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

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.

在多配制婚配系统中，雄性个体可能面临着提升繁殖成功率的性状与应对环境变化的需求之间的权衡取舍。为了获取配偶，雄性会投入资源以发育更大体型、演化更华丽的装饰结构，并开展高成本的行为活动，但这类投资可能与未来的环境条件不兼容。随着气候变化加剧，热胁迫可能会破坏那些曾长期带来交配机会的能量密集型生理通路。

我们以驼鹿（Alces alces）——一种对温度敏感的物种，其南部种群正处于耐热极限的边缘——为研究对象，探究与社会支配地位相关的性状如何与环境条件共同作用，进而影响雄性的交配行为与交配机会。我们推测，具备有利性状（如年龄、角的大小）的雄性会将更多资源投入繁殖活动，从而获得更高的交配机会；同时我们预期，高温会限制雄性的繁殖投入，且这一限制效应会随雄性年龄与角的尺寸增长而加剧。

我们量化了年龄介于1.5至11.5岁的雄性驼鹿的交配策略、繁殖投入与交配机会。研究中，我们采用隐马尔可夫模型（hidden Markov models）识别交配策略，借助加速度计数据量化运动投入，并以与雌性的近距离接触作为交配机会的替代指标。我们将上述交配动态建模为雄性年龄、角的尺寸与环境温度的函数。

研究发现，高温会放大年龄相关的繁殖运动时间分配与投入强度的差异。热胁迫对老年雄性的繁殖投入产生了不成比例的限制——而这类雄性恰恰是交配机会最高的群体。尽管高温改变了雄性的交配行为，但并未降低其交配机会。在所有温度条件下，壮年与老年雄性的交配机会均为最高，然而用于繁殖运动的时间与投入却随年龄增长而下降。

当前正在升高的秋季气温所代表的气候变化，可能会加剧不同年龄雄性间繁殖投入的差异，并抑制老年雄性的运动——而这类雄性通常是主要的繁殖者。交配行为与交配机会之间的脱节表明，运动并非一条可靠的繁殖通路，同时也凸显了节能策略的优势，尤其在对温度敏感的物种面临愈发严苛的环境条件时。本研究揭示了多配制婚配策略中运动投入的局限性，以及这一关键生活史阶段对环境变化的脆弱性。