A DIRECT COMPARISON OF PHOTOPERIODIC TIME MEASUREMENT AND THE CIRCADIAN SYSTEM IN EUROPEAN STARLINGS AND JAPANESE-QUAIL

The extent to which circadian rhythms are involved in photoperiodic ti me measurement in quail is enigmatic, and earlier investigations have produced results consistent with an hourglass clock or one involving d amped circadian oscillators. To address the problem further, the prese nt authors carried out a direct comparison between the clocks in quail and those in starlings. Starlings possess strongly self-sustaining ci rcadian oscillators. In Experiment 1, comparisons of testicular growth were made between the two species when birds were exposed to light:da rk (LD) 6:30, LD 6:18, and LD 18:6. Starlings grew their testes rapidl y under both LD 6:30 and LD 18:6, and they became photorefractory (und er LD 6:18, the testes remained undeveloped). Quail grew their testes rapidly under LD 18:6 but did not do so under LD 6:30 or LD 6:18. In E xperiment 2, entrainment of the activity rhythm under cycles of LD 6:3 0 was investigated by measuring the phase of the rhythm after release into constant darkness (DD). Birds were exposed to either 10 cycles or 11 cycles of LD 6:30 prior to DD. Starlings maintained their 24-h rhy thmicity under LD 6:30 and always free ran from the phase of the subje ctive day. By contrast, quail showed circadian activity approximately 24 h after every light pulse and free ran from the phase of the last l ight pulse received. In Experiment 3, phase response curves (PRCs) wer e generated to 6-h light pulses. The species were strikingly different ; starlings produced a Type 1 PRC, whereas quail produced a Type 0 PRC . More important, in quail the 6-h light pulse had the same effect reg ardless of circadian time and in almost every case activity free ran f rom the position of the 6-h light pulse. The results in quail are cons istent with the photoperiodic time measurement system being based on a weakly self-sustaining (rapidly damping) circadian system that is inv ariably reset by 6 h of light, whereas in starlings the pacemakers are strongly self-sustaining. The results support the notion that hourgla ss pacemakers can be highly damped circadian pacemakers.