Does adult nutrition affect adult longevity and fecundity?
Additional amino acids in the adults’ nutrition had no measurable effects on longevity and fecundity of Maniola butterflies. Given their relatively long adult life-span, we would have expected the Mediterranean females to benefit from additional nitrogen intake and live longer or produce larger quantities of eggs. An explanation for the lack of such a benefit could be that the butterflies had sufficient nitrogen intake at the larval stage and therefore supplementary amino acids did not affect adult butterflies, similarly as shown for Araschnia levana by Mevi-Schütz and Erhardt, 2005). In Mevi-Schütz and Erhardt’s study those butterflies which were raised on nitrogen-poor larval food resources reacted positively to nectar enhanced with amino acids, while butterflies from caterpillars raised on nitrogen-rich food did not react to increased amino acid intake. In Araschnia levana, a lack of amino acid intake at one life history stage can thus be compensated at a later stage.
The Maniola adults we used in our experiment did most probably not suffer from nitrogen limitation at the larval stages. Meadows from where they were collected were all situated in the proximity of fertilized farmland, or they were grazed by sheep and goats, which also produces additional nitrogen input. Moreover, although widespread across Europe, M. jurtina has been reported to suffer from too much nitrogen input at highly fertilized meadows, from where it usually disappears, moving to more natural and nitrogen-poor grasslands (Ebert and Rennwald, 1991; Schweizerischer Bund für Naturschutz, 1987). Araschnia levana typically occurs in nitrogen-rich humid habitats such as floodplain forests, where its larval host plant, Urtica dioica, thrives. It is thus also not nitrogen limited in its natural habitats. For Pieris rapae (Morehouse and Rutowski, 2010), nitrogen availability is the key factor for growth and development at the larval stage, whereas carbohydrates are less constraining (for the larvae!) and can be compensated by increased adult feeding with sugar. Larvae of the Meadow Brown, M. jurtina, do not appreciate too much nitrogen in their diet, and the same has been shown for a number of other grassland butterfly species (e.g. Fischer and Fiedler, 2000).
Additional amino acids in the nectar can thus be favourable for butterflies, but only under certain conditions (Boggs, 1986); in many cases nitrogen income during the adult stage has no effect on fecundity (Fischer et al., 2004; Moore and Singer, 1987; Hill and Pierce, 1989; Mevi-Schütz and Erhardt, 2003).
A very recent study on Coenonympha pamphilus (Cahenzli and Erhardt, 2012) found that females of this species increased the quality of their offspring (i.e. heavier larvae) when receiving additional amino acids. Only if larvae had been raised on scarce resources also the quantity of the eggs laid by C. pamphilus was increased through amino acid intake as adults. An amino acid enhanced diet might also have effects on offspring weight in Maniola jurtina, but this was not investigated in the frame of our study.
Coenonympha pamphilus, P. rapae and A. levana are all multivoltine species. Additional amino acids in the nectar may be more likely to have a positive fecundity effect in multivoltine, but not in strictly univoltine species. A number of studies observing no fitness effects of nectar amino acids on butterfly fecundity would support this idea (Murphy, 1983; Moore and Singer, 1987). It is evolutionarily plausible that species with only one generation per year need to be less dependent on fluctuations in resources than species with more generations per year. So it seems logical that multivoltine species react more strongly and instantaneously to improvement of adult food resources than univoltines, like M. jurtina.
However, there are also studies which detected no nectar amino acid effects for bivoltine (Mevi-Schütz and Erhardt, 2003) and multivoltine species (Hill and Pierce, 1989). In Lasiommata megera, total number of eggs laid depended on the emergence weight of the female and the amount of carbohydrates (i.e. nectar mimic without amino acids) ingested (Mevi-Schütz and Erhardt, 2003). Interestingly, O’Brien et al. (2004) showed for the univoltine butterfly Euphydryas chalcedona that egg provisioning (i.e. carbon intake) takes place before adult emergence and that the extent to which larval diet contributed to egg carbon can depend on the timing of the oviposition.