Exposure to toxic trace metals is negatively associated with testis mass in wild male roe deer

Although trace metal toxicity has been extensively studied in laboratory settings, its long-term effects on wildlife are poorly understood. Most wild animals are likely to be exposed to low doses of trace metals throughout their lives; however, whether such chronic exposure affects fitness-related traits in natural populations remains to be investigated. In this study, we tested the relationships between trace metal burden and sex hormones, primary and secondary sexual traits in wild male roe deer. Using samples collected from 71 hunted males, we measured the hair and liver concentrations of various toxic trace metals, faecal testosterone metabolites (FTM), testis mass and antler length. We focused on cadmium (Cd), cobalt (Co), nickel (Ni) and lead (Pb) in roe deer tissues. Using linear (mixed effects) models, we found that Cd and, to a lesser extent, Co concentrations in the liver, but not in hair, were negatively associated with absolute testicular mass, a proxy of sperm production. However, metal burden, measured in either the liver or hair, was not related to antler length or FTM levels, despite their respective roles in male competitiveness for access to females and the underlying adjustments for seasonal breeding. Hence, our findings highlight the consequences of exposure to Cd and Co on male testicular function in a wild ungulate. Any alteration in male competitiveness for fertilization is expected to have lasting fitness consequences, which could subsequently alter population dynamics.