Effect of dominance status on sex hormone levels in laboratory and wild-spawning male trout.

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 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-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 17,20 beta-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.

Androgen control of social status in males of a wild population of stoplight parrotfish, Sparisoma viride (Scaridae).

In the protogynous stoplight parrotfish (Sparisoma viride), large males defend territories that encompass the home-ranges of several mature females. However, high-quality habitat is in short supply, such that smaller, competitively inferior males do not defend territories. We investigated the role of 11-ketotestosterone (11KT) and testosterone (T) in the regulation of territorial behavior in a wild population of a protogynous reef fish, the stoplight parrotfish, at Glover's Reef, Belize. Radioimmunoassay of plasma samples from individuals of known social status revealed that nonterritorial males have lower levels of T and 11KT than territorial males. Nonterritorial males allowed access to vacant territories underwent pronounced increases in T and 11KT. When sampled 1 week after territory acquisition, levels of T and 11KT in new territorial males were significantly higher than the levels in established territorial males, but by 3 weeks after territory acquisition, there was no significant difference. We further investigated the hypothesis that such short-term increases in androgen levels are a response to intense male-male interactions during territory establishment. Simulated territorial intrusion promoted increased plasma levels of both T and 11KT while access to vacant territories without neighboring territorial males did not. These findings suggest that the endocrine system plays a role in fine-tuning the levels of territorial aggression exhibited by male stoplight parrotfish. We discuss these results in light of recent theory in behavioral endocrinology.

Hormonal control of sex and color change in the stoplight parrotfish, Sparisoma viride.

The stoplight parrotfish, Sparisoma viride, changes sex from female to male. In most cases, sex change is accompanied by dramatic change in coloration, from the female-like "initial phase" coloration to "terminal phase" coloration that is associated with males. However, some males do not change color at the same time they change sex, becoming female-mimic males (termed initial phase males). Using radioimmunoassays validated here for use with plasma from S. viride, we examined the hormonal profile of stoplight parrotfish undergoing sex change. As predicted, females were characterized by undetectable levels of 11-ketotestosterone (11KT), moderate levels of testosterone, and high levels of 17 beta-estradiol (E2). Fish that were found histologically to be undergoing sex change had elevated levels of 11KT and decreased levels of E2. Testosterone appeared to be unaffected. Males had the highest levels of 11KT and testosterone and low levels of E2. Because initial phase males eventually change color, we were able to take blood samples from males (sex confirmed histologically) before, during, or after color change. Thus, we also examined the hormonal correlates of color phase change. Initial phase males, like females, had undetectable levels of 11KT, moderate levels of testosterone, and significantly higher levels of E2 than either terminal phase males or males with transitional coloration. During color transition, 11KT levels rose sharply and levels of E2 declined. Males with terminal phase coloration had the highest levels of 11KT and testosterone and low levels of E2. Finally, administration of 11KT induced precocious sex change and color change in functional adult females. These findings suggest that 11KT plays a key role in instigating natural sex and color change, but that once sex change is complete, 11KT is not necessary for spermatogenesis or spawning behavior.