Interaction of lighting and other environmental variables on activity of hypothalamo–hypophyseal–gonadal system

Abstract

SINCE the pioneer study of Rowan¹, environmental lighting has been considered a predominant regulator of breeding activity in a variety of birds. A long daily photoperiod increases the levels of stored and circulating gonadotropins, and accelerates gonadal growth in Japanese quail^2,3, white-crowned sparrow⁴, American tree sparrow⁵, domestic pigeon⁶ and mallard⁷. Other environmental variables such as temperature, rainfall and courtship are also known to influence breeding⁸. The nature of the endocrine response to temperature and rainfall is unknown, but the effect of male courtship on female endocrine systems has been studied extensively in ring doves (Streptopelia risoria). It increases the plasma levels of oestrogen⁹, luteinising hormone¹⁰ (LH) and progesterone¹¹, which in turn, induce sequential behavioural changes in the female that culminate in egg laying^12,13. Using ring doves, I have investigated how environmental light and courtship interact to elicit the species-specific endocrine response. Since there is no significant change in testis size or weight during various stages of the breeding cycle of doves, this is not a suitable criterion for assessing the activity of the male endocrine state. Instead I used compensatory testicular hypertrophy, which is a response to environmental lighting in many species of birds, and investigated whether long daily photoperiod was required for this phenomenon in male ring doves, and whether courtship had any effect on this response.