Use of cast antlers to assess antler size variation in red deer populations: effects of mast seeding, climate and population features in Mediterranean environments

Resumen

Fundamental understanding of the factors influencing cervid antler size, development and investment has been traditionally drawn from harvest data. However, depending on the hunting tactic, harvest data may not represent a random sample of the population leading to possible inferential biases. Cast antlers may represent an alternative, cost-effective and non-invasive method. We used 4756 red deer ('Cervus elaphus' L.) cast antlers collected during a 10-year period to evaluate the relationship between annual antler gross score and three key environmental components that determine habitat quality and resource availability in Mediterranean systems: (1) population traits (density and male age structure), (2) acorn yield and (3) a proxy of plant productivity [Real Bioclimatic Index(RBI)]. Population traits and acorn yield were measured before antler formation (autumn/winter) whereas RBI was calculated before (autumn/winter) and during (spring) antler formation. Population traits explained the highest amount of variance in antler score, followed by acorn yield and spring RBI, while no effect was found for autumn/winter RBI. Antler gross score was negatively related to population density but positively associated with acorn yield, spring RBI and male age structure. Interestingly, a significant interaction between population traits and acorn yield suggests a disproportional effect of population traits on antler size during non-mast years (poor acorn crops), whereas no significant population effect was observed during mast years. Similarly, we found a positive effect of spring RBI on antlers when density was medium or low and/or age structure was balanced or older. These findings have important ecological implications in environments with high inter-annual resources variability where high population densities lead to strong intraspecific competition during years of low food availability (e.g. during non-mast years or drier springs), producing large antler size variation. Finally, although cast antlers reflect changes in environmental conditions we do not recommend their use unless reliable data on age structure is available.