El Ni&ntildeo, capture success, and wing fluctuating asymmetry in Drosophila sturtevanti Duda, 1927

Fluctuating asymmetry (FA) is widely used as a proxy for developmental instability, although reduced rather than increased FA, has occasionally been reported under particularly challenging conditions, possibly because environmental filtering removes less developmentally stable individuals. Here, we investigated temporal variation in wing shape asymmetry in males of Drosophila sturtevanti collected in 2022, 2024, and 2026 at the same sampling site, an urban Atlantic Forest fragment. Sampling in 2022 and 2026 were completed during single field expeditions in early April, whereas in 2024 repeated collections were required until mid-May because of markedly low capture success. We therefore examined whether this difference was associated with unusual environmental conditions and whether FA was higher in 2024, as expected under greater developmental perturbation. Daily temperature and relative humidity were examined from early March onward, encompassing approximately one month before adult emergence. The 2024 sampling period was warmer and drier than the corresponding periods in 2022 and 2026 and coincided with the final phase of the strong 2023-2024 El Ni&ntildeo event. Contrary to our prediction, FA was significantly lower in 2024. The magnitude of directional asymmetry remained similar among years, although the spatial pattern of the DA vector differed between 2024 and 2026. The coincidence of reduced capture success and reduced FA is consistent with the hypothesis that adverse conditions may impose demographic filtering, such that individuals expressing greater developmental instability are less likely to contribute to the sampled adult population.