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Annu. Rev. Entomol. 2008.53:145-160. Downloaded from arjournals.annualreviews.orgby CORNELL UNIVERSITY on 02/08/08. For personal use only.Speciation: theformation of a newbiological speciesobserver ant, a model ant moves from the nestto the food source 25% slower than when travelingalone. The model ant adjusts her speed,slowing down when the observer ant is delayedand accelerating when the observer isfollowing closely. Tandem running in the antT. albipennis was formally described as a caseof teaching (49), which is a highly restrictivecategory of social learning. Currently, onlyone other case of teaching in nonhuman animalshas been confirmed (107). In addition torecruiting nestmates to distant food sources,social bees can also copy the flower choiceof experienced foragers. Experiments in twolaboratories indicated that inexperienced observerbees (Bombus spp.) were more likely toland on the flower type visited by model beesthan on an unvisited alternative (64, 121).PROSPECTSThis review indicates that a variety of insectsrely on learning to enhance all major life activities,including feeding, antipredatory behavior,aggression, social interactions, courtship,and mate choice. Given that insect learningis now well documented, future research canfocus on a variety of topics concerned withthe evolution of learning and with the effectsof learning on insect ecology and evolution.These issues are outlined below.Ecological Variables Associated withEnhanced Learning AbilitiesThere is widespread interest in the evolutionof learning, cognition, and intelligence.To this end, work on vertebrates has successfullyidentified positive correlations betweenspecific ecological needs and either thenecessary learning abilities required to meetthese needs or the relative volumes of specificbrain parts housing the cognitive traitsthat process these needs (34). The strikinglack of comparable data from insects indicatesthat research is biased toward vertebratesor that negative data have remained unpublished.There is a large interspecies variationin the relative size of brain parts such as themushroom body (104), but that variation hasnot yet been tightly linked to insect learningand ecology. Fruitful lines of research withinthis area include testing the predictions that(a) nesting insects that shuttle between theirnest and food sources would have better spatiallearning and memory than closely relatednonnesting species (31), (b) social learning andbehavioral flexibility (innovation) are associatedwith larger volumes of relevant brainparts such as the mushroom body (66, 93), and(c) nectar feeders would be less likely to exhibittaste-aversion learning than would closely relatedfoliage feeders (90). A related promisingresearch program should examine the neurogeneticmechanisms underlying betweenspeciesvariation in learning and memory abilities(101, 122).Effects of Learning on Speciationin InsectsBehavior in general and learning in particularmay be major forces driving speciation (9, 74,119). This widespread assertion, however, isnot yet broadly supported by theory or data(34). Insects have been employed extensivelyin the study of speciation (24, 59, 69) and couldreadily be used for critical tests of the role oflearning in speciation.Insect learning can contribute to speciationin at least two ways. First, if adultscan learn to seek the substrate they fed onas larvae, and if adults typically mate atthe substrate, the resulting assortative matingcould lead to speciation (108). At least inD. melanogaster, parts of the mushroom body,the brain part involved in olfactory learning,remain intact during metamorphosis, potentiallyallowing memory to be maintained fromlarvae to adults (8). A few reports have indicatedthe transfer of memory from larvae toadults (53, 92, 111). Furthermore, two alternativelearning scenarios do not require memorythrough metamorphosis. First, in some insects,the larvae pupate next to the substrate;therefore eclosing adults can learn directly the154 Dukas
larval-substrate characteristics (60). Second,eclosing adults may smell substrate odors remainingon the pupae from the larval stage(10). In short, there is solid evidence that adultinsects can learn about their larval substrateand exhibit significant preference for this substrateover available alternatives. Further researchmay examine the role that such learnedpreference plays in speciation.The second way in which insect learningcan contribute to speciation involves matechoice. Work on D. melanogaster reviewedabove indicates that both males and femaleslearn in the context of mate choice. In females,learning about the variety of locallyavailable males might narrow the range ofacceptable mates and hence increase assortativemating (37). In males, learning decreasesinterspecific courtship (35). Learningin the context of sexual behavior is probablyprevalent in insects but current empiricaldata are limited (38). We also have toevaluate the importance of such learning inspeciation.Annu. Rev. Entomol. 2008.53:145-160. Downloaded from arjournals.annualreviews.orgby CORNELL UNIVERSITY on 02/08/08. For personal use only.SUMMARY POINTS1. Learning is probably a universal property of insects, which rely on learning for allmajor life functions.2. Genetically based individual variation in learning has been documented in a few insectspecies.3. The widespread assertion that insects may exhibit little learning owing to their smallbrain and brief life span has been rejected by recent theory and data.4. Social learning is currently known in social Hymenoptera but may be prevalent amongother insects.5. Because occurrences of learning are well documented in a variety of insects, futureresearch can build on that information to examine ecological features associated withenhanced learning abilities and how learning influences evolution.DISCLOSURE STATEMENTThe author is not aware of any biases that might be perceived as affecting the objectivity ofthis review.ACKNOWLEDGMENTSI thank Bernie Roitberg and Lauren Taylor for comments on the manuscript. My work hasbeen supported by the Natural Sciences and Engineering Research Council of Canada.LITERATURE CITED1. Abisgold JD, Simpson SJ. 1988. The effect of dietary-protein levels and hemolymphcomposition on the sensitivity of the maxillary palp chemoreceptors of locusts. J. Exp.Biol. 135:215–292. Aceves-Pina EO, Quinn WG. 1979. Learning in normal and mutant Drosophila larvae.Science 206:93–963. Alcock J. 2005. Animal Behavior. Sunderland, MA: Sinauer. 8th ed.www.annualreviews.org • Evolutionary Biology of Insect Learning 155
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