Adaptive plasticity in egg size helps butterflies to cope with desiccation.

Seasonality in many tropical regions exhibits unpredictable fluctuations in humidity, with extended dry conditions followed by intense rains. Insects often employ phenotypic plasticity in life-history traits, including egg size, to cope with these conditions. Egg size is a critical determinant of fitness. As a sessile life stage, eggs are particularly vulnerable to stressors such as desiccation. Plastic adjustments in egg size may therefore enhance offspring survival under variable humidity. However, whether insects can actively sense ambient humidity during oviposition and modify egg size remains unclear. We investigated this question in the tropical satyrine butterfly Mycalesis mineus by rearing individuals under low (60% RH) and high (85% RH) humidity conditions during both larval and adult stages. We found that adult oviposition humidity, but not larval humidity, induced plastic changes in egg size. Females laid larger eggs when maintained at low RH during oviposition. A parallel experiment in Melanitis leda, a species diverged from M. mineus approximately 54 million years ago, yielded similar results. In M. mineus, larger eggs provided significant fitness advantages, including reduced water loss, increased initial larval weight, and enhanced starvation resistance. There was no negative correlation between egg size and number, indicating that females can increase egg size without reducing fecundity. However, egg size declined with advancing female age, suggesting physiological constraints. Our findings demonstrate that environmental cues drive egg size plasticity in tropical butterflies and highlight its potential role as an adaptive mechanism to buffer offspring against unpredictable humidity fluctuations under climate change.