In a fluctuating environment, the optimal level of exploratory behavior depends on the proportion of current risks and benefits. The exploratory behavior is, therefore, often subjected to heterogenous selection. In populations of commensal rodents living in close proximity of humans, this pressure is further increased by pest management. We hypothesize that the black rat (Rattus rattus) responds to this pressure by either high behavioral flexibility or by development of personality types. The aim of this study was to analyze exploratory behavior and boldness of wild black rats and its changes over time to determine whether exploratory behavior is a personality trait in black rats. Studies on animals with unreduced variability are necessary for determination of normal range of behaviors. The behavior in the open field and hole board tests yielded 1 multivariate variable representing exploratory behavior and 1 representing boldness. The hole board test additionally provided an axis representing exploratory behavior. Exploratory behavior showed moderate to high repeatability, even though we observed a considerable effect of habituation. Exploratory behavior was also strongly correlated across contexts; therefore, our results suggest that the black rat responds to heterogenous selection pressure by developing personality types. We also found a strong effect of litter identity on some aspects of the exploratory behavior. Boldness was less repeatable, which we interpret as high behavioral flexibility in this behavioral trait. In concordance with our hypothesis, the personality types in exploratory behavior, but not in boldness, are possibly maintained by heterogenous selection pressure created by human pest management. (PsycINFO Database Record

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The relationship between physiological and behavioral stress markers is documented in several rodent species. However, there is no information regarding the role of adrenocortical activity in behavior of the black rat (Rattus rattus). Therefore, we hypothesize that the adrenocortical activity of black rats varies between individuals and is related to some of the behaviors in a novel environment. To test this hypothesis, we (i) validated a method for quantifying glucocorticoid metabolites from feces (fGCMs) with an enzyme immunoassay (EIA); (ii) examined variation and diurnal rhythms of feces and GCM production; and (iii) examined the relationship between GCM levels and exploratory behavioral traits. We fulfilled the first aim (i) by successfully performing an ACTH challenge test to validate the use of a 5α‐pregnane‐3β,11β,21‐triol‐20‐one EIA for measuring fGCMs. Second (ii) we detected considerable consistent interindividual variability in production of both feces and glucocorticoids. The peak production of feces occurred in the first hour of the dark cycle, the peak of fGCMs occurred approximately 3 h later. Lastly, (iii) there was no clear relationship between behavior in the hole board test and GCMs. Grooming, a typical behavioral stress marker, was negatively associated with stress reactivity, while head‐dipping in the hole‐board test (traditionally considered an exploratory behavior independent of stress) was not correlated with the GCMs. This study offers a first look at GCMs in the black rat, successfully validates a method for their measurement and opens possibilities for future research of the relationship between glucocorticoids and exploratory behavior in this species.

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