Males in polygynous systems may be facing a tradeoff 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 behavior 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 favorable 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 modeled 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 behavior, 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 behavior 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.

Animal Capture and Handling

We chemically immobilized male and female adult moose (≥1 year) via helicopter (Levine et al. 2022). For the duration of the study, we sought to maintain 15 collared animals of each sex (approximately 30% of the population). We determined age via cementum annuli (Matson’s Laboratory, Milltown, MT) using the right, incisiform canine (Swift et al., 2002). We fitted all moose with GPS collars (VERTEX PLUS; Vectronic Aerospace GmBH). All GPS collars had hourly fix rates and three orthogonal accelerometers collecting 5-min averaged activity data. In autumns of 2020 to 2022, we classified antler size of GPS-collared male moose through field observation and photography.

Movement Metrics

To quantify mating tactics (mate-searching and female-defense), we identified likely behaviors with a three-state hidden Markov model (momentuHMM package) parameterized with the step length and turn angle of movement. Our model classified hourly GPS locations as either encamped, tortuous, or exploratory states. To further specify and validate mating behaviors, we visually inspected which of the three states aligned with known interactions between GPS-collared male and female moose (i.e., female-defense). The encamped and tortuous states were both associated with bouts of female interaction. We therefore manually validated a bedded state (<20m step length; Verzuh et al., 2021) as the distance of typical GPS error for a stationary collar, grouped the encamped and tortuous states into a single female-defense state, and designated the remaining exploratory movement as mate-searching behavior. To quantify mating opportunity (proximity to females), we summed the number of hourly fixes in which a male was ≤50 meters from a collared female each day of the focal period (wildlifeDI package).

Antler Classification

Scoring systems for moose are different than other cervids because the palmate structure creates more diverse antler morphologies than the tine-based scoring of cervine antlers (Wright & Nesbitt, 2003). We used existing methods of classification derived for larger subspecies, Boone and Crockett standards, and harvest records for Shiras moose to inform our scoring system. The scoring system (1–5) was based on antler form, palm structure, standard size approximations (spread, height, and width), and qualitative estimates of size relative to the head and ears. We estimated measurements when possible, using the length of the ear (20cm) and distance between the pedicles (17.5cm).

Environmental Data

We used ambient temperatures from the center of the study area (43°52'N, 109°19'W; Kirwin Snotel 560) to calculate daily maximum temperature.