Profile repeatability: A new method for evaluating repeatability of individual hormone response profiles

Simulating physiological flexibility in the acute glucocorticoid response to stressors reveals limitations of current empirical approaches

Wild animals often experience unpredictable challenges that demand rapid and flexible responses. The glucocorticoid mediated stress response is one of the major systems that allows vertebrates to rapidly adjust their physiology and behavior. Given its role in responding to challenges, evolutionary physiologists have focused on the consequences of between-individual and, more recently, within-individual variation in the acute glucocorticoid response. However, empirical studies of physiological flexibility are severely limited by the logistical challenges of measuring the same animal multiple times. Data simulation is a powerful approach when empirical data are limited, but has not been adopted to date in studies of physiological flexibility. In this article, I develop a simulation that can generate realistic acute glucocorticoid response data with user specified characteristics. Simulated animals can be sampled continuously through an acute response and across as many separate responses as desired, while varying key parameters. Using the simulation, I develop several scenarios that address key questions in physiological flexibility. These scenarios demonstrate the conditions under which a single glucocorticoid trait can be accurately assessed with typical experimental designs, the consequences of covariation between different components of the acute stress response, and the way that context specific differences in variability of acute responses can influence the power to detect relationships between the strength of the acute stress response and fitness. I also describe how to use the simulation tools to aid in the design and evaluation of empirical studies of physiological flexibility.

Repeatability of daily profiles of baseline glucocorticoid metabolites measured in the urine and faeces of eastern rock sengis (Elephantulus myurus)

Repeatability of hormone concentrations is of great interest for studies investigating the evolution of hormonal traits. Particularly the repeatability of glucocorticoids (GCs) in response to a stressor is frequently investigated, but often only point (initial and/or response value), or single measures are used. A new method takes into account the entire individual hormone profile and generates an individual profile repeatability (PR) score. The method was developed for response profiles, but it may also be valuable for baseline values in species with diurnal changes in hormone concentrations. GCs are determined in a variety of matrices, and repeatability can vary considerably depending on the matrix. We investigated the repeatability of baseline GC metabolite (GCM) concentrations measured in urine (uGCM) and faeces (fGCM) of captive eastern rock sengis (Elephantulus myurus) using the more traditional linear mixed model approach and the PR method. GCMs were assessed over 24 h and measurements were repeated twice with two weeks between replicates. A diurnal rhythm in GCM concentrations associated with the activity period of the sengis was found in urine, but not in the faeces. Urinary GCM concentrations exhibited a moderate repeatability, whereas the repeatability of fGCM concentrations was low. Urinary GCM concentrations and their repeatability differed between the sexes; with higher concentrations and lower PR scores in females. No such sex differences were apparent for fGCM concentrations and the PR score was not able to characterise repeatability of fGCM concentrations, which were lacking a distinct profile. The PR score enabled a successful quantification of the repeatability of the diurnal uGCM profiles. Hormone profile, sex and sample matrix can affect hormonal traits considerably and the results may be obscured if these factors are not carefully considered.

Are individuals consistent? Endocrine reaction norms under different ecological challenges

Quantifying organismal capacity for compensatory mechanisms is essential to forecast responses to environmental change. Despite accumulating evidence for individual variation in physiological plasticity, the causes and consequences of this variation remain unclear. An outstanding question is whether individual reaction norms are consistent across different environmental challenges, i.e. whether an individual that is responsive to one environmental variable will be equally responsive to a different environmental variable. Additionally, are these reaction norms themselves consistent over time, i.e. repeatable? Here, we quantified individual baseline glucocorticoid responses in house sparrows, Passer domesticus, to sequential manipulations of temperature, wind speed and food unpredictability that were repeated in discrete blocks of sampling under both control and stressor-exposed conditions. Individuals significantly decreased their baseline corticosterone levels and increased their mass during treatment exposure. This response was consistent across environmental challenge types. There was high repeatability in the intercept and slope of the baseline corticosterone reaction norm between environmental challenges but broad credible intervals in the repeatability of the reaction norm slope, suggesting that although glucocorticoid levels during baseline conditions are repeatable, among-individual variation in the shape of the glucocorticoid response may be higher than within-individual variation. Within-subject variation in baseline corticosterone levels was mainly explained by within-individual variation in body mass during stressor exposure. Despite the high lability in physiological traits, endocrine plasticity is repeatable across environmental challenges and may be able to evolve as a result of genetic accommodation, in which selection acts on genetic variation of reaction norms.

Individual differences in hormonal responsiveness to social encounters: Commentary on Félix et al., 2020 and review of pertinent issues

The results of a recent study, Félix et al., 2020, give new information about the behavioral and endocrine correlates of individual differences in the potential for androgen response in male cichlid fish. We think the study raises issues that are pertinent to the study of hormones and competition in other species, particularly humans. Focusing mostly on androgen reactivity to social challenge, we emphasize the importance of inter-individual variability in physiology, personality, and motivation in studies of hormone responses to social encounters. Additionally, we give special attention to matters of "repeatability" and the timing of hormone sampling. We conclude with an appreciation of the value of comparative analysis in behavioral endocrinology.

Individual variation and the challenge hypothesis

In this paper I discuss how the challenge hypothesis (Wingfield et al., 1990) influenced the development of ideas about animal personality, and describe particularly promising areas for future study at the intersection of these two topics. I argue that the challenge hypothesis influenced the study of animal personality in at least three specific ways. First, the challenge hypothesis drew attention to the ways in which the environment experienced by an organism - including the social environment - can influence biological processes internal to the organism, e.g. changes to physiology, gene expression, neuroendocrine state and epigenetic modifications. That is, the challenge hypothesis illustrated the bidirectional, dynamic relationship between hormones and (social) environments, thereby helping us to understand how behavioral variation among individuals can emerge over time. Because the paper was inspired by data collected on free living animals in natural populations, it drew behavioral ecologists' attention to this phenomenon. Second, the challenge hypothesis highlighted what became a paradigmatic example of a hormonal mechanism for a behavioral spillover, i.e. testosterone's pleiotropic effects on both territorial aggression and parental care causes aggression to "spillover" to influence parenting behavior, thereby limiting behavioral plasticity. Third, the challenge hypothesis contributed to what is now a cottage industry examining individual differences in hormone titres and their relationship with behavioral variation. I argue that one particularly promising future research direction in this area is to consider the active role of behavior and behavioral types in eliciting social interactions, including territorial challenges.

An individual based, multidimensional approach to identify emotional reactivity profiles in inbred mice

BACKGROUND

Despite extensive environmental standardization and the use of genetically and microbiologically defined mice of similar age and sex, individuals of the same mouse inbred strain commonly differ in quantitative traits. This is a major issue as it affects the quality of experimental results. Standard analysis practices summarize numerical data by means and associated measures of dispersion, while individual values are ignored. Perhaps taking individual values into account in statistical analysis may improve the quality of results.

NEW METHOD

The present study re-inspected existing data on emotional reactivity profiles in 125 BALB/cJ and 129 mice, which displayed contrasting patterns of habituation and sensitization when repeatedly exposed to a novel environment (modified Hole Board). Behaviors were re-analyzed on an individual level, using a multivariate approach, in order to explore whether this yielded new information regarding subtypes of response, and their expression between and within strains.

RESULTS

Clustering individual mice across multiple behavioral dimensions identified two response profiles: a habituation and a sensitization cluster.

COMPARISON WITH EXISTING METHOD(S)

These retrospect analyses identified habituation and sensitization profiles that were similar to those observed in the original data but also yielded new information such as a more pronounced sensitization response. Also, it allowed for the identification of individuals that deviated from the predominant response profile within a strain.

CONCLUSIONS

The present approach allows for the behavioral characterization of experimental animals on an individual level and as such provides a valuable contribution to existing approaches that take individual variation into account in statistical analysis.

Across time and space: Hormonal variation across temporal and spatial scales in relation to nesting success

There is a renewed interest in investigating individual variation in hormone levels in relation to fitness metrics, as hormones act as mediators of life-history trade-offs. Hormone concentrations, however, are labile, responding to both internal and external stimuli, so the relationship between hormones and fitness can be non-consistent. One explanation of this inconsistent relationship is that a single hormone sample may not be representative of individual phenotypes in a free-living species. We addressed this issue by repeatedly sampling a free-living population of mountain white-crowned sparrows, Zonotrichia leucophrys oriantha, for baseline and stress-induced corticosterone (cort) and testosterone (T) across different stages of the breeding season. We measured (co)variation using three different methods, taking into account inter- and intra-individual variances, to determine whether hormone levels and the stress response are repeatable. We documented the temporal (over 3 months) and spatial (home-range) variation of individual hormone phenotypes and investigated how these components related to nesting success. At the population level, we found significant repeatability in male stress-induced cort concentrations but no repeatability in male or female baseline cort or male T concentrations. Using a new metric of intra-individual variance focusing on the stress response (profile repeatability), we found a wide range of variance scores, with most individuals showing high variation in their stress response. Similarly, we found a low level of repeatability of the reaction norm intercept and slope for the stress response across different life-history stages. Males with higher concentrations of stress-induced cort had more central home-ranges. Males with higher body condition had larger home-ranges; however, home-range size did not relate to male hormone concentrations or nesting success. We also did not find any significant relationship between variation in hormone levels and nesting success. We recommend that future studies combine both physiological and environmental components to better understand the relationship between hormones and fitness.

Eye region surface temperature dynamics during acute stress relate to baseline glucocorticoids independently of environmental conditions

Reactions to acute stressors are critical for survival. Yet, the challenges of assessing underlying physiological processes in the field limit our understanding of how variation in the acute stress response relates to fitness in free-living animals. Glucocorticoid secretion during acute stress can be measured from blood plasma concentrations, but each blood sample can only provide information for one point in time. Also, the number of samples that can be extracted from an individual in the field is usually limited to avoid compromising welfare. This restricts capacity for repeated assessment, and therefore temporal resolution of findings within- and between-acute stress responses - both of which are important for determining links between acute stress and fitness. Acute stress induces additional body surface temperature changes that can be measured non-invasively, and at high frequencies using thermal imaging, offering opportunities to overcome these limitations. But, this method's usefulness in the field depends on the extent that environmental conditions affect the body surface temperature response, which remains poorly understood. We assessed the relative importance of individual physiology (baseline glucocorticoid concentrations) and environmental conditions (air temperature and relative humidity) in determining the eye region surface temperature (Teye) response to acute stress, in wild blue tits (Cyanistes caeruleus) during trapping, handling and blood sampling. When controlling for between-individual baseline variation, Teye initially dropped rapidly below, and then recovered above baseline, before declining more slowly until the end of the test, 160 s after trap closure. One measure of the amplitude of this response - the size of the initial drop in Teye - was dependent on environmental conditions, but not baseline corticosterone. Whereas, two properties defining response dynamics - the timing of the initial drop, and the slope of the subsequent recovery - were related to baseline corticosterone concentrations, independently of environmental conditions. This suggests inferring the acute stress response using thermal imaging of Teye will be practical under fluctuating environmental conditions in the field.

How Truly Conserved Is the “Well-Conserved” Vertebrate Stress Response?

The vertebrate stress response is considered to be a highly conserved suite of responses that are evolved to help animals survive noxious environmental stimuli. The two major pathways of the stress response include the catecholamine release that is part of the autonomic nervous system and comprises the immediate fight-or-flight response, and the slower release of corticosteroids from the hypothalamic–pituitary–adrenal axis that help orchestrate longer-term responses. These two pathways are present in every vertebrate yet examined, and the anatomical and physiological architecture underlying these pathways are consistent. Despite these structural similarities, however, recent data indicate substantial temporal and species variation in the actual regulation of these pathways. For example, activation of both pathways varies seasonally in some species but not others, and responses of both pathways can be extensively modulated by an individual’s previous experience. Consequently, even though the anatomy of the stress response is highly conserved, the activation and functional output is not highly conserved. Given this variation, it is perhaps not surprising that it is proving difficult to correlate individual stress responses with differences in fitness outcomes. This review summarizes the challenge of making broad generalized assumptions about fitness consequences of the stress response given the functional variation we observe.

Meta-analytic insights into factors influencing the repeatability of hormone levels in agricultural, ecological, and medical fields

Interest in individual variation in hormone concentrations is rapidly increasing, particularly with regard to the evolutionary and practical implications. A key aspect of studying individual variation in any labile trait is estimating the degree of within- versus among-individual variation, but at present, we do not have a broad consensus on the extent to which hormone levels are repeatable and what factors might influence repeatability. To address this knowledge gap, we conducted a comprehensive meta-analysis of hormone levels that included 1,132 estimates of repeatability from 368 studies across three fields of study: agriculture, ecology, and medicine. We assessed the influence of sex, age class, sample type, hormone family, type of hormone measure, assay type, number of subjects, number of samples per subject, and sampling interval on repeatability estimates. Overall mean repeatability was 0.58, but estimates differed substantially among study disciplines, being lowest in ecology (0.34), moderate in agriculture (0.52), and relatively high in medicine (0.68). In addition, repeatability decreased slightly as sampling interval increased, and also tended to be higher for peak hormone levels than baseline levels. Overall, hormone levels are moderately repeatable, suggesting that they can potentially be useful indicators of individual variation. However, estimates of repeatability are quite variable among fields, so caution should be used when relying on single samples to assess individual variation.