Avian circannual systems: persistence and sex differences.

Birds show adaptation in their physiology and behavior to daily and seasonal periodicities in the environment. A circannual clock system gates seasonal events in birds to happen at the most appropriate time of the year, since a mistiming will delay the event until the arrival of the favorable time next year. The circannual clocks, the self-sustained endogenous system, are expressed under aperiodic conditions with a period close to 12months. In stonechats, it has been shown that circannual rhythms in testicular and molt cycles persist for 12years; this suggests that circannual clocks are functional throughout the life of an individual. Circannual rhythms are synchronized with both the photoperiodic and non-photoperiodic cues, and a synchronized circannual rhythm provides information on annual timing of the physiological event. The integration of rigid circannual timekeeping with the cyclic environment ensures phenotypic plasticity that is required for successful survival of a species in its habitat. There can be however sex differences in the circannual rhythm characteristics and in the relationship between circannual rhythms and external environment. In few cases that have been investigated, females appear playing a larger role in defining the breeding season in the year in relation to the environment. There is no evidence for the involvement of circadian clock in the generation of circannual rhythms. Therefore, future researches need to focus on finding molecular gears that possibly form the neuroendocrine loop and are translated into a seasonal event. Here, we briefly review limited information that is available on circannual rhythms and their relationship with the external environment from a few bird species, inhabiting tropical and temperate environments.

Rani S ，Kumar V 《-》

Avian circannual clocks: adaptive significance and possible involvement of energy turnover in their proximate control.

Wikelski M ，Martin LB ，Scheuerlein A ，Robinson MT ，Robinson ND ，Helm B ，Hau M ，Gwinner E ... -  《-》

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

Budki P ，Rani S ，Kumar V 《-》

Circadian rhythms are not involved in the regulation of circannual reproductive cycles in a sub-tropical bird, the spotted munia.

Circannual rhythms regulate seasonal reproduction in many vertebrates. The present study investigated whether circannual reproductive phenotypes (rhythms in growth of gonads and molt) were generated independently of the circadian clocks in the subtropical non-photoperiodic spotted munia (Lonchura punctulata). Birds were subjected to light:dark (LD) cycles with identical light but varying dark hours, such that the period of LD cycle (T) equaled 16 h (T16; 12 h L:4 h D), 21 h (T21; 12 h L:9 h D), 24 h (T24; 12 h L:12 h D) and 27 h (T27; 12 h L:15 h D), or to continuous light (LL, 24 h L:0 h D) at ~18°C. During the ~21 month exposure, munia underwent at least two cycles of gonadal development and molt; changes in body mass were not rhythmic. This was similar to the occurrence of annual cycles in reproduction and molt observed in wild birds. A greater asynchrony between circannual cycles of gonad development and molt indicated their independent regulation. Females showed reproductive rhythms with similar circannual periods, whilst in males, circannual periods measured between peak gonadal size were longer in T21 and T24 than in T16 or T27. This suggested a sex-dependent timing of annual reproduction in the spotted munia. Also, food availability periods may not influence the circannual timing of reproduction, as shown by the results on the rhythm in gonadal growth and regression in munia under T-photocycles and LL that provided differential light (feeding) hours. Further, a short-term experiment revealed that activity-rest patterns in munia were synchronized with T-photocycles, but were arrhythmic under LL. We conclude that circadian rhythms are not involved in the timing of the annual reproductive cycle in the spotted munia.

Budki P ，Malik S ，Rani S ，Kumar V ... -  《-》

Photoperiodic control of seasonality in birds.

Dawson A ，King VM ，Bentley GE ，Ball GF ... -  《JOURNAL OF BIOLOGICAL RHYTHMS》