Butterfly Female Proportion Increases in the Understorey During the Rainy Season in a Tropical Dry Forest

Vertical stratification is common among tropical forest butterflies. While the phenomenon has been broadly studied with respect to taxonomy and morphology, less is known about how seasons affect vertical distribution. Males and females often differ in morphology, reflecting divergent priorities such as mate location versus oviposition. No study has evaluated how seasons affect the vertical stratification of butterfly sexes, which is key to community dynamics. Here, we describe how the sex ratio of fruit-feeding butterflies varies between strata and seasons using bait trap data from a one-year study in a tropical dry forest in coastal Ecuador. We report a consistent seasonal shift in sex ratio across vertical strata for the ten most abundant species (89% of records). While the sex ratio was similar in the canopy and understory in the dry season, females became relatively more abundant in the understory and males relatively more abundant in the canopy during the rainy season. Firstly, new leaf flushes in young plants may attract females to the understory to search for oviposition sites. Secondly, males may spend more time in mate-location activities, in particular patrolling the forest canopy. These results highlight the complexity of drivers influencing vertical distribution in tropical dry forests.