We review evidence for and against the use of erythrocyte indicators of health status and condition, parasite infection level and physiological stress in free-living vertebrates. The use of indicators that are measured directly from the blood, such as haemoglobin concentration, haematocrit and erythrocyte sedimentation rate, and parameters that are calculated from multiple measured metrics, such as mean cell volume, mean cell haemoglobin content or mean cell haemoglobin concentration is evaluated. The evidence for or against the use of any given metric is equivocal when the relevant research is considered in total, although there is sometimes strong support for using a particular metric in a particular taxon. Possibly the usefulness of these metrics is taxon, environment or condition specific. Alternatively, in an uncontrolled environment where multiple factors are influencing a metric, its response to environmental change will sometimes, but not always, be predictable. We suggest that (i) researchers should validate a metric's utility before use, (ii) multiple metrics should be used to construct an overall erythrocyte profile for an individual or population, (iii) there is a need for researchers to compile reference ranges for free-living species, and (iv) some metrics which are useful under controlled, clinical conditions may not have the same utility or applicability for free-living vertebrates. Erythrocyte metrics provide useful information about health and condition that can be meaningfully interpreted in free-living vertebrates, but their use requires careful forethought about confounding factors.
Although the vertebrate stress response is essential for survival, frequent or prolonged stress responses can result in chronic physiological stress, which is associated with a suite of conditions that can impair survivorship and reproductive output. Anthropogenic habitat fragmentation and degradation are potential stressors of free-living vertebrates, and in theory could result in chronic stress. To address this issue, we compared haematological indicators of stress and condition in agile antechinus (Antechinus agilis) populations in 30 forest fragments and 30 undisturbed, continuous forest sites (pseudofragments) in south-eastern Australia over 2 years. In peripheral blood, the total leucocyte count was lower and the neutrophil/lymphocyte ratio and percentage of eosinophils in the total leucocyte population was higher in fragment than pseudofragment populations, indicating that fragment populations were probably experiencing higher levels of stress hormone-mediated and/or parasite infection-related chronic physiological stress. The total erythrocyte count and haematocrit were higher and mean erythrocyte haemoglobin content was lower in fragment than pseudofragment populations. This suggests that fragment populations showed possible signs of regenerative anaemia, a syndrome associated with elevated hypothalamus-pituitary-adrenal axis mediated stress. However, mean erythrocyte volume was also lower in fragments, and red blood cell distribution width did not differ between the study populations, findings which were not consistent with this diagnosis. Whole blood and mean cell haemoglobin concentrations were similar in fragment and pseudofragment populations. We suggest that where anthropogenic activity results in habitat fragmentation and degradation, chronic stress could contribute to a decline in agile antechinus populations. The broader implication is that chronic stress could be both symptomatic of, and contributing to, decline of some vertebrate populations in anthropogenically fragmented and degraded habitats.
Body condition influences activities and processes affecting fitness. Erythrocyte variables, particularly haemoglobin concentration and haematocrit, are increasingly being used as avian condition indicators. However, some potentially confounding factors should be considered when adopting this approach and there are some conflicting findings concerning such variables' reliability as condition indicators. We examined whether whole blood haemoglobin concentration (Hb), haematocrit (Hct) and four other erythrocyte and one leukocyte variable were significantly associated with body mass and four commonly used body condition indices (BCI) in 12-day old Welcome Swallow Hirundo neoxena nestlings. Haematocrit and Hb were significantly associated with body mass and all four BCIs and had significant repeatabilities (0.426 and 0.433, respectively); total erythrocyte count, mean corpuscular volume, mean cell haemoglobin, mean cell haemoglobin concentration and total leukocyte count were not associated with mass or the BCIs, despite mostly having significant repeatabilities (0.129-0.433) and being reliable condition indicators in some other bird species. The lack of association between four of these other erythrocyte variables and nestling condition indicated that some caution was appropriate in interpreting the functional significance of the Hct/Hb-condition relationships. It seems advisable to measure a suite of erythrocyte variables when trying to determine whether the commonly measured Hct and Hb are associated with body condition in nestling birds.
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