What are you actually measuring? A review of techniques that integrate the stress response on distinct time‐scales

Abstract

1. The field of stress physiology has rapidly expanded, particularly in those fields interested in identifying chronic stress in wild animals. Despite this expansion, stress remains difficult to assess and understand, due in large part to the temporal complexities of common stress measurement techniques. 2. While the stress response happens on a short timescale , chronic stress results over longer timescales. Therefore, the temporal dynamics of techniques used to assess 'stress' need to be fully understood in order to be applied correctly. 3. In this review, we provide information on 37 physiological and behavioural metrics that are commonly used to measure stress, especially in wild free-living vertebrates, with a particular focus on which timescale they integrate stress. 4. We organize these metrics into seven broad categories based on which physiological system they are most closely associated with (glucocorticoids, sympathetic/ parasympathetic nervous system, immune, metabolic, cellular/molecular, tissue development and behaviour). 5. We conclude by summarizing which kind of biological questions and variation each technique is most suitable for. 6. This review will enable researchers to understand the temporal dynamics of stress measurement techniques for better design of future studies.

“The stressed bird in the hand”: Influence of sampling design on the physiological stress response in a free-living songbird

“…Second, we found that these three physiological parameters showed different kinetic dynamics over the time course of the short-term stress response, reflecting different response latencies (Fig. 2) (Gormally and Romero, 2020). CORT levels increased markedly in most individuals, but the timing of the peak response underlies substantial between-individuals variation (Fig.…”

“…This relationship was previously demonstrated in CORT supplementation experiments, but also capture and restraint stress was shown to be sufficient to effect similar responses as is corroborated by a growing number of studies on vertebrates (Cīrule et al, 2012; Davis et al, 2008; Lentfer et al, 2015). The H:L ratio, however, is considered to mainly reflect long-term, rather than an immediate, short-term physiological stress response (Gormally and Romero, 2020; Maxwell, 1993; O’Dell et al, 2014) and there is a strong indication that an increase after capture becomes significant only 30 to 60 min post-capture and in a species-specific manner (Davis, 2005; Davis and Maney, 2018). Remarkably, our data show an unexpectedly fast decrease in the H:L ratio at 15 min after capture, followed by rapid recovery to baseline levels 30 min post-capture.…”

“…The number of studies on LCC in passerines is limited (Huber et al, 2020; Huber et al, 2017), and the observed patterns might be species-specific and underlie seasonal variation. Furthermore, Gormally et al (2019) have demonstrated that HPA axis regulation, immune function, antioxidant levels, DNA damage and stress-related alterations in behaviour change on distinct timescales in response to repeated stressors or differences in recovery time between the stress events (Gormally and Romero, 2020). We would, therefore, like to emphasise that we only recorded data over a short time frame and encourage follow-up studies to measure the recovery from capture and handling induced stress over a more extended period.…”

“…On the contrary, studies which applied different stress indicators, such as the change in total white blood cells (WBC), show that additional handling and blood sampling only has negligible effects (Bonnet et al, 2020; Davis, 2005). A possible explanation for this discrepancy may be that the different physiological stress systems and their measurable endpoints vary in their sensitivity to stressors and the timescale each parameter integrates stress (Gormally and Romero, 2020). These differences potentially impede the interpretation of data, aiming to reveal the effects of multiple blood sampling and handling on the individual stress response and the potential direct effects of sampling (e.g.…”

“…We, therefore, tested different blood sampling regimes and their impact on the outcome of short-term stress in a free-living passerine species, the great tit ( Parus major ). We used the well-established “capture-handling stress” protocol (Wingfield et al, 1982), as the underlying framework and included three stress metrics representing different physiological systems with varying response latencies towards handling-induced stress (Davis and Maney, 2018; Gormally and Romero, 2020; Johnstone et al, 2012). The basic protocol is designed to measure the activation and reactivity of the Hypothalamic-Pituitary-Adrenal- (HPA-) axis and the concomitant changes in circulating CORT levels in response to a defined stressor (i.e.…”