1 INTRODUCTION
The impact of ambient temperature on animal physiology , behavior , and life history traits , is well-established. However, the underlying mechanisms of how ambient temperature influences sexual selection, which is a key evolutionary force shaping reproductive traits and behaviors , remain unclear . The cost-benefit balance of sexual selection could be influenced by ambient temperature, which can impact the growth and maintenance of sexual weapons or ornaments , alter intrasexual competition and intersexual selection , influence the variance in reproductive success , and even impact mating systems . Furthermore, the cost-benefit balance of bearing ornamentation may be more significant than that of possessing weapons, as ornamentations often have limited direct benefits. Thus, understanding how ambient temperature influences preference for secondary sexually selected traits could have significant implications for understanding of animal behavior evolution and species adaptation to changing environmental conditions.
Due to the restricted resource expenditure and predation risks, male ornament characteristics are known to be costly to develop and maintain . Reduced sexual dimorphism has been linked to lower ambient temperatures, which may restrict the aesthetic expression of male ornamentation . Additionally, it has been demonstrated that the trade-off between immunological function and plumage expression for carotenoid-based decorations acts as a sexual cue for the quality of a particular bird . Additionally, according to experimental data, male zebra finches (Taeniopygia guttata ) tend to circulate more carotenoids in cooler climates rather than expressing them as carotenoid-based decorations . This points to a potential problem with the use of carotenoid-based decorations as accurate indicators of an individual’s character in colder climates.
Ambient temperature can impact animal courtship behavior in addition to influencing the display of sexual characteristics. Several studies have shown that male courting efforts are positively correlated with temperature . This suggests that males with low resource investment engage in intensive courting behavior. This might be because they have more energy to devote to mating displays or are more driven to locate partners. Additionally, ectothermic and endothermic animals’ metabolism and activity levels are greatly influenced by ambient temperature . These factors can also affect how much time these animals devote to mating and intrasexual competition .
Furthermore, ambient temperature can affect a female’s post-copulatory mate preference and even a species’ mating system. Parental care efforts needed significant inputs by patents as the environmental conditions became harsher, requiring longer incubation durations and intensive nest provisioning . Females in colder environments may be unable to compensate for these shortages, and instead pursue less-ornamented males with higher levels of direct parental investment because males with extravagant ornaments often provide limited parental care . Additionally, the increased necessity for parental care by both parents in colder environments may promote stable pair-bond and thus the evolution of biparental care. This transition to monogamy can be viewed as an adaptive strategy to better manage balanced parental investment and assure the survival of offspring in challenging environmental circumstances.
Shorebirds provide an ideal system within which to test the impacts of ambient temperature on mate choice. This is because shorebirds can be found breeding in a wide variety of wetland habitats across large temperature gradients, even on the intraspecific level. Specifically, we used four Charadrius plover populations from the Kentish plover (C. alexandrinus ) species complex which breed in locations with significant changes in ambient temperature throughout the mating season (Figure 1 and Song et al. 2020). Our study included three populations of Kentish plovers (C. alexandrinussensu stricto) and one population of white-faced plovers (C. dealbatus ), a former subspecies ofCharadrius alexandrinus that has recently been proven to be a full species that diverged from the former less than 600 000 years ago .
The goal of this study was to look at the effect of ambient temperature on male ornamentation, female mate choice, and parental investment across diverse populations. We specifically investigated if male ornamentation expression and its relationship to body size change with ambient temperature. Furthermore, we investigated how female mate choice at different temperatures responds to male ornamentation by assessing the connection between morphometric and ornament characteristics of males and females in mating pairings. Finally, we investigated whether female incubation investment is related to male ornamentation and if it varies with temperature. Our prediction is that under colder conditions, the trade-off between color expression and immune function on melanin and/or carotenoid may lead to a lower correlation between ornament and body size. Females may also prefer larger males with direct parental care over ornamented males with lower parental care, due to the increased need for parental investment in colder environments.