Regulation of immunocompetence by different androgen metabolites in a blenny with alternative reproductive tactics

In Parablennius parvicornis , small reproductive males with relatively low expression of secondary sexual characters (M− morphotype) parasite on the parental investment of the larger nest‐holder males which have fully developed secondary sexual characters (M+ morphotype). In comparison with M+ males, M− males have relatively low levels of androgens while having high blood cell percentages of lymphocytes and antigen responsiveness. Here we test the hypothesis that androgens are a causal factor for these differences in immunocompetence between morphotypes. After drawing an initial blood sample, males received a silastic implant containing either oil only (C), or oil with testosterone (T) or 11‐ketotestosterone (KT). Males were re‐caught 2 weeks later for drawing of the final blood sample. KT but not T induced the development of secondary sexual characters in M− males. M− males treated with KT showed lower swimming activity than the males treated with T or C implants, suggesting that KT also mediates behavioral changes in M− males. As expected, blood cell percentages of lymphocytes, but not of granulocytes, were higher in M− males than in M+ males. Overall, lymphocyte percentages increased in the C group which might have been a response to the surgery/treatment. In concordance with the hypothesis, lymphocyte percentages were suppressed in males treated with T in comparison with controls. However, no significant change was found in KT‐treated males. This suggests that androgens modulate central, morphological and immunological traits by partly independent androgen mechanisms in P. parvicornis. J. Exp. Zool. 305A, 2006 . © 2006 Wiley‐Liss, Inc.