Staying cool: the importance of shade availability for tropical ectotherms

Understanding how organisms mitigate the impacts of climate warming is one of the biggest challenges facing modern-day biologists. For tropical ectotherms, staying cool is critical for avoiding thermal stress, so individuals that are able to maintain territories in cool microhabitats are likely to gain fitness advantages. This study evaluated the importance of shade availability in the tropical fiddler crab, Austruca mjoebergi , by investigating temperature variation and behavioral responses (distribution, activity level, and time budget) in sun-exposed and shaded microhabitats. We found that the daily temperatures in sun-exposed microhabitats were significantly higher than in shady microhabitats, and that more male crabs held territories in the shade. Shaded males were active on the sediment surface for longer periods each day, allowing more time for feeding and more opportunities to detect mate-searching females. The greater number of males resident in the shade, and shorter distances between male neighbors in the shade, suggested that there was less available space for individuals to move into the shaded microhabitats. The behavioral differences between sun- and shade-living residents are likely to have consequences for male fitness. We highlight the importance of shade in providing thermal refugia for tropical ectotherms. Significance statement The impact of climate warming on tropical organisms could be less dire if individuals are able to access shade. Shade can offer thermal refugia for tropical organisms that may have important consequences for both behavior and fitness. Relatively low and stable temperatures in shaded areas allow male fiddler crabs to remain on the surface for longer compared to sun-exposed males, allowing for longer feeding periods and more mating opportunities.