Endocrine changes in photostimulated willow ptarmigan ( Lagopus lagopus lagopus) and Svalbard ptarmigan ( Lagopus mutus hyperboreus)

Abstract

Changes in plasma luteinizing hormone (LH), testosterone, thyroxine (T 4), and triiodothyronine (T 3) and the height of supraorbital combs were compared in captive willow ptarmigan ( Lagopus lagopus lagopus) and Svalbard ptarmigan ( Lagopus mutus hyperboreus) exposed to an artificial annual cycle of daylength simulating that at 70°N. Plasma LH and testosterone and comb height increased more slowly in Svalbard than in willow ptarmigan as daylength increased. In both species, plasma LH and testosterone fell abruptly, and the supraorbital combs regressed in June, marking the development of long-day refractoriness. Comparison with free-living Svalbard ptarmigan ( K.-A. Stokkan, P. J. Sharp, and S. Unander (1986) Gen. Comp. Endocrinol. 61, 446–451) at 80°N, showed a similar slow increase in reproductive function before the onset of the breeding season. However, maximum plasma LH levels and comb size were higher in free-living than in captive Svalbard ptarmigan. Furthermore, long-day refractoriness developed earlier in captive than in free-living Svalbard ptarmigan. In both species of ptarmigan, the development of long-day refractoriness was associated with increased plasma prolactin. This increase was larger and occurred earlier in the Svalbard than in the willow ptarmigan. Seasonal changes in thyroid hormones were not as marked as for the other hormones measured. In both species, plasma T 4 tended to increase and plasma T 3 to decrease as daylength increased. A small increase in plasma T 3 was seen after the development of long-day refractoriness in both species. It is concluded that captivity depresses the photoperiodic response of Svalbard ptarmigan more than that of willow ptarmigan. The association between increased plasma prolactin and the development of long-day refractoriness is similar to that reported for birds in other families. The changes in plasma thyroid hormones during the development of refractoriness cannot be readily related to changes seen in other species.