A comparison of the responses of captive willow ptarmigan ( Lagopus lagopus lagopus), red grouse ( Lagopus lagopus scoticus), and hybrids to increasing daylengths with observations on the modifying effects of nutrition and crowding in red grouse

Abstract

The photoperiodic responses of two races of Lagopus lagopus from different latitudes and of hybrids between them were compared under the same lighting conditions. The captive birds were descended from northern Norwegian willow ptarmigan ( Lagopus lagopus lagopus) and Scottish red grouse ( Lagopus lagopus scoticus). Under natural conditions, the red grouse begin to lay eggs 3 weeks earlier than the willow ptarmigan. Photosensitive birds were transferred from an 8-hr daylength to a lighting regime in which the daylength was increased by 1 hr per week for 11 weeks. At the start of the study, the red grouse had greater LH levels and comb sizes than the wilow ptarmigan or the hybrids. The critical daylength needed to stimulate LH secretion was less than 12 hr in both races but it was not clear whether it was greater in the willow ptarmigan than in the red grouse. However, the races differed quantitatively: the willow ptarmigan had lower LH levels at 12, 13, and 14-hr photoperiods than did the red grouse or the hybrids. In all the males, comb heights increased significantly ( P < 0.05) after the photoperiod was increased to 12 hr and reached their maximum in the red grouse, hybrids, and willow ptarmigan at photoperiods of 13, 14, and 16 hr, respectively. The female red grouse, hybrids, and willow ptarmigan laid their first eggs after the photoperiod was increased to 15, 17, and 19 hr, respectively. At the end of the study the red grouse but not the hybrids or willow ptarmigan were becoming photorefractory. A low-protein diet did not alter the timing of the onset of seasonal breeding in captive female red grouse although it did cause a reduction in the rate of lay and egg weight. The onset of seasonal breeding was delayed, however, when the birds were crowded. It was concluded that in L. lagopus, the photoperiodic response seems to be determined by inherited factors. Further, the absence of a clear difference between the critical daylengths in red grouse and willow ptarmigan raises the possibility that differences in the timing of the onset of seasonal breeding in these races may be caused by modifications in the neural or endocrine pathways “downstream” from the biological clock.