REPRODUCTIVE ENDOCRINOLOGY AND MECHANISMS OF BREEDING INHIBITION IN COOPERATIVELY BREEDING FLORIDA SCRUB JAYS (APHELOCOMA-C COERULESCENS)

Although the ecological and evolutionary bases of cooperative breeding have received close scrutiny, few studies have explored the physiological mechanisms responsible for delayed breeding by helpers. We chose the Florida Scrub Jay, Aphelocoma c. coerulescens, to determine whether breeders and nonbreeders have different hormone profiles during the reproductive season. We found that male nonbreeders had significantly lower testosterone levels and higher progesterone levels than did male breeders. Nonbreeder and breeder males had similar plasma levels of corticosterone. Elevated plasma levels of progesterone in nonbreeder males may interfere with the reproductive activity at the behavioral or physiological level. Alternatively, progesterone may enable these nonbreeding males to express parental behavior in the absence of a parent/offspring relationship, but the occurrence of the highest levels of progesterone during nest building and egg laying supports the former interpretation. In females, we found that nonbreeders had significantly lower levels of estradiol and corticosterone than breeders. The low levels of primary sex steroid hormones in male and female nonbreeders may physiologically and behaviorally inhibit reproductive activity and suggest that these individuals are reproductively incompetent. The absence of high levels of corticosterone in nonbreeders suggests that this reproductive inhibition is not due to glucocorticoid-mediated stress imposed by dominant breeders.