Journal of Comparative Psychology 1993. Vol. 107. No. 3. 250-260 Copyright 1993 by the American Psychological Association. Inc. 0735-7036/93/5,3.Ill) Shy-Bold Continuum in Pumpkinseed Sunfish (Lepomis gibbosus): An Ecological Study of a Psychological Trait David Sloan Wilson, Kristine Coleman, Anne B. Clark, and Laurence Biederman The shy-bold continuum is recognizedas a fundamental axis of behavioral variation in humans, but 3 major issues have not been addressed. First, the taxonomic distribution of shyness and boldness is unknown. Second, the ecological consequences of shyness and boldness have not been studied in natural populations. Third, no one has tried to predict and test patterns of shyness and boldness that might result from natural selection. We show that a shy-bold continuum, which influences diet, predator risk, and parasite fauna, exists in juvenile pumpkinseed sunfish (Lepomis gibbosus). Individual differences are relatively stable in nature but seem to disappear when the fish are held in social and ecological isolation in the laboratory. Thus, phenotypic stability may not reflect innate tendencies to be shy or bold but rather environmental conditions that maintain differences between phenotypically plastic individuals. In humans and at least some other species, individual or- ganisms vary along a behavioral continuum from extreme shyness to extreme boldness (for humans, Kagan, 1991; Kagan, Reznick, & Snidman, 1988; Matheny, 1989; Plomin & Dunn, 1986; Thomas & Chess, 1977; for primates, Stevenson-Hinde, Stillwell-Barnes, & Zung, 1980; Suomi, 1983, 1987, 1991; for canids, Bekoff, 1977; Fox, 1972; Goddard & Beilharz, 1985; MacDonald, 1983; for rats, Blanchard, Flannelly, & Blanchard, 1986; Blizard, 1981; Cooper, Schmidt, & Barrett, 1983; for goats, Lyons, Price, & Moberg, 1988). Shy individuals react to unfamiliar situ- ations by retreating or becoming quiet and vigilant. Bold individuals act normally or become actively exploratory in the same situations. Although psychologists regard the shy-bold continuum as a fundamental axis of behavioral variation, they have not addressed three major issues. First, the ecological con- sequences of shyness and boldness have never been stud- ied in a natural population of any species. Second, an evo- lutionary perspective is lacking. The biological basis of shyness and boldness is treated as a central issue, but ex- isting studies focus on heritability, phenotypic stability, David Sloan Wilson, Kristine Coleman, Anne B. Clark, and Laurence Biederman, Department of Biological Sciences, State University of New York at Binghamton. This research was supported by Grant DE-FG02-89ER60884 from the Ecological Research Division, Office of Health and En- vironmental Research, U.S. Department of Energy. We are very grateful to Bob Johnson for facilitating our exper- iments at Cornell University's experimental pond facility. We also thank Lee Dugatkin, Tim Ehlinger, Gary Mittelbach, Beren Robinson, Stim Wilcox, and the State University of New York at Binghamton's Ecology/Evolution/Behavior Group for interesting discussions. Correspondence concerning this article should be addressed to David Sloan Wilson, Department of Biological Sciences, State University of New York, Binghamton, New York 13902-6000. Elec- tronic mail may be sent to DWILSONOBINGVAXA.BITNET. and physiological mechanisms. No one has tried to predict and test adaptive patterns of shyness and boldness that might result from natural selection. Third, because re- search has been confined to humans and a few other mam- malian species, the taxonomic distribution of shyness and boldness is largely unknown. These unresolved issues in the psychological literature are precisely those that behavioral ecologists may be expected to address. Curiously, however, behavioral ecologists have not yet focused on shyness and boldness as an important subject that requires an ecological or evolutionary explanation (Clark & Ehlinger, 1987). In addition, it is important to dis- tinguish behavioral variation within populations from be- havioral variation between populations. For example, in some fish species populations that experience different de- grees of predation have been shown to differ in their be- havioral response to predators in ways that could be inter- preted as shy and bold (for the guppy Poecilia reticulata, Fraser & Gilliam, 1987; Magurran & Seghers, 1990; for the stickleback Gasterosteus aculeatus L., Giles & Huntingford, 1984; Tulley & Huntingford, 1987a, 1987b; for the minnow Phoxinus phoxinus, Magurran, 1986b, 1990). These and other studies explain differences between populations, but they tend to ignore the profound individual differences that can exist within populations (but see Altmann, 1980; Armit- age, 1986; Huntingford, 1976; Huntingford & Giles, 1987; Magurran, 1986a, 1986b; Murphy & Pitcher, 1991). Thus, the same gaps that exist in the psychological literature on shyness and boldness also exist in the behavioral ecology literature. In this article we examine shyness and boldness in pop- ulations of juvenile pumpkinseed sunfish. We focus on the following questions: (a) Does a shy-bold continuum exist in juvenile pumpkinseed sunfish? (b) How does shyness and boldness correlate with age, sex, and size? (c) What are the ecological consequences of shyness and boldness? (Possi- bilities include differences in spatial movement, feeding be- havior, growth rate, mortality, and parasite load.) (d) Are individual differences phenotypically stable? 250