Mouth- and Substrate-Brooding Cichlids Display Contrasting Trade-Offs and Diverse Evolutionary Pathways of Egg Investment

AbstractOptimal egg size is considered a classic and important concept in life history theory. Here, we used comparative analyses to investigate the ecological factors affecting egg size across 87 Lake Tanganyikan cichlid species, which employ either a mouth-brooding or a substrate-brooding strategy to raise their offspring. These two strategies differ substantially in resource availability, potentially leading to different selective regimes on egg size. We observed a strong negative correlation between egg size and egg number (clutch size) in mouth-brooding species but not in substrate-brooding cichlids. Our results suggest that the strength of the relationship between egg size and number differs between the two brooding strategies. Interestingly, in mouth-brooding species, we found that egg size increases when offspring exhibit external feeding behaviors (i.e., grazing behavior). We also demonstrate that substrate-brooding species that have relatively short periods of parental care typically have larger eggs compared with species with longer periods of parental care. Overall, our results demonstrate that behavioral and ecological differences, including parental care strategy and duration, play an important but often overlooked role in the evolution of egg size across species.