Abstract
Movement uses resources that may otherwise be allocated to somatic maintenance or reproduction. How does increased energy expenditure affect resource allocation? Using the butterfly Speyeria mormonia, we tested whether experimentally increased flight affects fecundity, lifespan or flight capacity. We measured body mass (storage), resting metabolic rate and lifespan (repair and maintenance), flight metabolic rate (flight capacity), egg number and composition (reproduction), and food intake across the adult lifespan. The flight treatment did not affect body mass or lifespan. Food intake increased sufficiently to offset the increased energy expenditure. Total egg number did not change, but flown females had higher early-life fecundity and higher egg dry mass than control females. Egg dry mass decreased with age in both treatments. Egg protein, triglyceride or glycogen content did not change with flight or age, but some components tracked egg dry mass. Flight elevated resting metabolic rate, indicating increased maintenance costs. Flight metabolism decreased with age, with a steeper slope for flown females. This may reflect accelerated metabolic senescence from detrimental effects of flight. These effects of a drawdown of nutrients via flight contrast with studies restricting adult nutrient input. There, fecundity was reduced, but flight capacity and lifespan were unchanged. The current study showed that when food resources were abundant, wing-monomorphic butterflies living in a continuous meadow landscape resisted flight-induced stress, exhibiting no evidence of a flight-fecundity or flight-longevity trade-off. Instead, flight changed the dynamics of energy use and reproduction as butterflies adopted a faster lifestyle in early life. High investment in early reproduction may have positive fitness effects in the wild, as long as food is available. Our results help to predict the effect of stressful conditions on the life history of insects living in a changing world.
Highlights
Many ecological situations increase the need for movement, for example when available resources are few or they are scattered across the landscape
Increased flight significantly affected energetics and shifted the timing of reproduction but did not affect total fecundity or lifespan in the butterfly Speyeria mormonia. This lack of an effect on total egg production in particular contradicted our primary hypothesis that the increased energetic demand of flight would result in reduced fecundity, since fecundity is compromised when food intake is reduced in this species
Both fecundity and food intake were higher in flown females compared to control females during early life
Summary
Many ecological situations increase the need for movement, for example when available resources are few or they are scattered across the landscape. Exercise has many positive effects [1,2,3], but increased activity can have negative consequences, such as increased oxidative damage or decreased lifespan [4,5,6] In this context, animal flight is a interesting trait as the energetic demand of flapping flight is extremely high [7, 8]. The energy consumed by movement could be allocated to other fitness-related processes: maintenance, reproduction and storage. Resource allocation among these processes is a fundamental part of life history theory. The total resource pool consists of diverse nutrients that are required at different life-history stages [10]
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