3.1 Mercury concentrations in Brandt's hedgehogs
Mercury levels in organs and spines of the Brandt's hedgehogs [median; mean ± SD] ranged from 6 to 270 ng/g dw [156; 150 ± 65 ng/g dw] in kidneys, from 2 to 264 ng/g dw [47; 66 ± 61 ng/g dw] in liver, from 3 to 108 ng/g dw [47; 44 ± 26 ng/g dw] in muscles, and from 1 to 94 ng/g dw [20; 27 ± 20 ng/g dw] in spines (Table 1). Levels of mercury measured in the kidneys were significantly higher (p < 0.05) than in other tissues and the ratio between the concentrations of Hg in liver and kidneys was < 1 (namely 0.3), indicating the absence of acute poisoning effects according to Delbeke et al., (1984).
A previous study has shown that mercury concentrations in bear hair samples above 6,000 ng/g dw would likely cause observed subclinical neurological effects in the animals (Dietz et al. 2011). Even more so, such neurological effects have been noticed also in mink, when the concentrations of mercury in the hair of this animal were measured up to 30,000 ng/g dw (Basu et al. 2007). According to previous studies, a mercury concentration of 1.1 mg/g in liver and kidneys is considered a threshold level for serious health effects in wild mammals (Eisler 1987), while levels of mercury up to 125 mg/kg dw in kidney tissues of carnivorous mammals were showed to cause fatal poisoning (Beyer and Meador 2011). In addition, 30 mg/g Hg in mammalian liver and kidney tissues is considered as an intoxication threshold, with levels up to 69 mg/g reported in the kidneys of wild and laboratory mammals whose deaths was attributed to mercury poisoning (Wren 1986; Lord et al. 2002; Rezayi et al. 2011). Finally, the U.S. EPA set the lowest guideline value for mercury in human hair at 1000 ng/g dw (Dietz et al. 2011). The concentrations of Hg measured in the organs and spines of the Brandt's hedgehog specimens analyzed in this study were considerably lower than all above-mentioned values, suggesting the absence of toxic effects for the considered wildlife.
The mean Hg levels in the liver of Brandt's hedgehogs (66 ng/g dw or 198 ng/g ww) were generally higher than the average mercury levels measured in liver tissues from the European hedgehog (Erinaceus europaeus) (60 ng/g ww), fox (Vulpes vulpes) (30 ng/g ww), porcupine (Hystrix cristata) (10 ng/g ww), stone marten (Martes foina) (110 ng/g ww), and badger (Meles meles) (180 ng/g ww) collected from the Italian Province of Pesaro and Urbino (Alleva et al. 2006), and higher than the multi-organ and hair Hg concentrations in Russian wild boars (Sus scrofa) (4–80 ng/g ww) (Eltsova and Ivanova 2021) (Table 2). Average Hg concentrations in the organs and spines of the Brandt's hedgehogs were instead comparable to or lower than those measured in tissues and hair of bank voles (Clethrionomys glareolus) (average concentrations ranging between 150 and 900 ng/g ww) and wood mice (Apodemus sylvaticus) (average concentrations ranging between 230 and 980 ng/g ww) collected in the UK (Bull et al. 1977), and golden jackal (Canis aureus) (up to 180 ng/g dw) from the region of Mazandaran, Iran (Malvandi et al. 2010) (Table 2). Finally, average Hg levels in the tissues and spines of the Brandt's hedgehogs were lower than those measured in raccoons (Procyon lotor) (up to 2990 ng/g dw) in the Polish Warta Mouth National Park near to mercury emission sources (Lanocha et al. 2014), Arctic foxes (Vulpes lagopus) (up to 8240 ng/g ww) from inland and coastal regions of Iceland (Treu et al. 2018), American martens (Martes americana) (up to 1228 ng/g dw) from Michigan, USA (Witt et al. 2020), and northern short-tailed shrew (Blarina brevicauda) (up to 39,000 ng/g ww) from Tennessee, USA (Talmage and Walton 1993) (Table 2). The overall mercury contamination of the Brandt's hedgehogs collected from the Sistan region of Iran resulted generally lower than of animals collected near known contamination sources, but nonetheless higher than levels in animals collected where no sources of Hg contamination have been reported. This suggests that the habitat of the Iranian hedgehogs is affected by mercury presence, likely deriving from the application of chemical fertilizers and pesticides.
3.2. Ecological factors affecting mercury levels
Several research studies showed that mercury levels in animal tissues and organs are potentially influenced by physiological and ecological factors, such as sex, age, size, feeding strategy, and habitat (Malvandi et al. 2010; Bilandžić et al. 2010; Zarrintab and Mirzaei 2017; Treu et al. 2018; Eyrikh et al. 2020).
In this study, sex showed a significant (p < 0.05) impact on mercury levels in the analyzed kidneys and muscle tissues (Table 1), suggesting that the mercury burden in the body of female hedgehogs might be reduced by transfer to the fetus through the placenta and to offspring during lactation, as widely described for other mammals (Yoshida et al. 1994; Frodello et al. 2000). Previous research also indicated that the levels of Hg in an organism are expected to increase with age and size, mostly due to the slower removal of this metal from the body and/or the longer time of exposure in older individuals (Braune et al. 2015). Also, in this study, the levels of Hg in selected hedgehog organs correlated with weight, length, and age. A significant positive correlation was observed between the levels of mercury in liver and kidney tissues and weight (r = 0.460, p < 0.05, r = 0.295, p < 0.05, respectively), between the levels of mercury in kidneys, muscle and spines with length (r = 0.471, p < 0.01; r = 0.291, p < 0.05; r = 0.342, p < 0.05, respectively), and between the levels of mercury in kidneys, liver and spines with age of the animals (r = 0.530, p < 0.01; r = 0.334, p < 0.05; r = 0.362, p < 0.01, respectively) (Table 3). As expected, the age of the animals positively correlated with their weight and length (p < 0.01), highlighting the positive relation between age and mercury accumulation in the animal tissues (Ben-David et al. 2001; Gerstenberger et al. 2006). The average age of hedgehogs analyzed in this study was 2.4 years, about one third of this species life expectancy, likely implying that mercury had enough time to accumulate in the individuals’ internal tissues.
To investigate if the habitat of the animals could also have influenced their contamination, the levels of mercury in organs and spines of Brandt's hedgehog specimens collected from an agricultural ecotype (n = 25) were compared with those from a forestry ecotype (n = 25). Median Hg levels in kidneys of hedgehogs from the agricultural ecotype (190 ng/g dw) were significantly higher (p < 0.05) than those from the forestry ecotype (126 ng/g dw) (Table 1), while no significant differences were observed comparing the Hg concentrations in the other tissues. The overall higher mercury levels of Brandt's hedgehogs collected from the agricultural ecotype could be likely associated with human presence in this area and the use of mercury in chemical fertilizers and pesticides (Benhaiem et al. 2008; Demesko et al. 2019). To date, urbanization and human-related land alteration (e.g., intensive agricultural activities) have been often associated with increasing metal contamination levels, including As, Cd, Cu, Pb, and Hg, in a wide variety of wildlife (Orlowski et al. 2008; Bilandžić et al. 2010; Flache et al. 2015). Our results strengthen the hypothesis that a higher bioaccumulation of harmful substances of anthropogenic origin in wild animal populations can be driven by the proximity of human settlements (Demesko et al. 2019; Dahmardeh Behrooz et al. 2020).
3.3. Correlations between mercury levels in different tissues
Significant correlations were observed between Hg concentrations in the analyzed hedgehog tissues (Table 3). Hg levels in liver tissues were significantly correlated with those in kidneys (r = 0.519, p < 0.01), followed by spines with kidneys (r = 0.337, p < 0.01) and muscles (r = 0.309, p < 0.05), respectively. This outcome agrees with the results of other studies in mammals, suggesting that the levels of mercury measured in hair and spines reflect those in organs and soft tissues (Ikemoto et al. 2004; Dainowski et al. 2015; Treu et al. 2018), and supports the use of non-destructive tissues for the monitoring of mercury environmental pollution (Dahmardeh Behrooz and Poma 2020; Dahmardeh Behrooz et al. 2020).
The stronger correlation found between the levels of mercury in liver and kidney (r = 0.519, p < 0.01) rather than between spines and organs/tissues could be mostly attributed to the active Hg metabolism in these two organs which are directly connected through the bloodstream (Treu et al., 2018; Boening, 2000). The reabsorption of Hg via enterohepatic recirculation in the animal body, as mentioned by Boening (2000), can thus explain the strong correlation observed between mercury levels in liver and kidney of the Brandt’s hedgehog. On the other hand, the absence of a significant correlation between spine and liver Hg levels could be due to the role played by factors such as age, sex, sampling location and the species-specific detoxification capacity of the Brandt’s hedgehog. Finally, a possible residual external contamination with Hg on animal hair and spines, even after washing steps, has been suggested as a possible additional source of contamination variability, potentially affecting the body-burden relationships (Morton et al. 2002; Li et al. 2008).
Since the specific kinetics of mercury accumulation and detoxification in organs and hair in different animal species are not fully understood yet, there is the need to further investigate Hg complex metabolic transformation processes, especially in terrestrial mammals. On the other hand, the strong correlation between the levels of mercury in the liver and kidneys and between hedgehog spines and kidney and muscle tissues suggests that Brandt's hedgehog spines can be a valuable non-invasive tool for environmental measurement and monitoring of Hg environmental pollution, but caution is advised when translating the outcomes deriving from this study to other species.