THE SIGNIFICANCE OF CIRCADIAN BASED PHOTOSENSITIVITY IN THE HOUSE SPARROW PASSER DOMESTICUS

Adult male House Sparrows, wild caught in December and January, were held on daily seven-hour artificial photoperiods for 14 days beginning in late February, with the aviary lights arranged to switch-on at 07.00 hrs. A control sample was then killed and the remaining birds divided into groups which received six hours of light daily (beginning at 07.00 hrs) plus one further hour of light given at variable times from the dawn. A further group received 16 hours of light given continuously (16L-8D). Pooled plasma samples from these groups were assayed for luteinizing hormone (using a radio-immunoassay technique) and the histological condition of all testes was examined. Although all subjects received the same total amount of photostimulation only those given schedules simulating long days responded with testicular growth and spermatogenesis. It is argued that this was because light can only be effective if received during relatively restricted phases of a circadian-based rhythm; only during these phases is the neuroendocrine apparatus sensitised to photostimulation. Increase in size of the testis tubules and meiotic divisions of primary spermatocytes, leading to secondary spermatocyte production and increase in testis weight, were considered to be indicators of FSH activity. Using these criteria a curve of FSH activity could be drawn and be compared with the quantitative curve for LH levels obtained from the plasma assays (a comparable assay technique for FSH is not yet available). Although LH levels remained approximately the same in all groups, and this correlated with the histological and histochemical evidence, FSH activity was more pronounced in those groups given light flashes at increased intervals after the beginning of the dark period. Thus LH activity occurred in the absence of spermatogenetic development in those groups given “short-day” schedules. Birds kept on a 16L-8D regime contained less plasma LH than anticipated and the possible existence of inhibitory mechanisms is discussed. It is feasible that when much FSH secretion begins LH production declines, but this explanation is speculative. The temporal pattern of LH production was markedly different in Greenfinches (for which the experimental results are described elsewhere) than in the House Sparrows, and the discussion considers the reasons for these differences in terms of the ecological and ethological adaptations of the two species.