The ability of wildlife to endure the effects of high temperatures is increasingly
important for biodiversity conservation under climate change and spreading urbanization.
Organisms living in urban heat islands can have elevated heat tolerance via phenotypic
or transgenerational plasticity or microevolution. However, the prevalence and mechanisms
of such thermal adaptations are barely known in aquatic organisms. Furthermore, males
and females can differ in heat tolerance, which may lead to sex-biased mortality,
yet it is unknown how sex differences in thermal biology influence urban phenotypic
divergence. To address these knowledge gaps, we measured critical thermal maxima (CTmax)
in male and female agile frog (Rana dalmatina) tadpoles captured from warm urban ponds
and cool woodland ponds, and in a common-garden experiment where embryos collected
from both habitat types were raised in the laboratory. We found higher CTmax in urban-dwelling
tadpoles compared to their counterparts living in woodland ponds. This difference
was reversed in the common-garden experiment: tadpoles originating from urban ponds
had lower CTmax than tadpoles originating from woodland ponds. We found no effect
of sex on CTmax or its difference between habitats. These results demonstrate that
aquatic amphibian larvae can respond to the urban heat island effect with increased
heat tolerance similarly to other, mostly terrestrial taxa studied so far, and that
phenotypic plasticity may be the main driver of this response. Our findings also suggest
that heat-induced mortality may be independent of sex in tadpoles, but research is
needed in many more taxa to explore potentially sex-dependent urban thermal responses.
