Sexual selection in the zebra finch Taeniopygia guttata: condition, sex traits and immune capacity

The aim of this study was to test two hypotheses: (1) that condition-depend ent secondary sexual traits reflect an individual's immune capacity and (2) that immune capacity and secondary sexual traits covary with primary sex t raits, specifically ejaculate quality. We used the zebra finch Taeniopygia guttata as a study species, since the traits that females find attractive i n males of this species, song rate and beak colour, are well established. A paired experimental design comprising 31 pairs of brothers was used; for e ach pair, one male was assigned to a control group provided with ad libitum food and no additional exercise, and the other male was assigned to an exp erimental group which experienced additional exercise and a reduced rate of food intake. After Il weeks, the experimental group differed significantly from the control group in a range of variables, including body mass, haema tocrit, granulocyte:lymphocyte (G:L) ratio and several primary sex traits, indicating that condition in this group was reduced. Birds in the experimen tal group showed a differential response to the treatment. We used the rank order in which birds could be captured by an experimenter as an index of c ondition. Birds easily caught were assumed to be in poorer condition than t hose which were more difficult to capture. Rank capture order was repeatabl e and was significantly correlated with the G:L ratio in the experimental g roup, but not in the control group. In the experimental group, rank capture order was correlated significantly with both secondary sex traits: birds i n better condition had redder beaks and a higher song rate. However, beak c olour and song rate did not covary significantly, suggesting that these two traits provide different types of information. Secondary sex traits did no t covary with primary sex traits or any sperm features. Thus, there was no evidence for Trivers' sexual-competence hypothesis or the phenotype-linked fertility hypothesis. We used four assays of immune capacity, two general ( G:L ratio and spleen mass) and two specific [antibody titres to sheep red b lood cells (SRBCs) and Brucella abortus (BA)I. The G:L ratio was significan tly higher in the experimental group, spleen mass (absolute and relative) d id not differ between the groups, anti-SRBC antibody titres were significan tly higher in the control group (contrary to expectation), and anti-BA anti body titres were close to being significantly lower in the experimental gro up. Within the experimental group, there was no evidence that antibody titr es covaried with secondary sex traits. Although we demonstrated that beak c olour and song rate were condition dependent, our experiment provided no ev idence that either of these traits covaried with immune capacity or sperm f eatures.