Animals and plants grow and reproduce surrounded by nutritional variation, where food is often scarce or key nutrients are lacking. Because the juvenile nutritional environment has major effects on the adult phenotype, linking nutrition and fitness is an increasingly important aspect of ecology, evolution and life-history theories (e.g. Boggs 2009; Raubenheimer, Simpson & Mayntz 2009; Morehouse et al. 2010).
Developing under nutritional constraints can be particularly challenging for animals with a complex life cycle, such as amphibians, holometabolous insects and many marine invertebrates. Such creatures are characterized by discrete larval and adult stages that often live in distinct habitats with different nutritional conditions. Thus, individuals have limited time to acquire the resources that are necessary to form the adult and to reproduce. Traditionally, life-history models propose that the effects of larval nutrition on adult fitness are mainly captured by two developmental traits: size and timing of transition out of the larval stage (e.g. Rowe & Ludwig 1991; Moran 1994; Awmack & Leather 2002; Roff 2002). Typically, nutrient limitation produces longer development time and/or smaller body size at metamorphosis, and these changes in developmental traits are correlated with reduced adult fitness. However, recent studies challenge these models by showing that larval nutrition also impacts adult fitness independently of these developmental traits (e.g. De Block & Stoks 2005; Stoks, De Block & McPeek 2006; Block & Stoks 2008). This occurs through ‘carry over’ or latent effects, in which traits originate from the larval nutritional experience and yet are only expressed in adulthood (Pechenick, Wendt & Jarrett 1998). Examples of latent effects due to restricted larval diets include decreased adult immune function (Fellous & Lazzaro 2010), adult shortage of energy storage molecules (Stoks, De Block & McPeek 2006) and increased oxidative stress (Block & Stoks 2008). Thus, by taking into account these additional latent effects, we can achieve a more complete understanding of how early nutrition might affect various components of adult fitness.
A second important issue when examining how juvenile nutrition alters adult fitness concerns the methods used to evaluate fitness. Measuring fitness is a challenging enterprise, especially because male fitness often depends not only on mating success, but also on different pre- and postcopulatory fitness components (Hughes 1998; Fedina & Lewis 2008). When resources are limited, fitness variation among individuals arises through different resource allocation patterns among various fitness components (Stearns 1992). However, the vast majority of studies looking at the fitness effects of larval nutrition have only focused on traits related to either precopulatory or postcopulatory fitness; remarkably few studies have considered both (but see Lewis, Sasaki & Miyatake 2011; Lewis et al. 2012). A more complete understanding of how nutrition affects fitness requires taking into account both pre- and postcopulatory fitness. An additional challenge is that fitness is ideally measured under natural conditions where keeping track of individuals is often a difficult task. Therefore, it is by considering both pre- and postcopulatory fitness components and by measuring these components in settings reflecting natural conditions that we obtain more precise estimates of fitness.
Herbivorous insects are an appropriate study system for examining the linkages between nutrition and fitness. Nitrogen is a key nutrient for all animal species, because it is required to build proteins, nucleic acids and many essential body structures (Mattson 1980; Bernays & Chapman 1994). However, because plant tissue contains only a small fraction of the nitrogen contained in animal tissue, nitrogen becomes a limiting element for most herbivores (Mattson 1980; Scriber & Slansky 1981; White 1984; Slansky & Rodriguez 1987; Bernays 1998; Awmack & Leather 2002). This results in a fundamental nutritional mismatch between herbivores and their food plants; caught in what has been called the herbivore's dilemma (Pierce & Berry 2011), these creatures must reconcile their nitrogen-rich lifestyle with their nitrogen-poor diet. The cabbage butterfly, Pieris rapae, is a model organism for studying the role of nitrogen limitation because of its particularly high demands for nitrogen, with adult bodies consisting of ~13% nitrogen at eclosion (Morehouse & Rutowski 2010a). This butterfly is also useful for testing how larval nutrition affects adult fitness because several male traits related to pre- and postcopulatory fitness components have previously been identified (e.g. Suzuki et al. 1977; Bissoondath & Wiklund 1996; Wedell & Cook 1999; Morehouse & Rutowski 2010b). Finally, Morehouse & Rutowski (2010a) found that larval nitrogen availability strongly affected larval growth and development. These authors also suggested that dietary nitrogen influences key male adult traits including male ornaments such as wing coloration and nutritive nuptial gifts that are passed to the females during copulation (Morehouse 2009; Morehouse & Rutowski 2010a). In this species, females prefer to mate with more colourful males (Morehouse & Rutowski 2010b) and wing coloration is based on pterins, a group of nitrogen-rich pigments (Kayser 1985). Pieris rapae males' nuptial gift consists of a protein-rich spermatophore (Bissoondath & Wiklund 1995) that increases male reproductive success by increasing female fecundity (Watanabe & Ando 1993), remating latency (Sugawara 1979; Kandori & Ohsaki 1996) and male paternity share (Wedell & Cook 1998).
In this study, I adopt a holistic approach to address the long-standing question of how adult fitness of an herbivorous insect is affected by the larval nutritional environment. I manipulated larval dietary nitrogen availability and measured its effects on adult fitness through three different pathways: development time, adult size and latent effects. In addition, I measured a suite of adult male fitness components in a setting that approximates natural conditions for the cabbage butterfly. Using this approach, I expected to gain a more comprehensive picture of how dietary nitrogen influenced male adult fitness in an organism with a complex life cycle.