SOCIAL RANK, INDIVIDUAL SIZE AND GROUP COMPOSITION AS DETERMINANTS OF FOOD-CONSUMPTION BY HUMBUG DAMSELFISH, DASCYLLUS-ARUANUS

The humbug, Dascyllus aruanus L. (Pomacentridae) is a small planktivorous coral-reef fish. Humbugs live in small, stable social groups that are organized into dominance hierarchies based on size. Influences of four individual and group attributes on food consumption were tested. The attributes studied were: (1) individual size, (2) social rank, (3) group size and (4) the summed size of higher ranked group members. Because these attributes are correlated, the composition of groups was manipulated in the field to test their separate effects on food consumption. Feeding rates were determined only by individual size, smaller fish fed at higher rates than larger ones. In contrast, the composition of the diet was influenced only by social rank; high-ranked fish ate a greater proportion of animal prey and copepods than low-ranked ones. The size of prey consumed was influenced by all four attributes, large and high-ranked fish ate larger prey than small and low-ranked fish. An increase in group size and the summed size of higher ranked group members was associated with a reduction in size of prey consumed. Larger fish fed further upstream than smaller ones. Influences of social rank on diet composition may be because large (high ranked) group members have priority of access to food arriving in the current. These influences may provide a mechanism for competition for food within groups. © 1991 The Association for the Study of Animal Behaviour.