Does testis size in cuckoos vary with paternal care?

Abstract

Male birds confront a tradeoff between increasing the number of offspring they sire and providing parental care. This behavioral tradeoff may be manifested as variation in testis size: larger testes, producing more sperm and testosterone, are advantageous for sperm and mating competition, thus ensuring paternity, yet high levels of plasma testosterone also inhibit parental care. There is good evidence that relative testis size is greater in species in which males compete more intensely to fertilize females, but it remains unclear whether, in turn, smaller testes prevail in species with greater levels of paternal care. Cuckoos provide an opportunity to test for an effect of paternal care on testis size because of the diversity of breeding systems in this family: parasitic species lack parental care altogether, whereas some coucals (Centropodinae) show exclusively or predominantly male care. In addition, coucals have a reduced left testis, which Ligon (1997) suggested may promote male care. We tested the idea that species with more paternal care have smaller testes in a phylogenetic analysis of 18 parasitic and 15 nonparasitic species and, specifically, quantified the reduction of the mass of the left testis in the Pheasant Coucal (Centropus phasianinus). Contrary to expectation, nesting cuckoos had larger testes than parasitic species and coucals compensated the reduction of the left testis by an increase of the right. These findings suggest that plasma testosterone is regulated independently of testis size and paternal care is not associated with testis size in cuckoos.