The mouthbrooding cichlid Tropheus moorii exhibits an exceptional degree of maternal investment. Females produce very large eggs, which are incubated for a period of six weeks, and they feed their young in the mouth while starving themselves. Only very few fish species are known to feed their young. We hypothesized that feeding may either (1) benefit females directly if young develop faster, or (2) provide a size advantage to young. To distinguish between these mutually non-exclusive hypotheses, we measured costs and benefits arising for mouthbrooding females when feeding their young. Field observations revealed that mouthbrooding females reduced bite rates, locomotion and territorial defence compared to non-brooding adults. Feeding rates correlated positively with locomotion, as fish moving around more also spent more time with territory defence and other social interactions. This suggests that buccal feeding is costly when compared to mere incubation without feeding the young. In an experiment, in which we controlled the access of females to food, we showed that these costs are apparently not counterbalanced by a benefit to females through a shorter incubation duration. Rather, fed young were larger, heavier and had higher burst-swimming speeds. The extreme maternal investment of T. moorii appears to yield fitness benefits to females by producing larger and stronger young, which consequently should have better survival chances in their natural environment.