Annual Gonadal Cycles and Pituitary Gonadotropins in Zonotrichia leucophrys gambelii

King, James; Follett, Brian; Farner, Donald; Morton, Martin

The annual cycles of gonadal weight and pituitary gonadotropic potency were investigated in free populations of Zonotrichia leucophrys gambelii in central Alaska (breeding) and in southeastern Washington (migrating or wintering). Some observations were made also on captives retained during the breeding season in outdoor aviaries in Washington. Prenuptial increase of testicular weight begins in mid-February in adults. In first-year birds testicular development begins significantly later. Ovarian enlargement also begins during February. Testicular maturation progresses essentially to completion during the period of migration, and testicular weight increases only slightly after arrival of the birds in the breeding area. Females have fully developed ovaries and oviducts at the time of their arrival in the breeding area about 15 days later than the first males. There is a minor increase in ovarian and oviducal weight to a maximum coinciding with the peak of oviposition in late May. This is followed by rapid regression of both ovary and oviduct. Testicular weight is maintained at a maximum until the broods were fledged in early July, and then decreases rapidly to minimal levels reached in early August. The onset of postnuptial molt in both sexes is correlated with gonadal regression. In captives retained outdoors in southeastern Washington, the testicular cycle is indistinguishable from that of the free-living birds in late winter and spring, but regression begins about two weeks earlier than in the natural population in Alaska. In captive females, ovarian development is arrested at a weight of about 40 mg, before the onset of significant follicular enlargement. Pituitary gonadotropin level is at an annual minimum in free-living birds during autumn migration. During winter, preceding detectable gonadal growth, the gonadotropin level approximately triples, suggesting a low level of synthesis with little or no release, and consequently a storage during this period. During the phase of accelerating gonadal growth in May, gonadotropin levels again approximately triple, presumably as a consequence of increase in both synthesis and release of the hormone(s). There is no additional increase in gonadotropic potency in birds in Alaska during the breeding season. In captive males, gonadotropic potency is the same as that of freeliving males in June; during testicular regression in captives in July the gonadotropin level is significantly decreased, indicating that the withdrawal of gonadotropic stimulus includes decrease of synthesis as well as cessation of hormone release. In captive females, pituitary gonadotropic potency is only about one-fourth that of reproductively active females in Alaska, suggesting that the reduced gonadotropic function in captivity involves a failure or partial failure of gonadotropic synthesis as well as release. Combined with the results of experimental investigations, our data show that testicular development to a fully functional status can be achieved by photostimulation, but that the period of testicular function is prolonged by stimuli associated with the breeding area. In females, in contrast, only the initial stages of ovarian development can be photoperiodically induced in cages. The full development of the ovary and oviduct does not occur with significant frequency in captivity. This gonadotropic blockade may result from fear or from deprivation of stimuli encountered during migration and in the breeding area.

Annual Gonadal Cycles and Pituitary Gonadotropins in Zonotrichia leucophrys gambelii

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