Shifts in nutrient allocation in a gift-giving butterfly: A hidden consequence of water balance?

As climate change intensifies drought, understanding how animals maintain fitness under water stress is key to predicting species persistence. Animals use diverse behavioural and physiological adjustments to avoid dehydration. However, the physiological and fitness costs of these mechanisms are often overlooked, despite their potential to shift nutrient acquisition and allocation. We hypothesized that maintaining water balance, through increased water intake and/or decreased water loss, leads to nutrient shifts and trade-offs in Pieris rapae butterflies. In this species, females receive a protein- and water-rich nuptial gift (NG), known to enhance fecundity and possibly mitigate dehydration. We quantified the impact of dry conditions on female hydration and fitness, using stable isotopes to trace nutrient allocation to storage, fecundity, and catabolism. We found that the NG, combined with reduced respiratory water loss, contributed to maintaining female water balance in dry conditions. Importantly, while dry environments did not impact potential fecundity, nutritional shifts and trade-offs that could affect long-term fitness were evident: females allocated more lipids to eggs at the expense of long-term storage, while reducing catabolism of NG-derived leucine. This interplay among water balance, nutrient allocation, and fitness emphasizes the importance of linking water balance mechanisms with broader nutrient-use strategies under environmental stress.