Alternative reproductive tactics are widespread in fishes, increasing the potential for sperm competition. Sperm competition has enormous impact on both variation in sperm numbers and sperm size. In cichlids, the sperm competition risk is very divergent and longer sperm are usually interpreted as adaptation to sperm competition. Here we examined whether sneaking tactics exist in Pelvicachromis taeniatus, a socially monogamous cichlid with biparental brood care from West Africa. The small testis indicates low gonadal investment which is typical for genetically monogamous species. In contrast, sperm length with up to 85？μm is extraordinarily long. We examined the reproductive behaviour of ten groups with a male-biased sex ratio under semi-natural conditions via continuous video recording. We recorded spawning site preferences and correlates of reproductive success and conducted paternity tests using microsatellites. Safe breeding sites that could be successfully defended were preferred. All offspring could be assigned to their parents and no multiple paternities were detected. Body size of spawning pairs predicted their spawning probability and offspring hatching rate suggesting benefits from mating with large individuals. Our study suggests low risk of sperm competition under the given conditions in P. taeniatus and thus first evidence for genetic monogamy in a substrate breeding cichlid. 1. Introduction Alternative reproductive tactics (ARTs) are widespread in many animal taxa (e.g., [1, 2]). In particular in externally fertilising fishes there is an enormous potential for ARTs [3–5]. So far, ARTs have been described for more than 170 fish species, 19 of which are cichlids [4]. Due to the diversity of ARTs and fertilisation mechanisms in fishes the potential for sperm competition is high. Sperm competition occurs when sperm of two or more males compete to fertilise a female’s eggs [6]. According to sperm competition theory, the strength of sperm competition should influence sperm traits: sperm quantity (e.g., sperm number, usually reflected by testis mass) and quality (e.g., sperm swimming speed [7–11]). The gonadosomatic index (GSI) is generally assumed to be a reliable indicator for sperm competition. It measures gonad mass relative to body mass [12]. Several empirical studies across taxa indeed report that males of polygamous species have a higher relative testis mass (e.g., in birds [13], primates [14], butterflies [15], and fishes [16–18]). Within species, sperm number is expected to increase in the presence of sneakers to maximise a male’s mating
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