Postnatal growth rate and gonadal development in circadian tau mutant hamsters reared in constant dim red light

The role of the circadian clock in the reproductive development of Syrian hamsters (Mesocricetus auratus was examined in wild type and circadian tau mutant hamsters reared from birth to 26 weeks of age under constant dim red light. Testis diameter and body weights were determined at weekly intervals in male hamsters from 4 weeks of age. In both genotypes, testicular development, subsequent regression and recrudescence exhibited a similar time course. The age at which animals displayed reproductive photosensitivity, as exhibited by testicular regression, was unrelated to circadian genotype (mean +/- SEM: 54 +/- 3 days for wild type and 59 +/- 5 days for tau mutants). In contrast, our studies revealed a significant impact of the mutation on somatic growth, such that tau mutants weighed 18% less than wild types at the end of the experiment. Our study reveals that the juvenile onset of reproductive photoperiodism in Syrian hamsters is not timed by the circadian system.     

The effect of brief light pulses on the photoperiodic reaction in the Djungarian hamsterPhodopus sungorus

Summary The effects of brief light pulses on photoperiodic reaction were examined in male Djungarian hamsters (Phodopus sungorus). Half of the animals came from long photoperiods and, accordingly, were in physiological summer state with large and functional testes. The other half had lived in short natural photoperiods before and was in physiological winter condition with involuted testes. These animals were exposed to long (LD 168) or to short (LD 816) photoperiods, or to short photoperiods in which the 16 h of darktime were interrupted by 1 min of light each night, either 4, 8 or 12 h after light off.Light pulses at midnight had the same effect as long photoperiods, they maintained gonadal size and activity or caused rapid recrudescence. If the light pulses were applied 4 or 12 h after light-off, however, a marked difference in effect ensued, depending on previous state and light regime. In animals coming from long photoperiods, these schedules induced regression, while in hamsters coming from short photoperiods the same schedules stimulated recrudescence, though significantly less than full long photoperiods or schedules with light-interruptions at midnight. Changes in body weight and pelage colour closely paralleled the changes in gonadal state.     

Termination of gonadal refractoriness in Turkish hamsters, Mesocricetus brandti

The Turkish hamster (Mesocricetus brandti) is a photoperiodic species. In this investigation, we characterized the photoperiodic requirements for termination of gonadal refractoriness, defined as the inability of the animal to respond to short-day treatment with gonadal regression. Paired testes weights were reduced to less than 20% of their original weight by 10 wk of 12L:12D treatment. This was followed by spontaneous testicular recrudescence (completed by Week 25 of 12L:12D treatment), the overt indication of refractoriness to short photoperiods. Next, the period of long-day exposure sufficient for termination of refractoriness was determined. Refractory males were exposed to 16L:8D for 5 to 20 wk. Ten weeks of 16L:8D treatment was enough for the animals to regain the sensitivity to a second challenge of 12L:12D treatment. Fifteen weeks of 20L:4D or 16L:8D terminated refractoriness in female Turkish hamsters; 20L:4D therefore was not interpreted as a short day by refractory hamsters. This was unexpected because in photosensitive animals this photoperiod acts like a short day, causing gonadal regression. These results suggest that Turkish hamsters are similar to Syrian hamsters in that both species require two or more months of long days in summer to recover sensitivity to the short days of the following fall.     

Effects of age and photoperiod on reproduction and the spleen in the marsh rice rat (Oryzomys palustris)

To examine the interactions between age and photoperiod on reproduction and spleen weights, we exposed adult male and female rice rats of various ages to photoperiods of 16:8-h light-dark photoperiods (16L:8D) or 12L:12D. After 10 wk, animals were killed and the following data were recorded: weights of testes, seminal vesicles, uterus, ovaries, body, and spleen and, in addition, vaginal patency. Young adult males displayed a greater degree of testicular and seminal vesicle regression in short photoperiods than did older males; the testes of most older males did not regress in response to short photoperiods. Spleen weight was unresponsive to short photoperiods in all males, but was affected by age. Females, however, exhibited reproductive organ regression and decreased vaginal patency in response to short photoperiods at all ages examined. Body weights were affected by photoperiod in young females, and, as in males, photoperiod had no effect on spleen weights. These data suggest that the reproductive response to photoperiod in adult male rice rats declines with age, whereas in adult females it does not.     

Effects of gonadal steroid hormones on hibernation in the Turkish hamster (Mesocricetus brandti)

Summary The Turkish hamster (Mesocricetus brandti) is a photoperiodic species in which exposure to a short day photoperiod induces gonadal atrophy. When hibernating male Turkish hamsters were administered testosterone via subcutaneous silastic capsules hibernation was abruptly terminated. The threshold serum testosterone level required to prevent hibernation was intermediate between the levels observed in males with fully-regressed and fully-functional testes. Dihydrotestosterone was as effective as testosterone in blocking hibernation. Estradiol had a partial inhibitory effect. Progesterone and corticosterone implants had no effect on hibernation. Similar results were obtained in female hamsters. The total period of hibernation for untreated males under laboratory conditions was usually 5–6 months. In most animals testicular growth began to occur toward the end of that period. As gonadal growth continued bouts of torpor decreased in frequency and duration, and hibernation was terminated when the testes approached reproductive size. Castrated males continued to hibernate much longer than intact animals. These observations suggest that in nature decreasing day length in the fall initiates gonadal regression which is followed by cold-induced hibernation. In the spring spontaneous recrudescence of the testes may lead to termination of the dormancy period. This mechanism may enable the species to prepare for the reproductive season with maximum efficiency.Supported by NIH Research Grant HD-10478 and by a grant from the Connecticut Research FoundationThe data in this paper were presented in part at the Annual Meeting of the American Society of Zoologists, Toronto, December 26–30, 1977     

Testicular apoptosis is down-regulated during spontaneous recrudescence in white-footed mice (Peromyscus leucopus).

Among individuals of many nontropical species, seasonal breeding is timed by tracking changes in the daily photoperiod. Transfer of rodents to short (< 12 h of light/day) day lengths for 6 to 14 weeks can induce regression of the testes mediated by apoptosis. After 16 to 20 weeks of short day exposure, reproductive function is "spontaneously" initiated, and testicular recrudescence is observed. The gonadal mechanisms that underlie testicular recrudescence are not fully understood. If the onset of testicular regrowth that occurs during spontaneous recrudescence reflects a down-regulation of apoptotic signals, then a decline in apoptosis should be noted concurrent with increased testis mass. This experiment sought to assess the role of apoptosis in the restoration of reproductive capacity to photoperiod-inhibited white-footed mice. Males were assigned to long (16:8 LD) or short (8:16 LD) photoperiods for 0, 14, 18, 22, 26, or 30 weeks. At each of these time points, testis mass and testosterone concentrations were assessed. In addition, apoptotic activity was measured using both in situ terminal deoxynucleotidyl transferase dNTP end labeling (TUNEL) and DNA laddering. Short photoperiod exposure induced maximal decreases in testicular parameters after 14 weeks (p < 0.05). After 26 weeks of short days, testis mass was no longer different between males housed in long days and those housed in short days. In contrast, the high incidence of apoptotic TUNEL labeling and DNA laddering observed at 14 weeks was reduced to long day values after 22 weeks of short day exposure. Together, our results establish that a decrease in testicular apoptosis coincides with testicular recrudescence in white-footed mice. The current study demonstrates a decline in the incidence of testicular cell death concomitant with changes in testis mass or length, elucidating a timeline of changes at the cellular level related to the onset of recrudescence.     

Timing of testicular recrudescence in siberian hamsters is unaffected by pinealectomy or long-day photoperiod after 9 weeks in short days

In this study, the authors asked whether pinealectomy or temporary exposure to a stimulatory photoperiod affects the timing of spontaneous testicular recrudescence in adult Siberian hamsters chronically exposed to short days (9:15 light:dark). In Experiment 1, hamsters were pinealectomized after 6, 9, or 12 weeks in short days. Pinealectomy after 9 or 12 weeks did not affect the timing of spontaneous gonadal growth (27.7 +/- 1.9 and 25.4 +/- 1.3 weeks, respectively) compared to sham-operated controls (28.6 +/- 0.9 weeks). Enlarged testes occurred earlier in animals that were pinealectomized after 6 weeks in short days (21.8 +/- 2.1 weeks). In Experiment 2, adult hamsters were exposed to short days for 9 weeks, transferred to long days (16:8 light:dark) for 4 weeks, and then returned to short days for 23 additional weeks. Although long-day interruption caused gonadal growth in 15 out of 19 hamsters, the temporary long-day exposure did not affect the timing of spontaneous gonadal growth following return to short days (28.2 +/- 0.9 weeks) in 10 of the 15, relative to the timing observed in control hamsters continuously maintained in short days (28.2 +/- 1.1 weeks). Four out of 19 hamsters did not show gonadal growth following long-day exposure. Spontaneous gonadal growth in these hamsters (28.0 +/- 1.4 weeks) also occurred at the same time as controls. The remaining 5 hamsters exhibited enlarged testes following long-day exposure (12.0 +/- 0.0 weeks) but were refractory to the second short-day exposure. All hamsters exhibited entrainment of wheel-running activity following the change in photoperiod. A final group of 13 animals were pinealectomized before long-day transfer. They exhibited gonadal growth (at 17.2 +/- 0.8 weeks) but failed to regress a second time when returned to short days. The timing of gonadal growth in these animals was delayed relative to the sham-operated hamsters temporarily transferred to long days (Experiment 2) but accelerated relative to the hamsters pinealectomized at 9 weeks, which remained continuously in short days (Experiment 1). The results of both experiments suggest that a pineal-independent process mediates the timing of spontaneous gonadal growth in Siberian hamsters chronically exposed to a short-day photoperiod.     

Photoperiodic and pineal effects on food intake, food retrieval, and body weight in female Syrian hamsters

Food intake, food retrieval, and body weight changes were measured in female Syrian hamsters maintained under long and short photoperiods, and in animals sustaining pinealectomy or sham surgery. Animals maintained under short photoperiods gained more weight, ingested more food, and brought more food from more distant compartments into their nest compartments than did animals maintained under long photoperiods. Pinealectomy prevented the short-day elevations in body weight and food retrieval during the period of gonadal recrudescence, but it did not produce significant changes during the period of gonadal regression; in contrast, pinealectomy prevented the short-day increases in food intake only during initial weeks of the period of gonadal regression: The effects of photoperiod or pinealectomy were not evident for any of the dependent measures during pregnancy. The role of the pineal gland in mediating short-day effects is discussed.     

Testicular response to melatonin or suprachiasmatic nuclei ablation in the spotted skunk

Testes of the Western spotted skunk enlarge and regress seasonally. The pineal hormone, melatonin, may be important in timing this seasonal reproductive activity. Likewise, the suprachiasmatic nuclei (SCN) have been implicated as possible neural regulators of pineal and reproductive events. These experiments were conducted to determine whether ablation of the SCN or constant administration of melatonin alters timing of the seasonal pattern of testicular regression and recrudescence. Male skunks (n = 24) were treated as follows: six received two empty Silastic capsules, six received two melatonin-filled Silastic capsules, six received sham lesions to the SCN, and six received lesions to the SCN (SCNx). All skunks were exposed to a natural photoperiod and had regressed testes at the onset of the experiment. Four of six males from the SCNx group had an average of 94 +/- 11.3% of these nuclei destroyed. Sham SCNx, animals with less than 40% of the SCN ablated, and males with empty capsules did not have fully enlarged testes until October. SCNx and melatonin-treated skunks exhibited a hastening of testicular recrudescence with maximal testis size being reached in June. Skunks with lesions to the SCN maintained enlarged testes for 5 months while all other groups exhibited rapid regression after attaining maximal testis size. Testicular regression occurred from July through October in animals receiving continuous melatonin, while controls exhibited recrudescence during this same period. Our data suggest that the SCN, melatonin, and thus the pineal gland, play a role in regulating the seasonal testicular cycle of the spotted skunk.     

A plastic interval timer synchronizes pubertal development of summer- and fall-born hamsters

Summer and fall decreases in day length induce reproductive regression in adult hamsters and delay reproductive maturation of their young. The following year pubertal development is triggered by an interval timer (IT) that renders animals refractory to inhibitory short day lengths after approximately 25 wk. Timing of gonadal and somatic development was examined among offspring born to Siberian hamsters in early-August vs. late-September day lengths. Pubertal maturation was delayed in both groups until late winter. Gonadal growth occurred at significantly later ages among August- vs. September-born males as did late-winter spurts in ponderal growth of both sexes. Timing of reproductive and somatic development depended on postnatal rather than prenatal photoperiod exposure and was unrelated to the circadian entrainment status of dams. When developmental patterns were assessed in relation to time of year, group differences were largely eliminated. Because the IT triggers these developmental events, its duration must be plastic. This plasticity facilitates a relative synchronization or entrainment of developmental milestones in hamsters born into different late-summer/early-fall photoperiods.     

Photosensitivity in body mass and testicular activity of brahminy myna, Sturnus pagodarum

This study analyzed photoperiodic sensitivity of brahminy myna (Sturnus pagodarum), which is a seasonally breeding bird species. During regression phase of the reproductive cycle (in early September), groups of myna were exposed to artificial photoperiods that were either close to or longer than those brahminy myna experiences at this time in wild at 29 degrees N. Following a 14-week exposure to such photoperiods (hours of light: hours of darkness; 13L:11D, 12L:12D, 11.5L:12.5D and 11L:13D), the birds were subjected to a longer day length (16L:8D) for another 9 weeks to test whether pre-treatment with varying photoperiods had an effect on subsequent long day photostimulation. There was a progressive increase in body mass under different pre-treatment photoperiods, with a faster increase in 11L:13D and 11.5L:12.5D than in 12L:12D and 13L:11D. When subjected to 16L: 8D, however, all groups showed decline in body mass. By contrast, the testes were not stimulated under 11L:13D and 11.5L:12.5D, and only one individual of the groups under 12L:12D and 13L: 11D showed a small testis recrudescence. All birds except one individual of the 13L:11D group, however, showed testis recrudescence when subjected to 16L:8D. These results suggest that body mass and testes have a different profile of photoperiodic response and appear to have two different threshold photoperiods for dissipating the post-reproductive refractoriness.     

Testicular recrudescence in intermediate day lengths reflects loss of photoperiodic memory in Siberian hamsters

Siberian hamsters transferred from a long (16 h light/day [16 L]) to an intermediate (13.5 L) day length (DL) undergo testicular regression within 2 months followed approximately 2 months later by "spontaneous" testicular recrudescence. Recovery of gonadal function after prolonged exposure to intermediate DLs is thought to reflect development of neuroendocrine refractoriness to intermediate-duration melatonin signals. The authors tested the alternative hypothesis that testicular recrudescence in 13.5 L occurs when the "memory" for the 16-L photoperiod fades and hamsters can no longer compare the 13.5-L to the prior 16-L day length. Adult hamsters transferred from 16 L to 13.5 L that underwent testicular involution were either maintained continuously in 13.5 L for 41 weeks or given a supplementary 2-week treatment of 16 L before being returned to 13.5 L. The supplementary treatment was administered either after hamsters had been in 13.5 L for 10 weeks and had involuted testes, or after 24 weeks, when the gonads had undergone recrudescence. The authors found that 16 L treatment administered at week 10 delayed final gonadal recrudescence by approximately 12 weeks; similar 16-L treatment at week 24 induced a second gonadal regression when animals were returned to 13.5 L. The most parsimonious hypothesis to account for these findings is that gonadal recrudescence in intermediate DLs reflects fading of the "memory" for prior long DLs rather than induction of refractoriness to melatonin signals generated in intermediate DLs.     

Refractoriness to the antigonadotrophic effects of melatonin in male hamsters and its interruption by exposure of the animals to long daily photoperiods

Exposure of adult male Syrian hamsters to artificially shortened photoperiods in the laboratory was followed by gonadal involution. Return of these animals to long days (light:ddark cycles of 14:10) resulted in regrowth of the reproductive organs, following which the animals were refractory to the normally antigonadotrophic effects of daily melatonin injections given late in the afternoon. When hamsters were kept under naturally shortening photoperiods beginning on October 19, their reproductive organs involuted and remained infantile until the following spring when they again recrudesced to adult size. These animals too were refractory to the inhibitory effects of daily afternoon melatonin injections. The refractory period to melatonin was interrupted by exposure of the animals to long days for 22 weeks.     

Spontaneous "regression" of enhanced immune function in a photoperiodic rodent Peromyscus maniculatus.

Short days inhibit reproduction and enhance immune function in deer mice (Peromyscus maniculatus). Their reproductive inhibition is sustained by an endogenous timing mechanism: after ca. 20 weeks in short days, reproductive photorefractoriness develops, followed by spontaneous recrudescence of the reproductive system. It is unknown whether analogous seasonal timing mechanisms regulate their immune function or whether enhanced immune function is sustained indefinitely under short days. In order to test this hypothesis, we housed adult male deer mice under long (16 h light day(-1)) or short (8 h light day(-1)) day conditions for 32 weeks or under long day conditions for 20 weeks followed by 12 weeks of short days. Mice under the long day conditions remained photostimulated over the 32 weeks, whereas mice housed under the short day conditions exhibited gonadal regression followed by photorefractoriness and spontaneous recrudescence. Mice transferred to short days at week 20 were reproductively photoregressed at week 32. Total splenocytes, relative splenic mass and mitogen-activated splenocyte proliferation were greater in those mice transferred to short days at week 20 than in those mice housed under either long or short day conditions for 32 consecutive weeks, and immune function in mice exposed to short days for 32 weeks was comparable with that of long day animals. These data suggest that short day enhancement of immune function is not indefinite. With prolonged (< or = 32 weeks) exposure to short days, several measures of immune function exhibit "spontaneous" regression, restoring long day-like immunocompetence. The results suggest that formal similarities and, possibly, common substrates exist among the photoperiodic timekeeping mechanisms that regulate seasonal transitions in reproductive and immune function.     

Testicular hormones modulate circadian rhythms of the diurnal rodent, Octodon degus

Sex differences have been identified in a variety of circadian rhythms, including free-running rhythms, light-induced phase shifts, sleep patterns, hormonal fluctuations, and rates of reentrainment. In the precocial, diurnal rodent Octodon degus, sex differences have been found in length of free-running rhythm (tau), phase response curves, rates of reentrainment, and in the use of social cues to facilitate reentrainment. Although gonadal hormones primarily organize circadian rhythms during early development, adult gonadal hormones have activational properties on various aspects of circadian rhythms in a number of species examined. Gonadectomy of adult female O. degus did not influence tau, phase angle of entrainment, or activity patterns in previous experiments. The present experiment examined the role of gonadal hormones in adult male degus' circadian wheel-running rhythms. We predicted that male gonadal hormones would have an activational effect on some aspects of circadian rhythms, particularly those in which we see sex differences. Phase angles of entrainment, tau, length of the active period (alpha), maximum and mean activity levels, and activity amplitude were examined for intact and castrated males housed in LD 12:12. Responses to light pulses while housed in constant darkness (DD) were also compared. Castration had no significant effect on tau or light-induced phase shifts. However, castration significantly increased phase angle of entrainment and decreased activity levels. The data indicate that adult gonadal steroids are not responsible for the sex differences in endogenous circadian mechanisms of O. degus (tau, PRC), although they influence activity level and phase angle of entrainment. This is most likely due to masking properties of testosterone, similar to the activity-increasing effects of estrogen during estrus in O. degus females.     

Role of the pineal and its hormone melatonin in the termination of photorefractoriness in golden hamsters

Continuous exposure of male hamsters to short day lengths induces testicular regression. This is followed many weeks later by spontaneous recrudescence of the testes with reinitiation of spermatogenesis and function of the accessory sexual glands. Hamsters at this stage of the annual reproductive cycle are refractory to short photoperiods--even continuous darkness will not induce another bout of testicular regression. Animals refractory to short days are also refractory to the pineal hormone melatonin and a number of investigators attribute spontaneous recrudescence and photo and melatonin refractoriness to a developed target cell insensitivity to endogenous melatonin from the pineal. Refractoriness is terminated by exposure to long days for at least 11 weeks. The pineal gland is reported to be essential for this process. We report here the effects of pinealectomy, daily melatonin injections, and constant-release melatonin implants on the ability of male hamsters to recover from the refractory state. In the absence of the pineal gland, refractory male hamsters did not discriminate (count?) 15 weeks of long days to terminate refractoriness. Daily melatonin injections at 1900 h, but not at 1200 h (lights 0600-2000 h) during the 15 weeks of long-day exposure blocked the recovery from refractoriness. Constant-release melatonin implants abolished the animals ability to measure 12 and 15 weeks of long days to terminate refractoriness. These results demonstrate that general target tissue insensitivity to melatonin cannot account for the refractory state in hamsters, that a multiplicity of target tissues may exist for melatonin to account for its varied roles throughout the annual reproductive cycle in hamsters, and that the pineal gland is intimately involved in the animals' ability to measure a prescribed duration of long days to terminate refractoriness.     

Circadian rhythms of self-selected lighting in golden hamsters: relation to gonadal condition

Golden hamster (20 males, 8 females) were maintained in isolation boxes for 4-7 weeks. The animals had access to wheels and selected their own lighting by pressing one bar to turn light-on and another bar to turn the light-off. All hamsters maintained circadian rhythms of wheel-running activity. Seventeen of 28 hamsters selected lighting with a circadian periodicty. For 9 hamsters, there was a significant positive correlation between wheel-running activity and self-selected darkness, while this correlation was significantly negative for 10 hamsters. Four hamsters had regressed testes at the end of the experiment. These 4 had significant positive correlations between activity and self-selected darkness, while none of the hamsters with significant negative correlations between activity and self-selected darkness had regressed testes. Light in phase with activity seems to be more important to the prevention of testicular regression than is the total daily amount of light.     

Effect of different photoperiods on gonadal maintenance and development in Mongolian gerbils (Meriones unguiculatus)

The role of photoperiod in adult testicular maintenance and body weight and juvenile development was assessed in male Mongolian gerbils (Meriones unguiculatus). Gerbils were raised on a 14L (14 hr of light) photoperiod. In the first study, adult gerbils with functional testes were transferred to thirteen different photoperiods (0L, 2L, 4L, 6L, 8L, 10L, 12L, 14L, 16L, 18L, 20L, 22L, or 24L) and body weights and testicular size were measured every week for 10 weeks. Body weights were similar in all groups. Testicular regression had occurred in animals housed on 0L, 2L, 4L, 6L, 8L, and 24L by week 10. In the second study, 14L-born prepubertal gerbils were transferred to thirteen different photoperiods as in the first study. Body weights and testicular development were examined for 10 weeks. At the end of 10 weeks the body weights of animals in all groups except 24L were similar to those of adults. Animals in 24L had a lower body weight gain. Exposure to 0L, 2L, and 24L inhibited testicular development and testes weights were significantly different from those of the other groups. These results demonstrate that maintenance of body weight in adult gerbils appears to be independent of photoperiodic signal. Exposure to very long (24L) and short photoperiods (< 10 hr) causes testicular regression in adult gerbils. Moreover, different photoperiods experienced in early life can influence prepubertal testis growth and body weight gain.     

Evidence of an annual rhythm in a small proportion of Siberian hamsters exposed to chronic short days

Siberian hamsters are photoperiodic seasonally breeding rodents. To date, there has been no evidence that Siberian hamsters exhibit an annual rhythm in reproductive, thermoregulatory, molt, or body mass changes. However, given that the termination of their winter cycle is under endogenous control, the authors thought it possible that under particular conditions, other aspects of their seasonal cycle may be endogenously mediated. Consequently, the authors monitored the reproductive condition, body mass, and molt of hamsters chronically exposed to short days (LD 9:15) for up to 2 years. All animals were taken from previous experiments and had exhibited gonadal regression, followed by refractoriness to shorts days and spontaneous gonadal growth, as well as a complete cycle of body mass and molt changes. Although some hamsters died during the study, the authors continued to monitor some animals for up to 106 weeks of short-day exposure. Of the 57 animals monitored, 4 (7.02%) exhibited a second cycle of reproductive regression and recrudescence. Furthermore, the timing of the second regression was similar in the 4 animals, occurring about 1 year after the initial short-day exposure (50.5 +/- 1.71 weeks). However, the timing of the second bout of recrudescence was more variable, occurring at about week 80 (79.5 +/- 8.01 weeks). Hamsters exhibited regressed gonads for about 7 months (29.0 +/- 8.02 weeks). Although the body mass of the 4 hamsters declined at the time of the second cycle of gonadal regression, it never recovered. No hamster exhibited a second molt cycle. These observations indicate that a small percentage of Siberian hamsters chronically exposed to short days can exhibit an annual cycle of reproduction.     

Photoperiodic control of testicular regression and moult in male House Sparrows Passer domesticus

Testis size, bill colour and moult were monitored in male House Sparrows Passer domesticus kept under a natural daylength regime between February and November. On three occasions (at the summer solstice, 25 days later and 39 days later), groups of birds were transferred to a daylength of 18 h of light and 6 h of darkness per day (18L: 6D), the natural daylength at the solstice. In birds under natural daylengths, the testes had regressed significantly by 2 5 days after the solstice. In those transferred to 18L:6D at the solstice, the onset of regression was delayed by about 4 weeks. Transfer to 18L: 6D after the solstice did not cause recrudescence; the testes continued to regress. In birds transferred to 18L: 6D at the solstice, moult was delayed by 4 weeks and progressed more slowly. These results suggest that photoperiodically induced gonadal regression in this species contains elements characteristic of both absolute and relative photorefractoriness.