The use of glucocorticoid hormones or leucocyte profiles to measure stress in vertebrates: What’s the difference?

Abstract

Measuring stress in animals is important in many ecological, zoological and veterinary research settings. A common method is to measure plasma levels of glucocorticoid hormones (cortisol, corticosterone, hereafter CORT). Over the past decade, an alternative method has become widely popular: assessing leucocyte profiles; in other words, the heterophil‐to‐lymphocyte ratio (H/L) in birds and reptiles or the neutrophil‐to‐lymphocyte ratio (N/L) in mammals, amphibians and fish. Recent studies have shown that although both indices reliably change after acute stress, they are not always correlated at baseline, and thus are not interchangeable. This lack of correlation has led to some confusion about which of these two measures is “better”; in other words, more biologically relevant and a truer readout of stress. In this review, we first document the dramatic expansion of the use of leucocyte profiles to assess stress over the past decade. We then review published evidence that may explain the discrepancies between leucocyte profiles and plasma CORT. We argue that these discrepancies stem from the nature of the stress reaction and the role of corticosterone (or cortisol) versus leucocytes during the stress response. The differing roles of the two responses translate to differences in the timing of each, which is important for researchers studying either acute or chronic stress. Although plasma glucocorticoids rise within minutes of the onset of acute stress and return to baseline within 1 or 2 hr after the stressor passes, H/L or N/L ratios remain low for at least 30–60 min or sometimes more. The temporal nature of these metrics differs also in the context of long‐term chronic stress. The glucocorticoid response wanes in repeatedly stressed animals, or those exposed to chronic environmental stress, which can often (but not always) lead to low baseline levels and more typically, an attenuated acute stress response. Meanwhile, H/L or N/L ratios appear to remain high under the same conditions. Understanding the temporal differences in the responses should help researchers decide which metric is the better method in their particular study, or whether both would be useful for elucidating different elements of the stress response.

Effects of early nutritional stress on physiology, life-histories and their trade-offs in a model ectothermic vertebrate

“…H:L ratios have been shown to positively correlate with various environmental stressors (increases in H:L with increasing magnitude of a stressor; reviewed by Davis et al, 2008) and with levels of circulating glucocorticoids (Gangloff al., 2017b;Goessling et al, 2015). However, levels of circulating CORT and H:L ratios are not always correlated within individuals (Davis and Maney, 2018;Goessling et al, 2015;Sparkman et al, 2014). We found no effect of early-life diet on H:L ratios and no trade-off with increasing growth rates in low-diet animals.…”

The method matters: The effect of handling time on cortisol level and blood parameters in wild cats

“…Literature data support our results for far‐east wild cats concerning stability of the N/L ratio within about 1 hr. Thus, based on analysis of literature data we also conclude that the N/L ratio does not vary if the samples are collected within 1 hr after the capture of the animal, so this valuable index may be used for estimation of long‐term stress (Davis & Maney, ) and physiological status (Thrall et al., ) in cats as it is used in other mammals and birds. At the same time, all other tested parameters should be used with caution.…”

“…Blood profile is affected by stress too (Dhabhar, Miller, McEwen, & Spencer, , ), so use of the total WBC number is problematic. The values of this parameter can increase or decrease depending on animal species (see review in Davis & Maney, ; Davis, Maney, & Maerz, ). In particular, an elevated level of neutrophils combined with the low level of lymphocytes in blood observed in some cat species (Kocan et al., ; Serieys et al., ; Weaver & Johnson, ), has been related to the release of glucocorticoids (Mayеr & Harvеy, ; Thrall, Weiser, Allison, & Campbell, ).…”

“…At present, for 17 of the 38 wild cat species blood profiles have been obtained and described in scientific literature (Akuzawa, Mochizuki, & Yasuda, 1987;Brown, Lappin, Brown, Munkhtsog, & Swanson, 2005;Caro et al, 1987;Dunbar, Nol, & Linda, 1997;Erasmus, 2008;Kocan, Blouin, & Glenn, 1985;Marco, Martinez, problematic. The values of this parameter can increase or decrease depending on animal species (see review in Davis & Maney, 2018;Davis, Maney, & Maerz, 2008). In particular, an elevated level of neutrophils combined with the low level of lymphocytes in blood observed in some cat species (Kocan et al, 1985;Serieys et al, 2013;Weaver & Johnson, 1995), has been related to the release of glucocorticoids (Mayеr & Harvеy, 2007;Thrall, Weiser, Allison, & Campbell, 2012).…”

Chronic stress in superb fairy‐wrens occupying remnant woodlands: Are noisy miners to blame?

“…The heterophil‐to‐lymphocyte (H:L) ratio is a reliable measure used to compare levels of chronic stress in birds (reviewed by Davis et al . , ; Goessling et al . ).…”