Bart Adriaenssens

Less is more: density influences the development of behavioural life skills in trout

Theory suggests that habitat structure and population density profoundly influence the phenotypic development of animals. Here, we predicted that reduced rearing density and increased structural complexity promote food search ability, anti-predator response and the ability to forage on novel prey, all behavioural skills important for surviving in the wild. Brown trout were reared at three densities (conventional hatchery density, a fourth of conventional hatchery density and natural density) in tanks with or without structure. Treatment effects on behaviour were studied on trout fry and parr, whereupon 20 trout from each of the six treatment groups were released in an enclosed natural stream and recaptured after 36 days. Fry reared at natural density were faster to find prey in a maze. Moreover, parr reared at natural density were faster to eat novel prey, and showed more efficient anti-predator behaviour than fish reared at higher densities. Furthermore, parr reared at reduced densities were twice as likely to survive in the stream as trout reared at high density. In contrast, we found no clear treatment effects of structure. These novel results suggest that reduced rearing densities can facilitate the development of behavioural life skills in captive animals, thereby increasing their contribution to natural production.

Personality and life-history productivity: consistent or variable association?

In an interesting article, Biro and Stamps [1] suggest that consistent individual variation in behavioural traits (animal personality) can be explained by associations with traits involved in life-history (LH) tradeoffs. Such associations would favour personality traits when individuals differ consistently in rates of biomass production through growth or reproduction (LH productivity). We agree that LH tradeoffs are likely to be important in shaping personality traits, but we suggest that correlations between personality traits and LH productivity can often vary between environments.

Predictability as a Personality Trait: Consistent Differences in Intraindividual Behavioral Variation

Although animal behavior is generally repeatable, most behavioral variation apparently occurs within rather than across individuals. With the exception of very recent interest in individual behavioral plasticity (consistent differences in responsiveness), this within-individual variation has been largely ignored despite its importance in the study of proximate and ultimate questions about behavior. Here, we repeatedly scored the undisturbed activity of 30 adult male mosquitofish across multiple observation bouts spanning 132 days ( observations per fish). We found that the behavior of some individuals was consistently more predictable in a given context than others. Repeatability for this “intraindividual variation” (IIV; ) was evident after accounting for individual differences in activity trends across days, and activity responses due to fine-scale temperature variation (i.e., individual plasticity in response to both variables). To our knowledge, this is the first evidence that predictability of behavior is a repeatable characteristic of individual animals. We suggest that IIV represents an important axis of consistent behavioral variation that has previously not been formally considered. Finally, individual differences in predictability may similarly exist for labile morphological and physiological traits but have seemingly not been studied.

Context dependency of trait repeatability and its relevance for management and conservation of fish populations

Animals show consistent differences in behaviour and physiology. Understanding these differences is vital for predicting the effects of gradual environmental change, such as climate change and ocean acidification. Here we review how such trait repeatability is relevant for conservation of wild fish.

Behavioural syndromes in juvenile brown trout (Salmo trutta); life history, family variation and performance in the wild

The main aims of this study were to investigate (1) the occurrence and strength of the shyness–boldness behavioural syndrome in brown trout fry, (2) whether this syndrome is associated with paternal migratory life history, (3) whether fry survival and growth in the wild is associated with paternal life history and/or boldness and (4) whether offspring performance showed maternal effects. Nine female migratory trout were each crossed with one migratory and one resident male and the offspring were raised in hatchery tanks until first feeding. The behavioural studies showed that fry that responded bold towards a novel object also accepted a novel food item earlier and responded more aggressively towards their own mirror image. Principal component analysis showed that this shyness–boldness syndrome (PC1) explained 34% of the behavioural variation. Offspring boldness was not affected by paternal migratory life history, but significant effects of the female parent suggested maternal and/or genetic effects. Deviations from this pattern (PC2), where some individuals behaved less aggressively and performed more approaches to food, explained an additional 17% of the variation in behaviour, and was significantly influenced by length and paternal migratory life history. Fry growth and survival in nature was not associated with boldness (PC1) or PC2, suggesting that alternative behavioural strategies can be successful in nature. However, female parent effects on variation in fry size persisted over the first growth period in the wild, suggesting that these effects may influence offspring fitness during early life when major selection occurs.

Generalist-specialist trade-off during thermal acclimation.

The shape of performance curves and their plasticity define how individuals and populations respond to environmental variability. In theory, maximum performance decreases with an increase in performance breadth. However, reversible acclimation may counteract this generalist–specialist trade-off, because performance optima track environmental conditions so that there is no benefit of generalist phenotypes. We tested this hypothesis by acclimating individual mosquitofish (Gambusia holbrooki) to cool and warm temperatures consecutively and measuring performance curves of swimming performance after each acclimation treatment. Individuals from the same population differed significantly in performance maxima, performance breadth and the capacity for acclimation. As predicted, acclimation resulted in a shift of the temperature at which maximal performance occurred. Within acclimation treatments, there was a significant generalist–specialist trade-off in responses to acute temperature change. Surprisingly, however, there was also a trade-off across acclimation treatments, and animals with greater capacity for cold acclimation had lower performance maxima under warm conditions. Hence, cold acclimation may be viewed as a generalist strategy that extends performance breadth at the colder seasons, but comes at the cost of reduced performance at the warmer time of year. Acclimation therefore does not counteract a generalist–specialist trade-off and, at least in mosquitofish, the trade-off seems to be a system property that persists despite phenotypic plasticity.

Telomere length covaries with personality in wild brown trout

The prevalence of consistent among-individual differences in behaviour, or personality, makes adaptive sense if individuals differ in stable state variables that shift the balance between the costs and benefits of their behavioural decisions. These differences may give rise to both individual differences in, and covariance among, behaviours that influence an individuals exposure to risks. We here study the link between behaviour and a candidate state variable previously overlooked in the study of state-dependent personality variation: telomere length. Telomeres are the protective endcaps of chromosomes and their erosion with age is thought to play a crucial role in regulating organismal senescence and intrinsic lifespan. Following evidence that shorter telomeres may reduce the lifespan of animals in a wide range of taxa, we predict individuals with shorter telomeres to behave more boldly and aggressively. In order to test this, we measured telomere length and behaviour in wild juvenile brown trout (Salmo trutta). We found individuals with shorter fin telomeres to behave consistently more boldly and aggressively under controlled conditions in the laboratory. No such relationship was found with muscle telomere length 3-4months after the behavioural assays. We suggest that telomere dynamics are an important factor integrating personality traits with other state variables thought to be important in the regulation of behaviour, such as metabolism, disease resistance and growth.

Individual variation in the compromise between social group membership and exposure to preferred temperatures

Group living is widespread among animal species, and comes with a number of costs and benefits associated with foraging, predator avoidance and reproduction. It is largely unknown, however, whether individuals sacrifice exposure to their own preferred or optimal environmental conditions so they can remain part of a social group. Here, we demonstrate that individual three-spine sticklebacks vary in the degree to which they forego exposure to their preferred ambient temperature so they can associate with a group of conspecifics. Individual fish varied widely in preferred temperature when tested in isolation. When the same individuals were presented with a choice of a warm or cold thermal regime in the presence of a social group in one of the environments, fish spent more time with the group if it was close to their own individually preferred temperature. When a group was in a relatively cool environment, focal individuals that were more social deviated most strongly from their preferred temperature to associate with the group. Standard and maximum metabolic rate were not related to temperature preference or thermal compromise. However, individuals with a higher standard metabolic rate were less social, and so energetic demand may indirectly influence the environmental costs experienced by group members. The reduced tendency to engage with a social group when there is a large difference between the group temperature and the individuals preferred temperature suggests a role for temperature in group formation and cohesion that is mediated by individual physiology and behaviour. Together, these data highlight exposure to non-preferred temperatures as a potential cost of group membership that probably has important but to date unrecognized implications for metabolic demand, energy allocation, locomotor performance and overall group functioning.

Bold perch live life in the fast lane

A schematic summary showing the links between behaviour and life-history observed by Nakayama, Rapp & Arlinghaus in wild Eurasion perch (Perca fluviatilis). [Colour figure can be viewed at wileyonlinelibrary.com]. In Focus: Nakayama, S., Rapp, T. & Arlinghaus, R. (2017) Fast-slow life history is correlated with individual differences in movements and prey selection in an aquatic predator in the wild. Journal of Animal Ecology, 86, 192-201. The pace-of-life syndrome hypothesis (POLS) suggests that individual behavioural variation co-evolves with life-history variation, causing individuals on a fast life-history trajectory to display more active or bold personalities than individuals following a slow trajectory. In the present study, Nakayama, Rapp & Arlinghaus () followed the detailed movement patterns of wild Eurasian perch using acoustic telemetry and studied their relationships with life-history traits inferred from scale samples. Consistent with POLS, individuals with greater reproductive effort changed more often between active and passive behavioural modes. Moreover, individuals growing fast as a juvenile stayed active longer and moved over greater distances when adult. This study shows compelling evidence for covariance between personality and pace-of-life in a natural population.