Competition level determines compensatory growth abilities

In many animal taxa, size-selective predation favors fast growth early in life. However, same-aged juveniles can diverge in size due to differences in genotype, environmental conditions, and parental effects and thus may vary in competitive ability. Under food scarcity, competitively inferior juveniles may suffer suppressed growth, whereas under benign conditions, small juveniles may exhibit growth compensation and perform as well as large ones. However, studies testing this while controlling for parental effects are lacking. Here, we hand-raised cichlids, Simochromis pleurospilus, from a wide range of egg sizes and manipulated their size by differential feeding. Afterward, high- and low-ration siblings were kept in groups assigned to either a high- or low-competition environment. We investigated how the degree of competition affected aggressiveness and growth of juveniles with different feeding histories. As predicted, when competition was high, high-ration offspring grew fastest. Interestingly, when competition was weak, low-ration juveniles grew at a similar rate as high-ration ones and many were able to catch up in size. High-ration fish were more aggressive than low-ration ones, and this effect was strongest under high competition. Additionally, in the high-competition environment, received aggression was negatively related to growth, and inflicted aggression correlated positively with the growth of the aggressor. These relationships were absent under low competition. Our findings suggest that the abilities to compensate for early growth depression depend on the prevalent level of competition. Aggression is likely used to monopolize food by juvenile S. pleurospilus; however, when competition is strong, aggression cannot compensate for a size disadvantage.