Manipulation of photo-refractoriness in the house sparrow Passer domesticus by circadian light regimes

Abstract

Three groups of house sparrows, Passer domesticus captured at the end of the breeding season when gonad regression was in progress, were held for 35 days on daily light cycles of 6L-18D, with a one hour light pulse given as an interruption of the dark period. The light pulse was given at different times in the different groups and at the end of this treatment half the specimens in each were killed. All testes were seen to be fully regressed with only spermatogonia present in the seminiferous tubules, and, furthermore, all birds contained dense amorphous tubule lipids. Although the spermatogenetic condition was similar in all three groups, the interstitial condition differed. In groups B (light pulse 12-hr from ‘dawn’) and C (light pulse 16-hr from ‘dawn’) the testis had acquired an expanded interstitium of juvenile Leydig cells lacking any lipid deposits, whereas group A (light pulse 8-hr from ‘dawn’) had a smaller interstitium which was becoming heavily lipoidal. Surviving members of each group were now given continuous 16-hr photoperiods (16L-8D) for a further 34 days. Group A responded with complete spermatogenesis, but in groups B and C the testes remained regressed, although the tubules in group C, but not group B, had become clear of lipids. The significance of these results is discussed and it is speculated that avian refractoriness may involve varying FSH LH titres consequent on their release being dependent on different photosensitive phases of a circadian rhythm. Such differential gonadotropin release has already been proved to occur during the period of post-refractory gametogenetic recovery.