Thermal constraints on microhabitat selection and mating opportunities

•Fiddler crabs often live in hot tropical environments.•Males can overcome thermal constraints through their large claw and their burrow.•The claw acts as heat sink and the burrow as cool refuge.•Male claw size explained 10% of body temperature.•Sexually selected traits, including ornaments, can also serve in thermoregulation.

Hot tropical environments constrain ectotherm mating opportunities when mate selection occurs on the surface. Thus, microhabitats and refugia can become a qualitative trait in mate selection. In fiddler crabs, the enlarged claw of males can act as a heat sink, which becomes advantageous when surface temperatures reach 50 °C during the day and crabs are actively seeking to mate. Uca mjoebergi females prefer male burrows found in the shade; therefore, we investigated the thermal constraints imposed on males and females in shaded and unshaded habitats. Crab surface activity decreased and body temperature increased as the day progressed, with more crabs active in shaded than sunny microhabitats. Body temperature was lower in male crabs found in burrows relative to crabs on the surface. Male claw size explained 10% of body temperature. Our results add further support to the hypothesis that thermal constraints imposed on males can be overcome by the large claw acting as a heat sink and the burrow acting as a refuge from heat. Classic sexually selected traits, including ornaments and behaviours, can have a secondary purpose in thermoregulation.