Effects of adult-derived carbohydrates, amino acids and micronutrients on female reproduction in a fruit-feeding butterfly

Abstract

It is generally believed that butterflies (and other holometabolous insects) rely primarily on reserves accumulated during the larval stage for reproduction, whereas the carbohydrate-rich adult diet is thought to mainly cover energy requirements. In at least some species though, realization of the full reproductive potential is extensively affected by post-eclosion nutrition. While the importance of carbohydrates is fairly well understood, the role of adult-derived amino acids and micronutrients is controversial and largely unknown, respectively. We here focus on the effects of different adult diets on female reproduction in the tropical, fruit-feeding butterfly Bicyclus anynana (Nymphalidae). Carbohydrates were the most important adult-derived nutrients affecting reproduction. Adding amino acids, vitamins or minerals to sucrose-based solutions did not yield a reproductive output equivalent to that of fruit-fed females, which showed the highest performance throughout. This suggests that either not yet identified compounds of fruit substantially contribute to reproduction, or that resource congruence (the use of nutrient types in a specified ratio) rather than any specific nutrient component is of key importance. Apart from adult income, realized fecundity depended on egg size and longevity, with the former dominating when dietary quality was low, but the latter when quality was high. Thus, the egg size-number trade-off seems to be affected by female nutrition.

Introduction

It is generally believed that Lepidoptera, as is the case in many holometabolous insects, rely primarily on nutrients accumulated during the larval stage for somatic maintenance and reproductive output (Leather, 1995; Telang et al., 2001; Mevi-Schütz and Erhardt, 2003a). In contrast to the protein-rich larval diet, adult diet (such as nectar or rotting fruit) is typically rich in carbohydrates and poor in amino acids (Watt et al., 1974; Baker and Baker, 1975, Baker and Baker, 1983; Romeis and Wäckers, 2000; O’Brien et al., 2004). However, some species supplement their adult diet with substrates ranging from pollen to mud, dung or carrion or by preferring amino acid-containing nectars (e.g. Gilbert, 1972; Boggs and Jackson, 1991; Braby and Jones, 1995; Smedley and Eisner, 1996; Beck et al., 1999; Rusterholz and Erhardt, 2000). Adult feeding mainly covers energy requirements (general maintenance including flight expenditures, e.g. Willers et al., 1987; O’Brien, 1999), but has also been found to affect reproductive output in at least some butterfly and moth species (e.g. Murphy et al., 1983; Leather, 1984; Hill, 1989; Boggs and Ross, 1993; O’Brien et al., 2000; Fischer and Fiedler, 2001; Fischer et al., 2004).

As insect eggs consist primarily of protein (Engelmann, 1999) female Lepidoptera are in high demand for amino acids. However, amino acids are generally scarce in their sugar-rich adult diet (O’Brien et al., 2002). Non-essential amino acids have been shown to be extensively synthesized by females using adult-derived carbon and endogenous nitrogen sources (O’Brien et al., 2002), whereas essential amino acids are entirely larval in origin and therefore likely ultimately limiting to reproduction. Hence, the role of amino acids seems to be particularly relevant for the understanding of nutritional constraints on insect reproduction (O’Brien et al., 2002).

However, the role of amino acids in the diet of adult butterflies is controversial (e.g. Beck et al., 1999; Mevi-Schütz and Erhardt, 2003b). Some species preferentially feed on nectars with high amino acid contents (e.g. Colias, Watt et al., 1974), whereas others do not (e.g. Battus philenor, Erhardt, 1991; Ornithoptera priamus, Erhardt, 1992). Selective feeding on nitrogenous diets may reflect the nutritional status of the butterflies (Rusterholz and Erhardt, 2000; Mevi-Schütz and Erhardt, 2003c), and seems to be restricted to females because of their higher demand for protein (e.g. Alm et al., 1990; Erhardt and Rusterholz, 1998; Rusterholz and Erhardt, 2000; Mevi-Schütz and Erhardt, 2003c). Nevertheless, no or only weak effects of amino acids on longevity and reproductive success of female butterflies were found in various species (Hill, 1989; Hill and Pierce, 1989; O’Brien et al., 2000; Romeis and Wäckers, 2002; Mevi-Schütz and Erhardt, 2003c), whereas fitness improved substantially in others (e.g. pollen-feeding Heliconius butterflies, Gilbert, 1972; Dunlap-Pianka et al., 1977; Euphydryas editha, Murphy et al., 1983; but see Moore and Singer, 1987 for contradictory results on this species).

In contrast to the majority of nectar-feeding butterflies in temperate zones, many tropical species feed on rotting fruits (Braby and Jones, 1995). In spite of further progress in understanding the role of nectar for reproductive output in butterflies (Boggs, 1997; O’Brien et al., 2002, O’Brien et al., 2004), studies on the impact of fruit-feeding on butterfly life histories are scarce (e.g. Fischer et al., 2004). Informal speculations that fruit could be a richer source of nitrogenous compounds than is nectar could not be confirmed by recent data; both resources appear to be rather similar in nutrient composition (Bosque and Pacheco, 2000; Omura and Honda, 2003; Fischer et al., 2004). However, rotting fruit may provide yeast to fruit-feeding Lepidoptera, which is an excellent source of protein to insect frugivores (Good and Tatar, 2001) rendering rotting fruit a potentially important source for amino acids.

Feeding on rotting fruits, however, does not only provide carbohydrates and some amino acids, but also detectable amounts of micronutrients such as minerals (e.g. potassium, magnesium and phosphorous) and a variety of vitamins (cf. University of Hohenheim, 1996), which could contribute to reproductive output (either indirectly via beneficial effects on overall performance or by a direct contribution to egg provisioning). Despite numerous studies focussing on larval nutrition (e.g. Vanderzant et al., 1962; Rodriguez, 1972; Murugan and George, 1992; Barbehenn et al., 1994, Barbehenn et al., 2001; Stamp, 1994; Woods et al., 2002; Perkins et al., 2004), studies on the importance of adult-derived minerals and vitamins for somatic maintenance and reproductive output are still scarce (e.g. Smedley and Eisner, 1996; Engelmann, 1999). Nevertheless, such adult-derived micronutrients appear to affect egg production in at least some insect species (e.g. Pappas and Fraenkel, 1977; Engelmann, 1999). Currently, the best studied example for the importance of adult-derived micronutrients for butterfly reproduction is the uptake of sodium by mud-puddling males (e.g. Boggs and Jackson, 1991; Smedley and Eisner, 1995; Beck et al., 1999).

Here we investigate the relative importance of different nutritional components on female reproduction in the tropical, fruit-feeding butterfly Bicyclus anynana (Butler, 1879) (Lepidoptera: Nymphalidae). We performed two separate experiments specifically focussing on the fitness effects (rather than preferences that may not necessarily indicate a fitness advantage) of adult-derived nutrients. While in experiment 1 the effects of carbohydrates and amino acids were investigated, experiment 2 focussed on the role of micronutrients (minerals, vitamins) for female reproductive output.