4.2 Phenotypic traits association between parents varies with ambient temperatures
Our findings suggest that the correlation between female body size and male ornamental coloration is influenced by temperature, indicating that temperature is an important factor in the mate choice of plovers. It is unlikely that this pattern is caused by the timing of when larger and/or more colorful males and females arrive, as we controlled for breeding start time in our analysis. Additionally, Kentish plovers typically have multiple broods during a single breeding season and often change mates between broods , making assortative mating driven by arrival time less likely to have a significant effect.
The condition-dependent mate preference hypothesis suggests that females in good condition are more selective in their choice of mates and exert stronger preferences . In addition, higher quality males tend to mate with higher quality females, and larger body size often predicts female fecundity . For instance, in a previous study, larger females of both Kentish plover and white-faced plover had larger egg sizes , which suggests a positive effect on offspring survival rate and fitness . Our findings show that the condition-dependent mate preference hypothesis applies only to warmer environments, as evidenced by the positive correlation between female body size and male ornamentation. However, this pattern was not observed in colder environments. In such conditions, male plover ornaments exhibited a trade-off with body mass, indicating that ornamentation is not a priority trait for self-maintenance. This deceptive signal of self-condition may decrease the attractiveness of males to fecund females, leading to a loss of ornament quality and potential mate status. Therefore, in colder environments, the reduced occurrence of ornamental traits on males’ quality can potentially limit the impact of local sexual selection, subsequently affecting mating systems. This could be suggestive a global pattern that sexual dimorphism and polygamy are higher in birds in tropical environments .
4.3 Post-copulatory investment
Males with showy ornamentation typically invest less in direct paternal care for the nest in order to pursue other mating opportunities. This explains why females usually prefer males with elaborate ornaments for their indirect genetic benefits rather than direct investment benefits. However, our results indicate that male incubation investment is not solely determined by its ornament quality, but rather varies depending on the interaction with ambient temperature. The decrease in total incubation investment with temperature suggests that in warmer environments, the incubation investment requirement is lower compared to colder environments. In harsh environments, male ornamentation provides fewer fitness benefits to females , and direct benefits such as male parental care are more valued by females. Thus, the negative correlation between male incubation investment and ornamentation only occurs in warmer environments where male ornamentation remains attractive to females, driving females to compensate and provide more care.
Previous studies have revealed that female investment in reproduction may be influenced by the attractiveness of their mate . Our path analysis shows that in warmer environments, females that mate with more ornamented males would increase their own incubation investment to compensate for males’ declining commitment in parental care. Furthermore, we discovered that females devoted more time in incubation at higher temperatures, but males invested more in cooler temperatures. This shows that in colder climates, male investment in incubation may be more significant than ornamentation. In harsh environments, parental care is crucial for offspring survival, which may lead to biparental care , showing a greater necessity for direct investment in such situations. Overall, our path analysis suggests that temperature influences male and female investment strategies in response to mating attractiveness and parental care demands.
The impact of environmental variables on mate choice has primarily been investigated in the context of the preference switch by females between sexual traits when their ability to sense each cue is compromised . Our study contributes to this understanding by revealing that female preference for indirect benefit signals, such as male ornamentation, may be influenced by the environment. These findings might provide light on how male sexually chosen traits are retained under female selection, a phenomenon known as the ’lek paradox’ . Previous studies have investigated how environmental heterogeneity can lead to variation in female mate choice, potentially explaining the lek paradox . However, they have primarily focused on mate choice intensity or thresholds for a specific male trait (reviewed by ). In contrast, our work demonstrates how abiotic variables such as ambient temperature can alter female preference for male ornaments. Our findings show that natural selection pressure constrains male ornamentation preferences, whereas females display a direct preference for ornamentation in stable habitats, such as those with warmer temperatures.