplifies the visual and aromaticcues given off by the prey carcasses, and so increases their attrac-tiveness to insects. Given this, it raises the question of why C. mul-meinensis bothers to aggregate and then decorate the aggregationswhen a solitary undecorated nest is just as effective. One clue forwhy this might be lies in the habitat; spiders in this study wereexposed to strong offshore winds. Tso and colleagues suggest thataggregating webs may help the spiders cope with wind exposure,and perhaps act as a barrier to physiological stress.

Spider architectural decision making, then, is clearly complex.The intertwining of an array of benefits that help make these highlystructured webs profitable is striking, and raises the further ques-tions of whymore animals don’t also augment their webs and otherstructures to gain similar benefits. As the authors note, this requiresa more detailed understanding of the costs involved, such as theenergetics of silk production and web construction, the predatorsencountered during web building and the time and energy divertedfrom other activities. With this in place, it will then becomepossible to determine the exact nature of the trade-offs betweenprofit and loss that help make complex structures worth the effort.

Louise BarrettExecutive Editor

Social Learning in Capuchins

The possibility that nonhuman animals possess cultural traditionshas longbeenof great interest. Cultural traditions are suggestedwhenpopulations show differences in behaviour and those differencespersist over generations. Foraging behaviours of primates providenotable instances, such as the potatowashing observed in some pop-ulations of Japanese macaques and the use of stone hammers andanvils in breaking nuts seen in some populations of chimpanzees.Long-lasting behavioural differences, however, can be explainedwithout invoking culture, for example, as being caused by geneticdifferences between populations or by differences in ecologicalcircumstances. A crucial prediction of the cultural tradition hypoth-esis is that the behaviours in question are passed from generation togeneration through social learning, something that is difficult to testin natural populations, butwhich can be tested using captive animalstransmitting novel behaviours. A paper in this issue (pp. 955–964) byJessica Crast, JessicaM. Hardy and DorothyM. Fragaszy presents sucha test with captive capuchin monkeys.

Crast and colleagues provided their subjects with an apparatuscontaining two reservoirs of juice (Fig. 2). Juice could be extracted

from one reservoir by pumping a lever and from the other byturning a recessed wheel. In the initial, baseline condition, infants7–18 months old were allowed access to the apparatus in a ‘crèche’from which adults were excluded. Next, in experimental phase 1,infants in the crèche again had access to the apparatus with bothreservoirs baited, while outside the crèche both infants and adultshad access to a second apparatus inwhich only one of the reservoirswas baited, which one depending on the group. After a 2-yearinterval, phase 2 trials were staged, in which a new crop of infantscould again move between a crèche, where they encountered anapparatus in which both foraging methods were effective, anda group enclosure, in which the infants together with adultsencountered an apparatus in which only one of the two methodswas effective.

Comparisons between experimental stages provided evidencethat social learning occurred. In the baseline condition, in whichinfants had no successful adults to observe, only two of 16 infantssolved the task. In phase 1, in which some adults also solved thetask, seven of the remaining 14 infants were successful. The greatersuccess of infants in phase 1 could be explained by increasing expe-rience as well as by social learning, so themore convincing compar-ison is between baseline and phase 2. In phase 2, with a new groupof naïve infants associating with already skilled adults, all 11 infantssolved the task, significantly more than in baseline. Phase 2 infantsalso solved the task faster, in a mean of 1.8 trials compared to 11.5trials in baseline. These differences provide convincing evidence ofsocial learning.

Results on which of the two methods were used by the infantsare also instructive. Of the 14 infants that were able to obtain juicein the crèche, 11 relied predominantly on the method that wasavailable to the adults in their group. This preference for the group’smethod of solution can be attributed in part to socially mediatedlearning of that method, but also in part to reinforcement of themethod in the group setting, where only that method wasrewarded.

The study by Crast and colleagues is particularly significant inthat it tracks acquisition of a novel behaviour across two genera-tions in relatively stable social groups. This study thus mirrorswhat must occur when cultural traditions are transmitted acrossgenerations in nature.

William A. SearcyExecutive Editor