Bower-building behaviour is associated with increased sperm longevity in Tanganyikan cichlids

We investigated the evolutionary relationship between spawning behaviour and sperm motility traits among Tanganyikan mouth-brooding cichlid species that have developed diverse mating behaviours and male sexual traits. Mouth-brooding behaviour is common among these fish, but different species demonstrate a range of spawning behaviours, bower construction, male sexual traits and timing of gamete release. We observed spawning behaviours and compared sperm motility traits of 28 Tanganyikan mouth-brooding cichlids to elucidate the evolutionary correlations between these traits. Sperm longevity was considerably longer in bower-building species that construct crater-shaped spawning sites compared with species that do not build bowers. Male bower builders released sperm in the pit of the bower prior to spawning, and the time from ejaculation to fertilization was longer. Conversely, most mouth-brooding cichlids deposited semen directly into the female buccal cavity, and spawned eggs were immediately picked up to be placed inside the cavity; thus, the time from ejaculation to fertilization was short. These observations suggest that increased sperm longevity is favoured in bower builders. Comparative phylogenetic analyses suggested that bower-building behaviour and greater time from ejaculation to fertilization are associated with the extension of sperm longevity, whereas sperm competition rank does not play a major role. In addition, bower-building behaviour preceded the emergence of increased sperm longevity. These results indicate that the extension of sperm longevity as a result of the emergence of bower builders may have acted as an evolutionary attractor for sperm longevity.