The response of the hypothalamo-gonadal system to environmental factors in the zebra finch, Poephila guttata castanotis

Summary

Studies of the hypothalamo-gonadal response of the zebra finch (Poephila guttata castanotis) to environmental factors were conducted by the use of adult birds captured in arid regions of South Australia. In these birds breeding activity is not photoperiodically induced but follows rainfall. The hypothalamo-hypophysial system participating in this response involves centres of the rostral and mediobasal (tuberal) hypothalamus. Parvocellular neurones in both hypothalamic portions are arranged in clusters. In the rostral hypothalamus magnocellular neurosecretory perikarya are spatially closely associated with the clusters which contain different types of parvocellular peptidergic neurones. Extended, complex neuropile formations, rich in synapses, encompass the neuronal clusters and provide important loci of neural-neuroendocrine interaction.

Experiments were undertaken to correlate indicators of breeding activity, such as increased size and mass of testes, spermatogenesis, and plasma levels of androgens and luteinizing hormone, with a greater availability of drinking water, increased air humidity, and green vegetation, all of which follow rainfall. Birds maintained in male/female pairs in controlled-environment rooms at 27° C on 14L10D received a diet of seeds (containing ∼ 10% water). Thirty-two birds were given drinking water at a basal level of 1 ml/week and one of eight different treatments, consisting of different combinations of drinking water, relative air humidity, and green grass, for two weeks (4 males/treatment). The size of the left testis was estimated at laparotomy and blood samples were taken from a wing vein before and after treatment. The observations suggest that, in moisture-deprived (1 ml water/week) adult zebra finches, an increase in testicular size is associated with a free supply of water; synergistic effects of supplying both water and green grass, or water and high humidity, were also observed. However, in six other groups (8–16 males/treatment), receiving a higher basal level of water (access for 3 min/day), an increase in testicular mass was associated with high humidity, while smaller non-significant increases followed the provision of free water or green grass. Spermatogenic activity was not significantly correlated with testicular size or mass. Leydig cells were sparse in all groups. Plasma androgen levels and luteinizing hormone levels were not obviously correlated with the treatments and were lower than values in zebra finches breeding in outdoor aviaries.