Resource heterogeneity but not inbreeding affects growth and grouping behaviour in socially foraging juvenile cichlid fish

1. Spatial food distribution determines resource profitability, defensibility and encounter rate of foragers. Clumped food distribution can promote aggressiveness and resource monopolisation, in turn increasing within-group variation in food intake and growth. However, the effects of food distribution may depend on foraging strategies. Little is known about the impact of spatial food heterogeneity on growth and grouping behaviour in social foragers in the absence of monopolisation. 2. Social foraging is present in many fishes, particularly at early juvenile life stages when fish are especially sensitive to environmental variation. Here, a heterogeneous food distribution may impair foraging success and growth and juveniles may increase sociability to attain social information about food resources. 3. We examined the impact of the spatial distribution of food as well as inbreeding on growth and social behaviour in juveniles of the cichlid fish Pelvicachromis pulcher. Inbred individuals often show poorer performance than outbred individuals (inbreeding depression), but inbreeding effects can be environment dependent. In the experiment, inbred and outbred fish were reared in a split-clutch design either under homogeneously distributed or spatially clumped food conditions for 8 weeks starting 1 week after juveniles could actively feed. We documented growth and performed a shoaling assay and a sociality test (choice between a large vs. a small shoal) after 6 weeks. 4. Spatial food distribution did not affect within-group body length variation, but individuals reared under clumped food conditions were smaller. Shoals of the different feeding conditions differed in social behaviour. Shoals of the clumped treatment group showed higher variation in inter-individual distances compared to shoals of the homogeneous treatment group. Furthermore, focal fish of the clumped treatment adjusted their association preference to the position of the groups' largest individual. We did not find significant inbreeding or environment-dependent inbreeding effects regarding growth or social behaviour. 5. Our study suggests that a clumped food distribution can impede localisation of food resources and thus growth in juvenile social foragers. Accordingly, in heterogeneous environments, the use of social information may be highly relevant to increase individuals' foraging success potentially explaining orientation on successful foragers, that is, large individuals. Inter-individual variation in juvenile's social behaviour may precede variation in food monopolisation capability and in growth emerging at later life stages.