📄 animal behavior and cognition

The role of animal personality in behavioural welfare diagnostics: a case study in Arctic charr (Salvelinus alpinus)

This study demonstrates that environmental complexity fosters stable, long-term expression of animal personality (specifically boldness) in Arctic charr, suggesting that monitoring behavioral variance through an animal personality framework offers a valuable approach for diagnosing fish welfare, even in the absence of direct covariation with growth rate or brain size.

Original authors: Philip, J., Laduree, G., Prat, A., Dellinger, M., Lobligeois, S., Benhaim, D.

Published 2026-08-28

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Philip, J., Laduree, G., Prat, A., Dellinger, M., Lobligeois, S., Benhaim, D.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Fish, like people, have distinct temperaments. Some are bold and quick to explore new surroundings, while others are cautious and prefer to stay hidden. In the scientific world, this consistent difference in personality is not just a curiosity; it is a measurable trait that can tell us a great deal about an animal's well-being. When animals are kept in captivity, such as in fish farms or research tanks, their environment plays a massive role in how they behave. A barren tank with smooth walls and no hiding spots is very different from a complex one filled with rocks, plants, and places to hide. Scientists have long suspected that these environmental details do more than just change how much a fish swims; they might change the very nature of the fish's personality, making it either stable and consistent or erratic and unpredictable. Understanding this link is crucial for improving the lives of animals in human care, ensuring they can express their natural behaviors rather than being forced into a state of constant stress or confusion.

In a recent study, researchers set out to test exactly how the complexity of a fish's home affects its personality over time. They focused on Arctic charr, a type of fish found in cold, mountainous regions, raising them in two very different conditions. One group lived in plain, white tanks that were completely empty, offering no shelter or visual interest. The other group lived in tanks filled with green plastic plants and black pebbles, creating a three-dimensional landscape with plenty of hiding spots. The scientists wanted to see if the fish in the empty tanks would become more stressed and erratic, or if the fish in the complex tanks would develop more stable personalities. To measure this, they tested the fish's "boldness" repeatedly over a long period. They placed each fish in a shelter inside a large, open arena and watched to see how long it took to swim out into the open space. They repeated this test four times over the course of many months, tracking the same individuals to see if their behavior remained consistent or if it changed wildly from day to day.

The results revealed a striking difference in how the fish held onto their personalities. In the complex environment, the fish maintained a consistent level of boldness over a long period. If a fish was naturally shy or naturally brave, it stayed that way for the entire 130-day interval between tests. Their behavior was predictable and stable, suggesting that the plants and rocks provided a sense of security that allowed their true personalities to shine through. In contrast, the fish living in the plain, empty tanks showed a very different pattern. While they appeared to have consistent personalities over a short period of just seven days, this stability completely fell apart over the longer term. After a few months, the fish in the empty tanks behaved erratically; a fish that was bold one week might be extremely cautious the next. The researchers found that the fish in the plain tanks had much higher fluctuations in their behavior, indicating that the lack of shelter forced them to constantly adjust their actions to cope with an exposed and stressful environment. This suggests that a barren environment does not just change the average behavior of a group, but actually suppresses the ability of individual fish to maintain a consistent personality.

The study also looked to see if these personality differences were linked to other physical traits, such as how fast the fish grew or the size of their brains. It is often thought that bolder fish might grow faster because they are more willing to take risks to find food, or that their brains might be structured differently to handle decision-making. However, the researchers found no evidence to support these ideas in this specific experiment. The fish in the complex tanks did not grow faster than those in the plain tanks, and there was no clear connection between a fish's boldness and the size of its brain or specific brain regions. The only physical difference observed was in the stability of behavior itself. The study suggests that the primary impact of the environment was not on the fish's body or brain size, but on the reliability of their behavior. The complex environment acted as a buffer, allowing the fish to express their natural traits consistently, while the plain environment seemed to disrupt this consistency, forcing the fish into a state of behavioral instability.

These findings offer a new way to think about animal welfare in aquaculture and research. Instead of just checking if fish are healthy or growing well, caretakers can look at the stability of their behavior as a sign of their well-being. When fish are kept in environments that match their natural needs, with places to hide and explore, they are able to maintain their individual personalities over time. When they are kept in empty, bare tanks, their behavior becomes unpredictable and unstable, which may be a sign of chronic stress. The study concludes that providing structural complexity is not just a luxury for fish, but a necessity for allowing them to live as their natural selves. By simply adding plants and rocks to a tank, we can help fish develop stable, consistent personalities, which is a fundamental part of a good life for these animals.

Technical Summary: The Role of Animal Personality in Behavioural Welfare Diagnostics in Arctic Charr

Problem Statement
The study addresses the challenge of diagnosing fish welfare in captive environments, specifically hatchery systems. While traditional welfare assessments often rely on mean behavioural responses, these averages may obscure the underlying behavioural architecture. The authors posit that "animal personality"—defined as consistent among-individual variation in behavioural traits across time and context—offers a superior framework for welfare diagnostics. A critical gap exists in understanding how structural environmental complexity influences the long-term stability (repeatability) of these personality traits and whether such traits covary with functional phenotypes like growth rate and brain size. The authors investigate whether a "plain" environment suppresses personality expression compared to a "complex" environment, potentially leading to maladaptive behavioural dynamics and compromised welfare.

Methodology
The researchers conducted a controlled experiment using Arctic charr (Salvelinus alpinus) from a single breeding stock.

Experimental Design: Fish were reared in two distinct environmental conditions: a "plain" treatment (bare white tanks) and a "complex" treatment (tanks containing green plastic plants and black pebbles). The study utilized a half-sib family design with three replicates per treatment.
Temporal Scale: To assess long-term consistency, boldness was tested over four repetitions spanning approximately 130 days. This included short-term intervals (7 days between Repetitions 1–2 and 3–4) and a long-term interval (130 days between Repetitions 2 and 3).
Behavioural Assay: Boldness was measured using an Open Field Test with a shelter. Individuals were placed in a shelter and allowed to voluntarily emerge into an open arena. Video tracking (Ethovision XT15) recorded variables including latency to emerge, time spent in zones, distance moved, and velocity.
Phenotypic Measurements:
- Growth: Specific Growth Rate (SGR) was calculated based on body mass and fork length measurements taken after each trial.
- Brain Size: Upon euthanasia at 470 days post-hatching, brains were dissected, and volumes of four regions (olfactory bulbs, telencephalon, optic tectum, cerebellum) were quantified using ImageJ and an ellipsoid volume formula.
Statistical Analysis:
- Personality Metrics: A Principal Component Analysis (PCA) generated a composite "boldness score." Variance components (among-individual $V_i$ and within-individual $V_w$ ) and repeatability were estimated using Bayesian linear mixed-effects models (MCMCglmm).
- Covariation: Bayesian multivariate models were used to estimate the covariance and correlation between boldness, growth rate, and brain regions within each treatment.

Key Results

Mean Boldness: There was no significant difference in mean boldness between the plain and complex treatments.
Repeatability and Variance:
- Short-term (7 days): Repeatability estimates were similar between treatments, indicating that individuals maintained consistent behavioural differences over short periods regardless of environment.
- Long-term (130 days): A significant divergence emerged. In the complex environment, repeatability remained stable and high ( $R \approx 0.26$ ), with low within-individual variance. Conversely, in the plain environment, repeatability collapsed ( $R \approx 0.05$ ) after the long interval, accompanied by significantly higher within-individual variance.
Trait Covariation: The study found no credible evidence of covariation or correlation between boldness and growth rate, nor between boldness and total brain size or specific brain regions in either treatment. A weak, non-significant trend toward a positive correlation between boldness and telencephalon size was observed in the plain treatment, but the credible intervals included zero.

Key Contributions

Environmental Complexity and Personality Stability: The study provides empirical evidence that environmental complexity fosters the long-term stability of animal personality. While plain environments do not alter the mean level of boldness, they appear to suppress the expression of consistent individual differences over time, leading to high behavioural plasticity (within-individual variance) rather than stable personality.
Variance Partitioning as a Welfare Diagnostic: The authors demonstrate that partitioning behavioural variance (specifically the ratio of among-individual to within-individual variance) is a more sensitive indicator of environmental quality than mean behavioural responses. The collapse of repeatability in plain environments suggests a "personality suppression" effect.
Decoupling of Traits: The research challenges the assumption that behavioural traits, growth, and brain size necessarily covary in a coordinated manner under these specific rearing conditions, highlighting the context-dependency of phenotypic syndromes.

Significance and Claims
The paper claims that the animal personality framework, specifically through the lens of variance partitioning and repeatability, offers a valuable tool for diagnosing fish welfare. The findings support a "nature-based welfare" perspective, suggesting that complex environments allow fish to express stable, species-specific behavioural patterns, whereas plain environments may induce chronic stress or context-dependent behavioural modulation that overrides intrinsic personality.

The authors modestly conclude that while their metrics did not detect behaviour-brain or behaviour-growth associations, the stability of personality in complex environments serves as a robust indicator of welfare. They explicitly state that their design was not intended to estimate narrow-sense heritability and recommend future studies focus on refined neuroanatomical measures to further explore potential behaviour-brain links. The primary significance lies in shifting the diagnostic focus from average group responses to the temporal dynamics and stability of individual behavioural variance.

Technical Summary: The Role of Animal Personality in Behavioural Welfare Diagnostics in Arctic Charr

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