We investigated the relationship between male social status and hormone levels in salmonids spawning under laboratory and field conditions. In small groups of rainbow trout (Onchorhynchus mykiss) spawning in the laboratory, dominant males had higher plasma levels of testosterone (T) and 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P) compared with subordinates. Steroid levels increased in subordinate males that became dominant after dominant males were experimentally removed; higher steroid levels in dominant males appears to be a result rather than a cause of their social status. In free-ranging brown trout (Salmo trutta) sampled in the field, we found higher levels of 11-ketotestosterone (11KT) but not T in dominant males. No significant differences in levels of either androgen were found between dominant and subordinate male brook trout (Salvelinus fontinalis) sampled at the same field location. Furthermore, in marked contrast with the laboratory fish, there were no significant differences in plasma 17,20β-P between dominant and subordinate males in either species of fish in the wild. The different findings in the laboratory and field may indicate species differences in behavioral endocrinology among brook, brown, and rainbow trout. Alternatively, the greater differential in hormonal profile of dominant and subordinate males in the laboratory may reflect the relative uniformity of the laboratory environment; this simple environment may allow competitively superior males to more completely dominate less competitive tank-mates and to exclude them from female sexual cues. In any case, these results suggest that the relationship between steroid hormones and spawning behavior in male salmonids is likely more complex than suggested by experiments conducted solely on laboratory-held rainbow trout.
Bibliographical noteFunding Information:
The authors thank Mr. L. Lemke of the Fisheries branch of the British Columbia Ministry of the Environment for providing access to fish at the Pennask River Collecting Station, Mr. Charles Bell and the Belwin Foundation for granting us access to Valley Creek, Mr. S. McAdam and Mr. D. Johnson for assistance in collecting samples and observing fish, Dr. J. G. Dulka for assistance in extracting blood samples, and Dr. N. E. Stacey for kindly commenting on the manuscript. Partial support for this project was provided by N.S.E.R.C., the Minnesota Department of Natural Resources, and the Minnesota Agricultural Experiment Station (contribution No. 21,999).