| Abstract: | The blood androgen levels of both high- and low-ranking swordtail males show a reduction to one third of initial levels after social isolation but are returned to normal following a 20-minute exposure to a small male of the same species behind a transparent partition (standard-opponent test). Experiments to determine the cause of this effect revealed that the social contact involved in the test was not responsible, but that rather the presence of the fish in a new environment (the test-aquarium) for 20 hours itself sufficed to restore the normal androgen concentrations. The blood corticoid levels of both high- and low-ranking males are also reduced to one third of initial levels by social isolation. The normal level of this hormone was, however, restored following a standard-opponent test only in the case of the high-ranking males; the corticoid levels of the low-ranking males remaining depressed. Transfer to a new environment in itself did not account for the effect on the high-ranking males. This result suggests that the pituitary-adrenal systems of high- and low-ranking males are differentially responsive to the social situation represented by the standard-opponent test. |
