Towards a comparative approach to the structure of animal personality variation

Latent personality traits underpinning observed behavioral variation have
been studied in a great many species. However, a lack of standardized
behavioral assays, coupled to a common reliance on inferring personality
from a single, observed, behavioral trait makes it difficult to determine
if, when, and how conclusions can be directly compared across taxa. Here,
we estimate the among-individual (co)variance structure (ID) for a set of
four behaviors expressed in an open field trial, putatively indicative of
boldness, in seven species of small freshwater fish. We show that the ID
matrices differ in terms of the total amount of variation present, and
crucially the orientation, and as a consequence, biological interpretation
of the first eigenvector. Specifically, loading of observed traits on the
main axis of variation in ID matched a priori expectations for a shy-bold
continuum in only three of the seven cases. Nonetheless, when the ‘shape’
of the matrices was compared in higher dimensions, there was a high level
of similarity among species, and weak evidence of phylogenetic signal. Our
study highlights the present difficulty of trying to compare empirical
inferences about specific personality traits across studies. However, it
also shows how multivariate data collection and analysis allows the
structure of behavioral variation to be quantitatively compared across
populations or species without reliance on ambiguous verbal labels. It
also suggests that the field may have much to gain from greater uptake of
phylogenetically informed comparative approaches when seeking to test
evolutionary hypotheses about the origin and maintenance of personality
variation.