The influence of operational sex ratio on the mating behaviour of reindeer (Rangifer tarandus L.) during an experimental rut: the effect of male age

The operational sex ratio (OSR), the ratio of sexually active males to sexually receptive females, is one of the main measures used to predict the intensity and direction of mating competition, influencing the opportunity for sexual selection. Here we conducted the first experimental study to investigate how OSR and male age impacts the intensity of mating competition and the male activity budget in a large mammalian species, reindeer (Rangifer tarandus), under semi-natural conditions. We manipulated OSR on two levels in two enclosures, a female biased treatment (3♂:6♀ = OSR 0.5) and a sex balanced treatment (3♂:3♀ = OSR 1), over two years with males from two age groups (1.5 or 2.5-years-old), and with females of various ages. We found some support for prevailing OSR theory, notably with female intrasexual competition occurring at lower frequencies in OSR = 1. An emerging trend was noted for male intrasexual competition, with higher frequencies of occurrence in OSR = 1 for our 2.5-year-old males. Courtship behaviour was also found to occur at higher frequencies of occurrence among the 2.5-year-old males, however, there was no effect of OSR. Regarding the male activity budget, we found that 1.5-year-old males followed a more income breeding strategy, with no difference in foraging behaviour between OSR treatments. In contrast, our 2.5-year-old males had a significantly lower frequency of foraging behaviour in OSR = 1, presumably resulting from a higher occurrence of intrasexual competition. Vigilance behaviour was also found to occur at higher frequencies in OSR=1 for the 2.5-year-old males, likely due to females being more limiting. Our results suggest that age is an important factor with respect to how male reindeer respond to differing OSR values. Future research aiming to expand the generalizability of experimental OSR theory to a wider range of taxa and ecological conditions need to consider other factors that will impact competition for mates, such as prime reproductive age