Abstract The mating system of the Australian lycaenid butterfly, Jalmenus evagoras, is highly unusual com- pared to most other Lepidoptera. Characteristics of this system, which has been termed an ‘explosive mating strategy,’ include the formation of an intensely competi- tive mating aggregation of males, a highly male biased operational sex ratio, a lack of discrimination and mate choice by both sexes, a high variance in male mating success, and female monogamy. In this study, we tested the hypothesis that multiple mating by males imposes physiological costs resulting in smaller spermatophores, and that this results in a fitness cost to females. We found that male J. evagoras transferred only 2.2% of their eclosion weight during their first mating, consistent with the hypothesis that males of monandrous species produce a relatively small investment. The wet weight of the ejaculate declined by an average of 27% at the sec- ond mating and the dry weight by 29%, and an intermat- ing interval of 5–9 days was needed for the ejaculate to return to the size at the first mating, regardless of male size or age. Wet ejaculate mass increased proportionally with male size, though dry mass was proportionally larg- er in smaller males. Ejaculate mass tended to increase with male age at both first and second matings. Female characteristics, in general, did not affect ejaculate mass, although the wet weight of the ejaculate was positively associated with female weight at the second mating. Copulation duration increased from 2.4 h to approxi- mately 3 h at the second mating, and to over 4 h at the third and fourth matings. Fecundity was positively corre- lated with female size but not with mating history, copu- lation duration, or any other characteristics measured for either males or females. Female longevity declined sig- nificantly as the number of times the male partner had previously mated increased. We conclude that despite the small male investment in ejaculate, the costs of multiple mating may nonetheless be significant, as indicated by the reduction in ejaculate mass, an increase in copulation duration, and reduction in female lifespan with increas- ing mating number. Key words Copulation duration · Explosive mating system · Ejaculate size · Lepidoptera · Mating history Introduction Mating behavior in the Lepidoptera has been increasing- ly used in recent years to examine how ecological cir- cumstances constrain or promote sexual selection and the evolution of different mating systems. In many but- terfly species, males do not defend either mates or a ter- ritory, conforming to a category of mating system known as scramble competition polygyny (Emlen and Oring 1977). Within this category, two alternate types of mat- ing behavior have been distinguished, based on the avail- ability of females. When females are spread out in time and/or space, a prolonged searching polygyny may be fa- vored, in which males fly about broadly in search of re- ceptive females in an effort to outrace their competitors (Thornhill and Alcock 1983). Under these circumstanc- es, males may be expected to indulge in complex court- ship behavior to stimulate unreceptive females and both Communicated by M.A. Elgar L. Hughes ( ✉ ) 1 · N.E. Pierce Museum of Comparative Zoology, Harvard University Cambridge, MA 02138, USA B. Siew-Woon Chang 2 · D. Wagner 3 Department of Ecology and Evolutionary Biology Princeton University, Princeton, NJ 08544, USA Present addresses: 1 Department of Biological Sciences Macquarie University, North Ryde, NSW 2109, Australia e-mail: lhughes@rna.bio.mq.edu.au Tel.: +61-2-98508195 Fax: +61-2-98508245 2 Department of Organismic and Evolutionary Biology Harvard University, 22 Divinity Avenue, Cambridge MA 02138, USA 3 Department of Biological Sciences University of Nevada, Las Vegas, NV 89154, USA Behav Ecol Sociobiol (2000) 47:119–128 © Springer-Verlag 2000 ORIGINAL ARTICLE Lesley Hughes · Belinda Siew-Woon Chang Diane Wagner · Naomi E. Pierce Effects of mating history on ejaculate size, fecundity, longevity, and copulation duration in the ant-tended lycaenid butterfly, Jalmenus evagoras Received: 22 January 1999 / Received in revised form: 28 July 1999 / Accepted: 18 September 1999