Pectoral fin size in a fish species with paternal care: a condition-dependent sexual trait revealing infection status

1. The three-spined stickleback, Gasterosteus aculeatus L., is a territoria l fish with exclusive male parental care. Males oxygenate the eggs with fan ning movements of their pectoral fins. The present authors investigated whe ther the apparent sexual differences in the functional demands of the pecto ral fins have resulted in sexual differences in fin size. If males have rel atively larger pectoral fins, females may use this as a signal to aid their mate choice for good fathers. Therefore, further objectives were to study the condition-dependency of relative pectoral fin size in males and the rel ationship with male parasite load. 2. Reproductively active males possessed relatively larger pectoral fins th an females in both wild-caught and laboratory-bred fish. 3. In the field, caring males with relatively large pectoral fins were in b etter physical condition and had more food in their stomachs. 4. Relatively small pectoral fins and poor body condition were associated w ith infection by the intestinal parasite Pomphorhynchus laevis (Acanthoceph ala), the prevalent parasite species in the study population.