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ABSTRACT Originating from human psychology, the concepts of “optimism” and “pessimism” were transferred to animal welfare science about 20 years ago to study emotional states in non‐human animals. Over time, “optimism” and “pessimism” have developed into valuable welfare indicators, but little focus has been put on the ecological implications of this concept. Here, we aim to bridge this gap and underline the great potential for transferring it to behavioural ecology. We start by outlining why “optimism” and “pessimism” can be considered as aspects of animal personalities. Furthermore, we argue that considering “optimism”/“pessimism” in a behavioural ecology context can facilitate our understanding of individual adjustment to the environment. Specifically, we show how variation in “optimism”/“pessimism” can play a crucial role in adaptation processes to environmental heterogeneity, for example, niche choice and niche conformance. Building on these considerations, we hypothesise that “optimists” might be less plastic than “pessimists” in their behaviour, which could considerably affect the way they adjust to environmental change. [ABSTRACT FROM AUTHOR]

The gut microbiota-immune-brain axis is a feedback network which influences diverse physiological processes and plays a pivotal role in overall health and wellbeing. Although research in humans and laboratory mice has shed light into the associations and mechanisms governing this communication network, evidence of such interactions in wild, especially in young animals, is lacking. We therefore investigated these interactions during early development in a population of common buzzards (Buteo buteo) and their effects on individual condition. In a longitudinal study, we used a multi-marker approach to establish potential links between the bacterial and eukaryotic gut microbiota, a panel of immune assays and feather corticosterone measurements as a proxy for long-term stress. Using Bayesian structural equation modeling, we found no support for feedback between gut microbial diversity and immune or stress parameters. However, we did find strong relationships in the feedback network. Immunity was negatively correlated with corticosterone levels, and microbial diversity was positively associated with nestling body condition. Furthermore, corticosterone levels and eukaryotic microbiota diversity decreased with age while immune activity increased. The absence of conclusive support for the microbiota-immune-brain axis in common buzzard nestlings, coupled with the evidence for stressmediated immunosuppression, suggests a dominating role of stress-dominated maturation of the immune system during early development. Confounding factors inherent to wild systems and developing animals might override associations known from adult laboratory model subjects. The positive association between microbial diversity and body condition indicates the potential health benefits of possessing a diverse and stable microbiota. [ABSTRACT FROM AUTHOR]

To ensure good welfare of animals in human hands, it is essential to modify housing conditions according to the animals’ needs. Traditionally, the effects of such modifications are studied by means of group-level comparisons, thereby widely neglecting consistent inter-individual differences (i.e., so-called ‘animal personalities’). However, as animals with distinct personality types might differ in their environmental needs and hence react differently to the same environment, such systematic inter-individual differences might have important welfare consequences. This becomes particularly apparent under laboratory conditions, where animals are typically housed under highly standardized and barren environments. Against this background, we here aim to investigate personality-dependent welfare consequences in response to different housing conditions in laboratory mice. Female C57BL/6J mice were characterized for their personality type in exploration behavior and the most and the least explorative individuals were set up in either simple or in highly complex housing conditions that included constantly changing environmental enrichment items. We monitored individual welfare by studying behavioral, physiological, and immunological outcome measures. Besides personality dependent differences in immune parameters and overall improved welfare under complex housing conditions, we indeed found hints that individual mice were differently affected in their welfare depending on the specific combination of personality type and housing condition. Specifically, highly explorative mice appeared to be more adversely affected by simple housing, but also profited more from complex housing compared to low explorative mice. These findings indicate that welfare promoting adjustments do not necessarily benefit all individuals equally and therefore, call for a shift of perspectives in the evaluation of animal welfare. [ABSTRACT FROM AUTHOR]

Adolescence is a sensitive phase when cues from the social environment can shape consistent individual differences in behavioral and hormonal profiles, but the effect of the social partner on these phenotypes is not well‐understood, especially for females. We therefore aimed to investigate whether the age class and sex of the housing partner affect social behavior and cortisol concentrations in female guinea pigs during adolescence and estimate the repeatability of cortisol concentrations in this life stage. Female guinea pigs were housed upon weaning for 4 weeks with either an adult female, adolescent female, or adolescent male. Social behavior was observed in the home enclosure and a preference test and social interaction test were carried out at the end of the experiment. Cortisol concentrations (baseline and response to a challenge) were measured prior to the start of the experiment and 2 and 4 weeks after pair formation. We hypothesized that the social partner would affect behavior and cortisol concentrations. We found that females housed with an adult female were less aggressive and more consistently displaced by their housing partner than females housed with an adolescent female. Sex of the housing partner did not affect focal female behavior. Regarding hormones, baseline cortisol concentration was neither significantly repeatable nor significantly affected by the housing partner. However, cortisol responsiveness was highly repeatable both 1 h and 2 h after the onset of a challenge. The age class of the housing partner affected cortisol responsiveness, with higher cortisol concentrations measured at 1 h in females housed with an adult female. These results indicate that adolescent female guinea pigs exhibit consistent individual differences in cortisol responsiveness while retaining plasticity to adjust to cues from the social environment. Taken together, we show in a female rodent that the social environment can significantly shape behavioral and endocrine parameters during adolescence. [ABSTRACT FROM AUTHOR]