There was a strong response to selection, as evidenced from the log2 antibody titers, to the challenge with sheep erythrocytes in males used in this study [low line (mean ± SE) (n): 0.00 ± 0.00 (28); control line: 4.11 ± 0.50 (27); high line: 14.31 ± 0.45 (29); F2,83 = 391.5, rsPc > 0.9999, P Escherichia coli, Newcastle disease virus, bronchitis virus, bursal disease virus) varied between selection lines in a similar fashion (20). Furthermore, selection lines differed in size of spleen and bursa (35), cellular immunity in vivo (36), and mortality after infection with Marek’s disease (37). Thus it seems safe to assume that immunocompetence was successfully manipulated.
Comb size varied significantly between selection lines (F2, 79 = 9.78, rsPc > 0.9997, P t53 = 3.96, P 1A). Thus, there was a negative association between immunocompetence and the degree of sexual ornamentation, as predicted by the immunocompetence handicap hypothesis.
Artificial selection on a particular trait often affects many other traits, and the variation in comb size between selection lines could reflect a general effect on body size, rather than a specific effect on sexual ornamentation. Indeed, body size varied between selection lines (F2, 84 = 4.03, rsPc = 0.98, P t57 = 2.55, P 1B). However, further analysis showed that selection lines differed in comb size even when body size was controlled for statistically (Fig. 1C; results of ANCOVA: selection line: F2, 78 = 7.22, rsPc = 0.999, P F1, 78 = 6.27, P n): 3,465 ± 49 (28); control line: 3,333 ± 56 (27); high line: 3,305 ± 47 (29); F2,84 = 2.87, rsPc = 0.939, P F2, 83 = 1.61, rsPc = 0.40, P > 0.3). Thus we conclude that artificial selection on immunocompetence affected sexual ornamentation independent of selection effects on body size or condition.
According to the immunocompetence hypothesis, the tradeoff between immunocompetence and sexual ornamentation is caused by a dual effect of testosterone on immune function and ornamentation (7). Other compounds could also play this role, and corticosterone has been suggested as an alternative (38), but previous work has shown that corticosterone level does not vary between the selection lines used in our study (39). Testosterone varied significantly between selection lines (F2, 77 = 4.93, rsPc = 0.99, P t50 = 3.08, P 2). Thus testosterone may regulate resource allocation to immune function and sexual ornamentation, as suggested by the immunocompetence handicap hypothesis.