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 essential for predicting ecological resilience. Terrestrial animals use diverse behavioural and physiological strategies to avoid dehydration, yet the associated physiological and fitness costs remain poorly understood. Because water balance is tightly linked to nutrient acquisition and metabolism, mechanisms that enhance hydration may alter how animals allocate key macronutrients across vital functions. Here, we investigated how maintaining water balance - via increased water intake or reduced water loss - shapes nutrient allocation and trade-offs in the cabbage white butterfly (Pieris rapae), a species in which males transfer nutrient- and water-rich nuptial gifts to females during mating. Using controlled humidity treatments and stable-isotope tracing, we quantified how the hydric environment and mating status influence female allocation of nutrients - including nuptial gift-derived amino acids - to storage, fecundity and catabolism. We found that females in dry environments maintained water balance largely by acquiring nuptial gifts and by reducing respiratory water loss. However, dry conditions still altered nutrient allocation: females invested more lipids into eggs at the expense of long-term storage, and they reduced catabolism of an essential amino acid derived from the nuptial gift. These results show that mechanisms supporting water balance can indirectly reshape nutrient-use strategies, revealing physiological trade-offs that may influence longer-term fitness. More broadly, our findings highlight the tight coupling between water and nutrient economies and emphasize the need for a nutrient-explicit framework for understanding how animals cope with increasing aridity.