Persistence of circannual rhythms under constant periodic and aperiodic light conditions: sex differences and relationship with the external environment

Abstract

Summary The timing and duration of gonadal phases in the year indicates that breeding cycles are regulated by endogenous mechanisms. The present study on tropical Spotted Munia (Lonchura punctulata) investigates whether such mechanisms are based on circannual rhythms, and whether circannual rhythms between sexes differ in their relationship with the light environment. Birds were subjected to 12 h light per day (12L:12D), alternate days of light and darkness (24L:24D, LL/DD) and continuous light (LL), with L= 22 lux and D = <1 lux, for 28 months (mo) at constant temperature (18±1°C). Groups kept on natural day lengths (NDL) served as controls. Measurement of body mass, gonads, and molts of the primary wing feathers and body plumage at regular intervals showed that birds underwent repeated cycles in gonads and molt, but not in the body mass. In NDL, gonadal phases in both sexes cycled with 12 mo periods. In other conditions, males cycled with similar periods of about 11 mo, but females cycled with relatively large period variations, about 10 to 13 mo. Gonadal recrudescence – regression phase was longer in males than in females and, in both sexes, in the second year as compared to the first year. The molt of wing primaries was more closely coupled to gonadal maturation in groups on NDL and 12L:12D than in groups on LL and LL/DD, but this relationship drifted apart in the second year. Body plumage molts were relatively more highly variable in both the frequency and pattern. It is suggested that annual breeding cycle in spotted munia is regulated by the self-sustained circannual rhythms, which probably interact with the annual photoperiodic cycle to synchronize breeding cycles to calendar year. Both sexes seem to have independent timing strategies, but females appear to share a greater role in defining the reproductive season in relation with the environment.