Exposure assessment of metals in cave-dwelling bats: Worldwide relevance of local knowledge

Bat populations are decreasing worldwide, mainly due to anthropogenic influence, climate changes, urbanisation, agricultural intensification, and industrial exploitation of natural resources. Colonies have been displaced from caves and other natural roosts, leaving mines (including metal extraction mines) as key remaining roosts for cave-dwelling bats in many regions. Consequently, mines have become important for global conservation efforts. The persistence of metals in these environments might affect biota even after exploitation activities have ceased. This study investigates potential metal exposure in cave-dwelling bats roosting in abandoned mines. Non-invasive samples of fur, wing, and faeces from 140 individuals of four insectivorous cave-dwelling bat species (Rhinolophus ferrumequinum, Rhinolophus euryale, Rhinolophus hipposideros, and Miniopterus schreibersii) were collected in four abandoned mines in northern and central Portugal, frequently used as hibernation roosting sites. Soil, rock, and water samples were also collected in each mine to characterise the exposure source. Concentrations of 13 metals (As, Ag, Cd, Co, Cr, Cu, Mn, Ni, Pb, Se, Zn, Sn, and W) were measured by ICP-MS spectrophotometer. Metals were detected in the wings and fur of all bat species, confirming metal exposure. The highest concentrations were mainly found in the wing membranes. M. schreibersii presented the highest metal concentrations in fur. Differences in metal concentrations between species might be related to foraging behaviour and habitat use, linked to distinct anthropogenic activities. These results reinforce the importance of monitoring metal exposure in cave-dwelling bats roosting in abandoned mines, as this exposure may help to understand the decreases in bat populations and to support measures to prevent their decline.