Environmental Biology of Fishes 63: 281–287, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. Male reproductive success in a promiscuous armoured catfish Corydoras aeneus (Callichthyidae) Masanori Kohda, Kanako Yonebayashi, Miyako Nakamura, Nobuhiro Ohnishi, Satoko Seki, Daisuke Takahashi & Tomohiro Takeyama Department of Bio- and Geosciences, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan (e-mail: maskohda@sci.osaka-cu.ac.jp) Received 23 December 1999 Accepted 16 July 2001 Key words: male mating competition, female choice, mating success, body size, sexual size dimorphism, sperm competition Synopsis Among a variety of fish mating systems, promiscuity with random-mating seems to be most prevalent. However, detailed studies of promiscuity have been rare due partly to the peculiar difficulty in examination of male mating and reproductive success in the random mating. Females of the armoured catfish Corydoras aeneus (no sexual dimorphism other than size of males < females) spawn 10–20 egg-clutches with multiple males at a time, but an entire egg clutch is inseminated by sperm of a single male. We studied mating system of this fish in aquarium. Males had neither mating territories nor monopolized females, never being aggressive against rival males. Evidence of female preference for certain male traits including size was not detected. Females mated a male in proportion to his relative courtship frequency among males. Courtship frequency was not related to male size, and male mating success was not different between small and large males. Clutch size and insemination rate were different neither between small and large males nor between frequently and less frequently courting males. Thus, the male reproductive success will not be related to the male size, but directly to courtship frequency, indicating the random mating in this fish. There seemed to be fecundity advantage with size in female, and the consequent sexual difference in energy allocation will be responsible to the sexual dimorphism. We also discuss the low male-GSI in this promiscuous fish in which sperm competition hardly occurred. Introduction Sexual size dimorphisms are seen in many fish species; males being larger in some fishes and vice versa in others (Andersson 1994). In many cases, the dimor- phisms are well explained by sexual selection which includes intrasexual selection (e.g., competition for mates) and intersexual selection (e.g., mate choice). For example, if females prefer large males, this pref- erence can be an evolutionary driving force leading to large males, and preference for small males can lead to small. In species in which males compete for females, or territories or other ecological resources to attract females, the contest competition favors large males (Barlow 1991, Andersson 1994). In contrast, when males mate randomly, early- or small maturing males will be favored (Singer 1982, Warner 1975, 1984, Andersson 1994). In this mating system, large males will not gain an advantage from its size trait in mat- ing, which has been theoretically accepted (Warner 1984, Andersson 1994). However, a few empirical studies have provided evidence for random mating (Warner & Harlan 1982, Endler 1983, Conover 1984, Warner 1984, Andersson 1994), because confirmation of reproductive success (RS) of each male is quite dif- ficult when multiple males participate in reproduction at a time. Females are larger than males in many catfishes (Breder & Rosen 1966, Burgess 1989). Corydoras aeneus is a small catfish distributed in South America.