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.