Unraveling the Relative Importance of Oral and Dermal Contaminant Exposure in Reptiles: Insights from Studies Using the Western Fence Lizard (Sceloporus occidentalis)

Abstract

Despite widespread recognition of significant data deficiencies, reptiles remain a relatively understudied taxon in ecotoxicology. To conduct ecological risk assessments on reptiles frequently requires using surrogate taxa such as birds, but recent research suggests that reptiles have significantly different exposure profiles and toxicant sensitivity. We exposed western fence lizards, Sceloporus occidentalis, to the same quantities of three model chemicals via oral (gavage) and dermal (ventral skin application) exposure for either 24 or 48 hours. Three phthalate esters (di-methyl phthalate [DMP], di-iso-butyl phthalate [DIBP], and di-n-octyl phthalate [DNOP]) were chosen as model chemicals because they represent a gradient of lipophilicity but are otherwise structurally similar. Overall, the more lipophilic phthalates (DIBP and DNOP) were found to have higher concentrations in tissues than the less lipophilic DMP. Significant differences in tissue concentrations between DIBP and DNOP were tissue-dependent, suggesting that delivery to a site of action following exposure is not only a simple function of lipophilicity. In dermal treatments, DMP usually had fewer detections (except in ventral skin samples), suggesting that lipophilicity (log Kow>2) is a requirement for uptake across the skin. In general, tissue residues were greater in oral treatments than dermal treatments (significant in adipose and liver tissue), but differences were driven strongly by differences in DMP which did not appear to be absorbed well across skin. When differences in tissue residue concentrations between oral and dermal exposure did occur, the difference was not drastic. Taken together these results suggest that dermal exposure should be considered in risk assessments for reptilian receptors. Dermal exposure may be an especially important route for reptiles as their ectothermic physiology translates to lower energetic demands and dietary exposure compared to birds and mammals.

Scientific Opinion on the state of the science on pesticide risk assessment for amphibians and reptiles

“…The lack of reptile investigations limits confidence in understanding of the relative importance of oral and dermal exposures, but a low metabolic demand suggests that dermal exposure may be a larger portion of total exposure in reptiles as opposed to estimates for birds (e.g. Weir et al., , , Salice and Weir, ). Some routes may be eliminated from consideration; for example, it is likely that inhalation sources are of relatively low concern due to relatively low respiratory rate (Weir et al., ).…”

Tissue distribution, metabolism and hepatic tissue injury in Chinese lizards (Eremias argus) after a single oral administration of lambda-cyhalothrin

“…The contribution of environmental contaminants, especially pesticides, to reptile decline has not been well-addressed (Wagner et al, 2015). Several studies have focused on the toxic effects of metals in turtles (Meyer et al, 2014) or conducted ecological risk assessment of industry chemicals on lizards (Weir et al, 2014). As little information on reptilian ecotoxicology is available, birds are often used, perhaps inappropriately, as surrogates in risk assessment (Weir et al, 2015).…”

Direct and indirect effects of petroleum production activities on the western fence lizard (Sceloporus occidentalis) as a surrogate for the dunes sagebrush lizard (Sceloporus arenicolus)

“…Therefore, a before‐sprint speed was less important, especially when the lizards were tested 2 d following exposure. Two days has been shown to be an adequate time for contaminants to be absorbed by lizards after dermal exposure . Following dosing of herbicide formulations, lizards were held for 48 h (without a basking lamp) to provide an opportunity for effects to manifest.…”