Shy-bold Continuum Research Articles

Individual differences and reproductive success in yellow-bellied marmots

Mirror-image stimulation (MIS) was used to determine the individual behavioral phenotypes of 90 adult, 132 yearling, and 135 young yellow-bellied marmots (Marmota flaviventris). Linear typal analysis (LTA) was used to group individuals based on similarities in their MIS scores. Principal component analysis (PCA) was used to evaluate the patterns of variation in behaviors and discriminant function analysis (DFA) was used to determine if there were consistent differences in marmot behavior. LTA produced two interpretable groups that were significantly and negatively correlated. Marmots in each group formed a shy-bold continuum. Bold animals were those that approached the mirror early and made nose contact with the image whereas shy animals approached the mirror late in the run, spent time in the back half of the arena, and frequently chirped at the image. Behaviors shifted with age; young were predominantly bold and adults were predominantly shy. This shift appears to be developmental and not genetically determined. Reliability and predictability analyses indicated that there is a 75% probability that a MIS run correctly identifies an individual's behavioral phenotype. The probability increases to 90% for those individuals with high loading scores. Play, agonistic, and amicable behaviors of female yearlings recorded in the field were significantly correlated with their MIS behavioral phenotypes whereas only play of male yearlings was so correlated. Correlations among eight life-history traits of adult females were independent of behavioral phenotypes. Shy females were more likely to be associated with female non-kin, produced more daughters, and were more likely to be immigrants. Among female yearlings, shy phenotypes were likely to become recruits and bold phenotypes were more likely to disperse. Bold adult females recruited yearlings regardless of their behavioral phenotype whereas shy adult females recruited primarily shy yearlings and the majority of bold yearlings dispersed. Among adult males, shy phenotypes were more likely to be territorial-colonial and to produce more young. Lifetime reproductive success (LRS) of females was strongly related to the number of years of residency, which was strongly correlated with the frequency of reproduction and the production and recruitment of offspring. The demographic patterns were independent of behavioral phenotype. Behavioral phenotype impacts demographic trends mainly by increasing the recruitment of daughters by bold females. LRS of males depends on the length of residency. Shy males have a higher LRS because they are territorial-colonial for a longer time and produce more young. Bold individuals appear to be those that are sub- ordinate and attempt to affiliate with other marmots whereas shy individuals appear to be dominant and less likely to be affiliative. By being more aggressive, the shy marmot is more likely to achieve residency as a yearling or adult whereas the more affiliative adult is more likely to recruit offspring, but affiliative yearlings are less likely to become recruits, especially when the adults are the dominant, “shy” individuals.

Shyness and boldness in humans and other animals

The shy-bold continuum is a fundamental axis of behavioral variation in humans and at least some other species, but its taxonomic distribution and evolutionary implications are unknown. Models of optimal risk, density- or frequency-dependent selection, and phenotypic plasticity can provide a theoretical framework for understanding shyness and boldness as a product of natural selection. We sketch this framework and review the few empirical studies of shyness and boldness in natural populations. The study of shyness and boldness adds an interesting new dimension to behavioral ecology by focusing on the nature of continuous behavioral variation that exists within the familiar categories of age, sex and size.

Shy-bold continuum in pumpkinseed sunfish (Lepomis gibbosus): An ecological study of a psychological trait.

The shy-bold continuum is recognized as 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.