Abstract
The leukocyte profile has the potential to be a reliable method to measure health conditions and stress in wild animals, but limitations occur because current knowledge on reference intervals is largely incomplete, especially because data come from studies on captive animals involving few individuals from single populations. Here we propose a general framework for achieving reliable leukocyte reference intervals, encompassing a set of internal and external factors, potentially affecting the leukogram. To do so, we present a systematic survey of the hematology of the common wall lizard, Podarcis muralis, involving 794 lizards from 54 populations over the whole geographic range of the species in Italy. Reference intervals for white blood cell (WBC) and leukocyte differential count were obtained by using linear mixed models in a Bayesian framework. The application of the procedure clearly showed that both internal (sex and size) and external (latitude and season) factors are a source of variation of leukocyte profile. Furthermore, the leukogram of common wall lizard has a strong variability among populations, which accounts for more than 50% of the whole variation. Consequently, some common assumptions used in studies on captive individuals are no longer supported in wild populations, namely, i) any group of individuals is a representative sample, ii) any population is representative of all others, iii) geographic clines do not occur over the species range, and iv) seasonal variation has limited effects. We encourage researchers aimed at the definition of leukocyte reference intervals for wild populations of reptiles to involve a large number of populations over a wide geographic range in ad hoc statistical models to disentangle local and geographic effects on leukocyte profile variation.
Highlights
The Reference intervals (RI) for white blood cell counts (WBC) as derived by the mode and the 95% prediction intervals of posterior distributions are sensibly asymmetric compared to the mean (Table 3)
RI is of primary importance, since the reliability of these values depends precisely on how they were obtained [5], and, as a cascade effect, the validity of all decisions that are taken from these intervals
We proposed a general method for defining RI for leukograms having a dual advantage: i) encompassing a set of internal and external factors, potentially affecting leukogram’s RI, and ii) assessing how much noise each of these factors causes to the RIs
Summary
Characterizing the physiological responses of wild animals to stressors, including humaninduced landscape changes, is an important question with deep implications for both animal. To be reliable for wild individuals, RI should be assessed by adopting general guidelines for the selection of reference individuals, which must embrace the variability among populations of the focal species [5] Without this information, disclosing if the cell count of a given individual is high or low compared to the leukocyte profile of healthy individuals is, impossible. The approach used to date in ecological and conservational research has been borrowed from veterinary research and has involved samples generally around 50–70 units, from a single locality, without replication (Table 1) Such a kind of approach can provide reliable data when dealing with pets or farm animals, which live in strictly controlled conditions that sensibly reduce all those sources of variation in leukocyte profile proper to natural conditions, such as habitat heterogeneity, climatic variability, food distribution, and competition with other species, just to name a few. The first attempt dates back to the work by Duguy [8, 9], whereas the first review on RI for leukocytes in reptiles is by Frye [10], who reported the leukocyte differential count for 24 species of reptiles
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