Evidence against the Spermatophore as Paternal Investment in Checkerspot Butterflies (Euphydryas: Nymphalidae)

Abstract

Two butterfly species, Euphydryas editha and E. chalcedona, were examined in a laboratory experiment to test whether the reproductive output of a mating is affected by the weight of the male-donated spermatophore. Ejaculate weight was determined by weighing males before and after mating. Reproductive output was measured as the number of eggs laid and the percentage that hatched. Spermatophore weight did not influence reproductive output in either species. We found no evidence that the spermatophore constitutes a significant paternal investment in these species. INTRODUCTION The relative parental investments of males and females play a key role in shaping mating systems (Trivers, 1972). In several species of Lepidoptera, there is evidence that males not only contribute sperm but actually make an investment by donating nutrients via the spermatophore at mating (Goss, 1977; Borgia, 1979; Boggs and Gilbert, 1979; Marshall, 1980; Boggs, 1981; Rutowski, 1982b). A substantial paternal investment has several implications. First, such an investment taxes a male's nutrient/energy budget for reproduction, thereby limiting his potential for polygyny (Thornhill and Alcock, 1983). Second, a male that makes a substantial investment should attempt to inseminate females with high reproductive potentials (Gwynne, 1981; Rutowski, 1982b). Third, females should choose males on the basis of potential investment (Thornhill, 1976; Gwynne, 1982). Female butterflies should assess potential nutrient donations on the basis of male characteristics that should influence spermatophore size: age, size and number of previous matings (Rutowski, 1982a). The spermatophore has often been interpreted as paternal investment, yet butterflies have never been tested to see whether this donation actually increases the reproductive output of matings, and hence could constitute a basis for sexual selection. In fact, in the moth Plodia interpunctella, radioactively labeled substances transferred to females at mating by the male do enter unfertilized eggs, but female fecundity is not a function of ejaculate weight in this moth and hence the ejaculate cannot be considered a form of paternal investment (Greenfield, 1982). Here we attempt to determine whether the spermatophore constitutes paternal investment in Euphydryas butterflies. We examine three male characteristics that may affect spermatophore weight (Rutowski, 1982a), then test for an effect of spermatophore weight on female reproductive output. We also test the effects of female age and size on reproductive output, to examine whether either may be a basis for male choice. MATERIALS AND METHODS Postdiapause larvae of Euphydryas editha were collected at Morgan Hill, Santa Clara Co., California, placed in a growth chamber (14 hr light at 24 C, 10 hr dark at 14 C), and fed Plantago lanceolata. Euphydryas chalcedona larvae from Jasper Ridge and Los Altos Hills, San Mateo Co., Calif., were similarly raised, and fed Diplacus aurantiacus and 'Present address: Department of Entomology, University of California, Davis 95616. 2Present address: Zoology Department, Duke University, Durham, North Carolina 27706.