The ontogeny of personality in the wild guinea pig

• We examine the development of personality across two different ontogenetic stages.
• We manipulate the early environment to create differences in personality types.
• Some but not all personality traits are plastic.
• Correlations between plastic personality traits tighten across ontogeny.
• Correlations between nonplastic traits change at certain ontogenetic stages.

The role of ontogenetic processes for the emergence of personality has received only little attention in the past. One reason for the lack of experimental studies on personality development may be that trait consistency over time is one of the cornerstones of the definition of animal personality, whereas, also by definition, ontogeny deals with change over time. Single traits or correlations between traits may be consistent or inconsistent throughout ontogeny; however, the proximate and ultimate causes are not well understood. Environmental factors acting upon individuals during early ontogeny potentially influence personality development substantially. Early environments may severely constrain but also adaptively shape individuals. We examined the personality development of cavies, Cavia aperea, when exposed to increasing and decreasing photoperiod before and after birth. We determined how these predictive environmental cues influenced the development of three behavioural and two physiological traits, their temporal consistency and the correlations between them. We found remarkable plasticity in the development of personality in the cavy, despite a relatively high degree of temporal consistency in most traits. There were stable correlations, some of which became tighter over time, between basal cortisol levels, resting metabolic rate and fearlessness across two different ontogenetic stages. However, we also found that some correlations emerged only after maturation or disappeared over time. Whereas exploration behaviour was tightly correlated with basal cortisol and boldness was correlated with resting metabolic rate, both correlations disappeared in mature animals. Instead, a correlation between exploration and boldness was evident in mature animals. These results call for a broader incorporation of developmental aspects into personality research.