1 Nectar sugars enhance fitness in male Coenonympha pamphilus butterflies by increasing longevity or realized reproduction Fabian Cahenzli and Andreas Erhardt F. Cahenzli (f.cahenzli@gmx.ch) and A. Erhardt, Dept of Integrative Biology, Section Conservation Biology (NLU), Univ. of Basel, St. Johanns Vorstadt 10, CH-4056 Basel, Switzerland. Te principal components of floral nectar are water and the sugars sucrose, fructose and glucose. Several studies have shown the importance of nectar sugars for female butterfly fecundity, whereas to date little attention has been paid to the effect of nectar sugars on male butterfly reproduction. Clear evidence for an effect of nectar sugars on male realized reproductive success is still missing. In this study, we fed male Coenonympha pamphilus butterflies nectar mimics with low (5%), medium (20%) or high (30%) total sugar concentrations with a sucrose:glucose:fructose ratio of 2.7:1.1:1. Sugar solutions were made mimicking Knautia arvensis, an essential nectar plant for C. pamphilus and many other European butterflies. Realized male reproduc- tive success for each treatment was measured indirectly via nuptial gifts, by recording reproductive parameters and by characterizing time patterns over the oviposition period of their female partner. Male butterflies fed high-concentrated nectar sugars had a longer lifespan than males fed low-concentrated nectar sugars. In contrast, offspring of males fed medium-concentrated nectar sugars had a higher hatching mass than progeny of males fed low-concentrated nectar sugars, indicating a tradeoff between somatic maintenance and reproduction in the use of nectar sugars. Tus, allocation patterns of nectar sugars differed according to sugar concentrations in adult food. Te method used in this experiment took into account the indispensable role of female butterflies in passing male nutrients to offspring. With this comprehensive approach, we can show the general importance of nectar sugars for male butterfly fitness and support previous findings suggesting a coevolutionary process between butterflies and flowers dependent on butterfly pollination. Oikos 000: 001–007, 2012 doi: 10.1111/j.1600-0706.2012.20190.x © 2012 Te Authors. Oikos © 2012 Nordic Society Oikos Subject Editor: Anna Traveset. Accepted 18 January 2012 Floral nectar is the most common and widespread adult butterfly food source (Gilbert and Singer 1975). In contrast to the protein-rich larval diet, nectar is composed mainly of sugars, although other nectar constituents, in particular amino acids, can also play an important role for butterfly fit- ness (Watt et al. 1974, Baker and Baker 1983, O’Brien et al. 2004, Mevi-Schütz and Erhardt 2005). Te concentration and composition of nectar sugars varies greatly between dif- ferent plant species, but sucrose, fructose and glucose are the three main sugars found in nectar (Baker and Baker 1983). Te sucrose: hexoses ratio in floral nectar correlates with dif- ferent pollinator types, and butterfly-pollinated flowers have high proportions of sucrose in their nectar (Baker and Baker 1983). Correspondingly, butterflies seem to generally prefer sucrose over fructose and glucose as shown in several pref- erence tests (Erhardt 1991, 1992, Rusterholz and Erhardt 1997). However, demands for nectar may vary between dif- ferent butterfly species due to different energy and nutri- tional needs (Boggs 1997). In Lepidoptera, one role of adult feeding is to cover energy requirements for general maintenance. For example, sucrose is used as fuel for flying (Willers et al. 1987, O’Brien 1999), and nectar sugars influence longevity (Murphy et al. 1983). Te remaining sugars, those carried over from the larval stage and those acquired as adults, can be utilized in reproduction. Several studies have shown the importance of nectar sugars on female butterfly fecundity (Norris 1935, Stern and Smith 1960, Murphy et al. 1983, O’Brien et al. 2004). In contrast, to date little attention has been paid to the effect of nectar sugars on male butterfly reproduction. Many insects have a mating system where the male trans- fers nutrients in spermatophores to the female during mat- ing, often referred to as ‘nuptial gifts’ (Tornhill and Alcock 1983). Spermatophores of Lepidoptera can be quite large (Rutowski et al. 1983). Since butterfly spermatophores contain water, sugars, lipids, hydrocarbons, amino acids, hormones and sperm (Marshall 1982, 1985), they are a costly physiological product composed of limited resources (Svärd 1985, Svärd and Wiklund 1986, Oberhauser 1988, Ferkau and Fischer 2006). Furthermore, radiotracer studies demonstrated that female butterflies use sugars from the spermatophore for both egg production and incorporation into somatic tis- sue (Boggs 1997). Tis suggests that nectar sugars may also have an effect on male butterfly reproduction. Adult food limitation reduced the longevity and tended to reduce fresh