Animal personality denotes consistent differences among individuals in behavioural traits across time and contexts. Applying this theory to behavioural research allows separation of among- and within-individual variation, which can be valuable when assessing welfare in captive animals. This study uses Arctic charr (Salvelinus alpinus) as a case study to explore how housing conditions shape personality expression and whether behavioural traits covary with functional phenotypes relevant to welfare.
The authors situate animal personality within a nature-based welfare framework that prioritizes the expression of species-typical behaviours and the maintenance of consistent behavioural patterns over time. Under this perspective, persistent individual differences in behaviour are not noise but meaningful signals: they can indicate how individuals cope with environmental conditions and reveal welfare compromises when behavioural structure or stability is disrupted.
The primary aims were to examine (1) how environmental complexity affects personality expression in Arctic charr, and (2) whether a focal behavioural trait—boldness—covaries with functional phenotypes, specifically growth rate and brain size. The authors frame such covariation as part of broader animal personality syndromes, where behavioural traits are linked to other phenotypic dimensions and may support adaptive responses to environmental conditions.
Environmental complexity is expected to influence both the mean expression of behaviours and their consistency. In many species, increased structural complexity promotes species-specific behaviours and reduces maladaptive traits. The authors propose that environmental complexity may generate alternative behavioural and functional phenotypes through multivariate phenotypic plasticity, meaning that multiple traits can change together in response to the environment.
The study found that mean levels of boldness did not differ between complex and plain environments. However, the temporal structure of boldness differed markedly between treatments. In the complex environment, boldness exhibited remarkably consistent repeatability over a longer-term period. In the plain environment, repeatability estimates collapsed after seven days. These results are presented as direct evidence that environmental complexity fosters stable behavioural trait expression, whereas a simplified environment may suppress detectable personality.
Despite hypotheses that behavioural traits such as boldness could covary with functional measures, the authors report no support in their dataset for associations between boldness and either growth rate or brain size. The absence of detected covariation in this study indicates that, for these measures and under the tested conditions, boldness did not predict the examined functional phenotypes.
The authors interpret their results as consistent with the idea that environmental complexity can promote alternative behavioural and functional phenotypes via multivariate phenotypic plasticity. While they found no empirical support for covariation with growth or brain size, the study emphasizes that partitioning behavioural variance using an animal personality framework remains a potentially powerful tool for welfare diagnostics. Specifically, tracking the consistency and structure of behavioural variance may reveal welfare-relevant changes that mean-level measures alone could miss.
The article reports the main outcomes and interpretations but does not provide further methodological detail in the abstract. Specifics on experimental design, sample sizes, the precise metrics and timing of boldness assays, and statistical approaches are not provided in the source abstract and therefore cannot be restated here. The absence of detected covariation with growth rate and brain size may reflect context-, species-, or trait-specific relationships; the authors note that such relationships are often species- and context-dependent.
The study provides evidence that environmental complexity supports stable expression of personality traits in Arctic charr, as shown by sustained repeatability of boldness in complex environments and a collapse of repeatability in plain environments after seven days. Although no associations were found between boldness and growth rate or brain size, the authors argue that the animal personality framework—by partitioning behavioural variance—offers a useful approach for diagnosing fish welfare. Competing interests were declared as none, and funding was provided by Rannis (195615-051).