Thermal conditions alter the mating behaviour of males in a polygynous system

Abstract Males in polygynous systems may be facing a trade‐off between the traits that enhance reproductive success and the need to cope with environmental change. To secure mates, males invest into large bodies, lavish ornaments and costly activities, but these investments may be incompatible with future environments. As climatic change intensifies, thermal stressors could be disrupting the energy‐intensive pathways that historically have yielded mating opportunity. We evaluated how traits associated with social dominance interacted with environmental conditions to shape mating behaviour and opportunity in moose ( Alces alces ), a heat‐sensitive species in which southern populations live at the edge of their thermal tolerance. We anticipated that males with favourable characteristics (e.g. age, weaponry) would allocate more to reproduction, resulting in increased mating opportunity. We expected that warm temperatures would limit reproductive effort, especially as age and weapon size increased. We quantified mating tactics, effort, and opportunity for male moose ranging in age from 1.5 to 11.5 years. We used hidden Markov models to detect mating tactics, accelerometer data to quantify movement effort, and proximity to females as a proxy for mating opportunity. We modelled these mating dynamics as a function of age, weapon size, and ambient temperature. Warm temperatures exaggerated age‐related differences in time and effort allocated to reproductive movement. Heat disproportionately limited reproductive effort in old males, the ages that also had the greatest mating opportunity. Even though warm temperatures altered mating behaviour, they did not reduce mating opportunity. Across temperatures, mating opportunity was highest in prime‐age and old males, yet time and effort devoted to reproductive movement decreased with age. Climate change, which is increasing autumn temperatures, may increase variation in reproductive effort across ages and depress the movement of old males, who are typically the primary breeders. The discrepancy between behaviour and opportunity suggests that movement is not a reliable pathway to reproduction and emphasizes the advantages of energy‐saving strategies, especially as environments become more taxing for heat‐sensitive species. We reveal the limitations of movement effort in polygynous mating strategies and the susceptibility of this critical life history stage to environmental change. Read the free Plain Language Summary for this article on the Journal blog.