Endogenous control of spawning in a marine copepod

Daily pattern of egg release has been studied in the marine calanoid copepod, Labidocera aestiva (Wheeler) under cycling light-dark (12L-12D, 12D-12L) conditions, and constant light (24L) or darkness (24D). Under cycling conditions maximum egg release occurred during the dark phase. Rhythmic spawning was also evident under 24D, but not under 24L. The results suggest that the pattern of nocturnal egg release is regulated by an endogenous rhythm entrained by light.     

The behaviour of Bulinus africanus: A circadian profile

Six different behaviour patterns have been studied across the circadian spectrum in the tropical freshwater snail Bulinus africanus. Snails entrained to a circadian light/dark regimen show a diurnal rhythm of locomotor activity, and feeding, photokinetic responsiveness, egg laying and hatching all follow a similar pattern. The rhythms are endogenous and persist under constant conditions. In contrast defaecation occurs mainly during the dark period. This antiphase relationship may be due in part to a cycle of absorption and fragmentation in the digestive gland cells, but it is likely that other factors are involved.     

Retinal and extraretinal photoreceptors mediate entrainment of the circadian locomotor rhythm in lizards

Summary The circadian locomotor activity rhythms of 7 species of lizards can readily be entrained (synchronized) toLD12: 12 (30–50 lux: 0) fluorescent light cycles after complete surgical removal of both eyes. Removal of the parietal eye and pineal organ does not prevent entrainment of blinded lizards. Appropriate control experiments established that lightper se, and not low amplitude temperature cycles or other obvious environmental variables, was the entraining stimulus for blinded lizards. In some cases, blocking the penetration of light to the brains of blinded lizards caused them to free-run (express their endogenous circadian rhythm) in the presence of a dim green light cycle, to which they had previously entrained, suggesting that the brain is the site of the extraretinal photoreceptor(s) mediating entrainment. The extraretinal photoreceptor(s) is capable of intensity discrimination since changing the intensity of aLD 12: 12 fluorescent light cycle caused a change in the phase-relationship between the entrained activity rhythm and the light cycle in a blinded gekko. The lateral eyes are also involved in mediating entrainment since removal of the lateral eyes of thoseSceloporus olivaceus which previously entrained to a dim green light cycle [LD 12: 12 (0.05 lux: 0)] caused them to free-run. Also, blinding had noticeable effects on the entrained activity patterns of some species of lizards.I thank Michael Menaker, Jeffrey Elliott, Sue Binkley, Joseph Silver, Ed Kluth, George Wyche, Bruee Rouse, Nancy Leshikar, Lili Mostafavi, Janet Alvis, Celeste Cromack, A. L. Mackey and Jean Rogers for their suggestions and technical assistance. Support for this work was provided by NIH grant HD-03803-02 (to M. Menaker); NSP grant GB-8138 (to M. Menaker); NSF traineeship GZ-1336 (to H. Underwood); and MH traineeship 5T01GM00836-09 (to H. Underwood).     

Effects of photoperiod and intermittent lighting on reproduction in pheasant hens

Forty 28-wk-old ring-necked pheasant hens were equally distributed among 5 treatment groups and exposed to the following light schedules for 36 wk: Treatment 1 - 16L:8D; Treatment 2 - 1L:11D:4L:8D; Treatment 3 - 1L:13D:2L:8D; Treatment 4 - 1L:14D:1L:8D; and Treatment 5 - 1L:14.5D:0.5L:8D. The number of days from stimulatory lighting to the first egg was significantly (P<0.05) greater under the intermittent schedules (20.3, 29.5, 40.3, 44.4, and 57.7 d, respectively) when the subjective daylength was shorter than 13 h. Despite the delay in initiation of laying, average egg production was higher under intermittent lighting (23.0, 36.0, 48.6, 43.8, and 42.3% or 58.0, 93.0, 122.5, 110.4, and 106.6 eggs). Patterns of oviposition indicated a tendency in the birds exposed to intermittent lighting to have synchronized laying, with the period opposite the longest scotoperiod provided by their light schedule; thus subjective daylengths of 13, 11, 10 and 9.5 h, respectively, were created. Fertility was significantly (P<0.05) lower under intermittent lighting and was apparently associated with a high proportion of eggs in the late stages of oviducal development at the time of insemination.     

Stable daily light regimens as inductive factors of endogenous testicular cycles in the European starling,Sturnus vulgaris

European starlings (Sturnus vulgaris ) maintained under chronic 12L:12D exhibit testicular cycles with a periodicity of 9–10 months. These circannual testicular cycles incorporate all of the physiologically distinct phases observed during gonadal cycles in starlings under temperate-zone photoperiods. Starlings maintained under chronic 6L-18D also undergo testicular cycles but these cycles: (a) have a relatively short periodicity (about 6 months); (b) include periods of testicular involution, though not to the minimal quiescent level for this species; and (c) do not include the physiologically distinct photorefractory phase separating testicular cycles in starlings under chronic 12L:12D and under temperate-zone photoperiods. While it is possible that testicular cycles in starlings under certain daily light regimens of fixed duration are a function of an endogenous circannual reproductive rhythm, we believe that the testicular cycles generated under both 12L:12D and 6L:18D are the product of gonadotropin secretion rates controlled by circadian (not circannual) oscillations periodically entrained by these chronic photoperiods.     

The influence of light on cone disk shedding in the lizard, Sceloporus occidentalis

The lizard, Sceloporus occidentalis has an all-cone retina. In lizards maintained on a 12-h light:12-h dark (12L:12D) cycle, a burst of cone outer segment (COS) shedding occurs 2 h after light offset (1400 h circadian time) (Young, R.W., 1977, J. Ultrastruct. Res. 61:172-72). In this investigation, we studied the effect of different lighting regimes on the pattern of cone disk shedding in this species. When lizards entrained to a 12L:12D cycle are kept in constant darkness (DD), the shedding peak is advanced approximately 2 h and the magnitude of shedding is reduced to 30% of control. COS increased in mean length from 12 micron in controls to 14 micron after one cycle in DD and maintained this length during a second cycle in DD. In constant light (LL), disk shedding was damped to approximately 10% of control values. Shedding synchrony in LL was also perturbed and therefore cyclic shedding bursts could not be distinguished. During LL there was a much larger increase in COS mean length than in DD. After one cycle of LL, COS length was 15 micron and after two cycles COS length exceeded 17 micron. When lizards entrained to 12L:12D are shifted to a 6L:18D regimen, the first shedding cycle is biphasic. The first peak of 5% shedding occurs 2 h after light offset whereas a second larger peak (13%) occurs according to the entrained schedule (1400 h). This manipulation separates out a dark-triggered and circadian shedding component, which is normally superimposed in lizards entrained to a 12L:12D cycle. When entrained lizards are placed in 36 h of LL followed by light offset, the peak shedding response after light offset is double the control response (53% vs. 27%). After 30 h of LL (lights off 90 degrees out of phase), there is a biphasic shedding response similar to the 6L:18D regimen although this time the dark-triggered shedding component is greater in magnitude then the circadian component. COS turnover is estimated by extrapolating from COS mean length increases during LL. From this method we obtained a 2.7-micron increase in COS length during each day in LL. If COS growth is not augmented during LL, this would yield a 4-5-d turnover time for the average 12.5-micron COS.     

Differential hormonal action of the bag cell neurons on the arterial system ofAplysia

Summary The peptide-secreting bag cell neurons ofAplysia californica activate a long-lasting, complex behavior called egg laying. During egg laying some organ systems (reproductive) are more active than others (digestive) suggesting that blood flow to these tissues may change in accordance with their activities during egg laying. To examine this possibility we used a semi-intact preparation of the three major arteries innervated by the abdominal ganglion. We found that electrically stimulated bursts of bag cell activity triggered a long-lasting (>1 h) increase in contractile activity in two arteries, the anterior and gastroesophageal, but did not affect contractions of the third (abdominal) artery. The arterial responses were not affected either in form or duration by denervation of the arteries, suggesting that the increase in contractile activity was mediated by hormonal actions of bag cell transmitters on vasoconstrictor muscles. In intact animals this differential action on the arterial system may cause a long-term decrease in blood flow to relatively inactive tissues (digestive and locomotory organs) while increasing circulation to tissues involved in egg production (ovotestis and oviduct).Abbreviations ASW artificial sea water - BCA bag cell activation - ELH egg laying hormone     

Adult emergence rhythm of the egg-parasitoid Telenomus busseolae

The adult emergence rhythm of Telenomus busseolae, an egg parasitoid of Sesamia nonagrioides, was examined when parasitoids were exposed to different light-dark regimes. Most of the adult parasitoids emerged throughout the whole period of the photoperiodic cycle. Peak male emergence occurred 2–5 hours earlier than that of females. Adult emergence was asynchronous in continuous darkness or light. However, regimes of alternative light and dark phases such as L4:D20, L8:D16, L12:D12, L16:D8 and L20:D4 h generated a population rhythm with a period length of 24 hours. The peak of the emergence activity moves from the scotophase to the middle of the photophase with an increase of the photophase from 4 to 20 h. Rhythmical activity of adults was synchronised within 2 cycles when immature stages of parasitoid grow under continuous light conditions (LL) and then transferred to L12:D12. Moreover, emergence rhythm persisted and continued in a free-run with a period length of less than 24 hours by transferring a rhythmic culture from L12:D12 h to LL or RR (continuous red light) conditions, indicating the existence of a circadian rhythm. The ecological implications of the expression rhythm relate to better survival of the parasitoids.     

Effect of exposure to long photoperiod during the rearing period on the age at first egg and the subsequent reproductive performance in geese

The objective of this study was to investigate the effects of photoperiods longer than 14 h of light:10 h of dark (14L:10D) during the rearing period on the age at first egg laying (AFE) and the subsequent reproductive performance in geese. Sixty-six White Roman geese (18 male and 48 female) were divided into three groups and subjected to different lighting schemes, i.e. natural lighting (NAT; 23 degrees 51'N, 120 degrees 33'E), 14L and 18L. Birds in 14L and 18L groups were exposed to 14L:10D or 18L:6D, respectively, beginning at 19 weeks of age and followed by a photoperiod of 10L:14D from 40 weeks of age. The natural photoperiod, including both dawn and dusk was between 11.5L:12.5D and 14.5L:9.5D. The results showed that the AFE was postponed (P < 0.05). Average weight of the first three eggs laid and the fertility of these eggs were improved (P < 0.05) for the geese in 14L and 18L groups when compared to those raised under natural lighting conditions. Meanwhile, the duration of laying were shifted from spring to autumn, with the peak laying rate in September and November instead of March. It was concluded that geese exposed to the photoperiod longer than 14L:10D for 21 weeks during the rearing period would suppress their AFE. Thereafter, the onset of laying could be induced by being transferred to the photoperiod of 10L:14D. The manipulation of photoperiodic regimes used in this study might have a potential benefit for geese farmers through improved weight and fertility of eggs.     

Circadian Temperature and Activity Rhythms in Mice under Free-Running and Entrained Conditions; Assessment after Purification of the Temperature Rhythm

Six female mice were studied separately for six weeks, first in constant light (300 lx), and then on a 12 : 12 L : D schedule (light on 07:00–19:00–h). Food and water were available ad libitum. Abdominal temperature and spontaneous locomotor activity were measured every 10 min. In constant light, the animals free-ran with both temperature and activity records showing circadian rhythms that were significantly greater than 24 h; by contrast, in the LD schedule, the circadian rhythms had become entrained and showed a stable phase relation to this schedule. The direct masking effects upon raw temperatures caused by bursts of activity were clearly seen, and could be removed by a process of ‘purification’. A comparison of the activity profiles during the entrained and free-running phases showed that the imposed light-dark cycle resulted in decreased activity in the light, increased activity in the dark, and bursts of activity at the light-dark and dark-light transitions. Masking effects due to the activity profile were present in the raw temperature profile, and many could be removed by purification using the activity profile; however, there was evidence that other masking effects, independent of activity, were present also. The efficacy of thermoregulatory compensation, as assessed from the rise of core temperature produced by spontaneous locomotor activity, was, in comparison with the free-running condition, increased in the dark phase and decreased in the light phase; this would appear to be one way to limit the temperature rise that occurs in the active phase of the rest-activity cycle.     

Serotonin modulates locomotory behavior and coordinates egg‐laying and movement in Caenorhabditis elegans

Biogenic amines have been implicated in the modulation of neural circuits involved in diverse behaviors in a wide variety of organisms. In the nematode C. elegans, serotonin has been shown to modulate the temporal pattern of egg‐laying behavior. Here we show that serotonergic neurotransmission is also required for modulation of the timing of behavioral events associated with locomotion and for coordinating locomotive behavior with egg‐laying. Using an automated tracking system to record locomotory behavior over long time periods, we determined that both the direction and velocity of movement fluctuate in a stochastic pattern in wild‐type worms. During periods of active egg‐laying, the patterns of reversals and velocity were altered: velocity increased transiently before egg‐laying events, while reversals increased in frequency following egg‐laying events. The temporal coordination between egg‐laying and locomotion was dependent on the serotonergic HSN egg‐laying motorneurons as well as the decision‐making AVF interneurons, which receive synaptic input from the HSNs. Serotonin‐deficient mutants also failed to coordinate egg‐laying and locomotion and exhibited an abnormally low overall reversal frequency. Thus, serotonin appears to function specifically to facilitate increased locomotion during periods of active egg‐laying, and to function generally to modulate decision‐making neurons that promote forward movement. © 2001 John Wiley & Sons, Inc. J Neurobiol 49: 303–313, 2001     

An entrainment model for semilunar rhythmic swimming behaviour in the marine isopod Eurydice pulchra Leach

Entrainment experiments have been carried out with geographically widely separated populations of the sand beach isopod Eurydice pulchra Leach subjected to periods of simulated tidal agitation imposed concurrently with a 24-h light: dark (L: D) cycle. Circatidal swimming rhythms of greatest amplitude were induced when agitation was applied with the subjective timing, within the L: D cycle, of local spring high tides. This occurred in a normal L: D regime and also when the L: D regime was phase shifted through 90°. Animals previously maintained in constant darkness (D: D) and subsequently exposed to simulated tidal disturbance at various times in constant darkness were unable to modulate the amplitude of circatidal swimming activity. Isopods previously maintained in a normal L: D cycle and subsequently subjected to artificial tidal agitation in constant darkness were, however, able to modulate circatidal activity. This indicates that E. pulchra is capable of detecting tidal agitation and daily light cues and using them in conjunction with its circadian “clock” to modulate its endogenous circatidal rhythmicity. The free-running semilunar rhythm of swimming activity entrained only when the timing of agitation within the day/night cycle mimicked the pattern of local spring high tides. Agitation with the timing of neap high tides entrained no free-running circa-semilunar activity pattern.     

Effect of Changing the Light/Dark Schedule, the Time of Onset of the Light or Dark Period, or the Daylength, on Rhythms of Epidermal Cell Proliferation

Rhythms of labeling and mitotic indices were studied in the hindlimb epidermis of the anuran tadpole Rana pipiens under different light/dark (LD) cycles and daylengths in order to examine the role of the various parameters of the lighting regimen in setting the periods of the rhythms and the timing of the cell proliferation peaks. Altering the time of, or inverting, the 12 h light period on a 24 h day resulted in phase shifting of basically bimodal circadian rhythms with peaks in the light and dark. Thus the cell proliferation rhythms were entrained to the LD cycle. These rhythms also entrained to noncircadian schedules since they lengthened on a 15L : 15D cycle and shortened on a 9L : 9D cycle, although the bimodal characteristic of a peak in the light and a peak in the dark remained. Studies of 18L: 6D and 6L : 18D cycles in which either the time of onset of light or dark was changed relative to the 12L: 12D control indicated that the onset of dark may regulate the timing of the labeling index peaks while the onset of light may determine the time of occurrence of mitotic index peaks. Control of the timing of labeling and mitotic index peaks by different parameters of the LD cycle suggests a mechanism for cell cycle regulation by the environmental lighting schedule. Analysis of the rhythms on all the cycles studied suggested that labeling index rhythms equal the length of, or twice the length of, the dark period. Mitotic index rhythms equal the daylfength or a multiple of the length of the dark period.     

Motility of zooxanthellae isolated from the Red Sea soft coral Heteroxenia fuscescens (Cnidaria)

Unicellular dinoflagellate algae are among the best examples of organisms that exhibit biological clocks. This study examined the effect of light regime on rhythmicity of motility in the symbiotic dinoflagellate Symbiodinium sp., freshly isolated from the soft coral Heteroxenia fuscescens (Ehrenberg). Freshly isolated algal cells, placed under a 12-h L:12-h D cycle, exhibited motility with a diel rhythm. This motility occurred only during the period of illumination and lasted 8-9 h, with a peak at 2.5-4 h after lights on. Algal cells placed in an inverted light regime inverted their motility pattern. The response to the L/D regime was very precise, and even a 1-h shift backward or forward affected initiation of motility and time of its maximal peak. When placed in either constant light or dark, algal motility ceased until the L/D cycle was restored. These findings suggest that the rhythm is entrained by light cues and is not due to an endogenous circadian rhythm. Further, we provide evidence that the presence of juvenile hosts does not affect the algal motility pattern. These results offer the first evidence for the lack of impact by the host on rhythmicity of motility of free-living algal cells. The motility pattern found in freshly isolated algae may indicate the presence of light-induced diel rhythmicity in yet-to-be described free-living Symbiodinium.     

Circadian Temperature and Activity Rhythms in Mice under Free-Running and Entrained Conditions; Assessment after Purification of the Temperature Rhythm

Six female mice were studied separately for six weeks, first in constant light (300 lx), and then on a 12 : 12 L : D schedule (light on 07:00-19:00-h). Food and water were available ad libitum. Abdominal temperature and spontaneous locomotor activity were measured every 10 min. In constant light, the animals free-ran with both temperature and activity records showing circadian rhythms that were significantly greater than 24 h; by contrast, in the LD schedule, the circadian rhythms had become entrained and showed a stable phase relation to this schedule. The direct masking effects upon raw temperatures caused by bursts of activity were clearly seen, and could be removed by a process of 'purification'. A comparison of the activity profiles during the entrained and free-running phases showed that the imposed light-dark cycle resulted in decreased activity in the light, increased activity in the dark, and bursts of activity at the light-dark and dark-light transitions. Masking effects due to the activity profile were present in the raw temperature profile, and many could be removed by purification using the activity profile; however, there was evidence that other masking effects, independent of activity, were present also. The efficacy of thermoregulatory compensation, as assessed from the rise of core temperature produced by spontaneous locomotor activity, was, in comparison with the free-running condition, increased in the dark phase and decreased in the light phase; this would appear to be one way to limit the temperature rise that occurs in the active phase of the rest-activity cycle.     

Maternal effects of photoperiods on glycogen metabolism related to induction of diapause in Cotesia vestalis (Hymenoptera: Braconidae) from Jilin, China

Most insects must accumulate glycogen before entering diapause. Photoperiod influences diapause beyond the maternal generation in Cotesia vestalis (Haliday) (Hymenoptera: Braconidae). In this study, we measured glycogen content and the activity and messenger RNA (mRNA) expression of glycogen synthase and glycogen phosphorylase at different G₀ (25 °C under light:dark photoperiods of 8L:16D, 12L:12D, and 16L:8D) and G₁ life stages (13 °C under 8L:16D). Glycogen content in G₀ and G₁ increased with shorter light periods, except for G₀ adult males, which showed no significant difference under the three photoperiods. Compared with those under 16 h, in those under 8 h light, glycogen synthase activity was significantly higher at all tested stages, except for G₀ pupa and adult male, for which it was identical; mRNA expression was higher at G₀ larva and prepupa, lower at pupa, and identical at rest stages. Glycogen phosphorylase activity was significantly higher for G₁ egg, lower at G₁ prepupa, and identical at all other stages; mRNA expression was higher at G₀ and G₁ larval stages, and similar at rest stages. These findings suggest that glycogen, being regulated by the two enzymes, may be a consequential factor in the transmission of maternal information and diapause induction of C. vestalis.     

Masking of circadian activity rhythms in canaries by light and dark

Canaries (Serinus canaria) were kept singly in cages placed in an artificially illuminated, soundproof cabinet. Perch-hopping activity was recorded by means of a computer system. In three series of experiments, the activity rhythms of the birds were entrained to 24 hr by light-dark (LD) cycles with 4, 12, or 20 hr of light (L), respectively. The intensity of illumination was 10 lux in L and 0.25 lux in darkness (D). Under LD 4:20 and 12:12, the intensity of D was increased daily at the same zeitgeber time to 1 lux for 1 hr (L pulse) during about 8 consecutive days. This sequence was followed by 8 days without L pulses before giving another series of L pulses at a different zeitgeber time. Under LD 20:4, the intensity of L was decreased to 1 lux for 1 hr (D pulse). The activity of all birds was more or less increased by the L pulses (positive masking) and decreased by the D pulses (negative masking). The level of masking activity during the L and D pulses depended on the circadian phase at which the pulses were administered. Positive masking by L pulses was minimal about 5 hr after the beginning of D, and increased steadily thereafter. Negative masking by D pulses was maximal at the beginning and the end of L, and minimal during the middle.     

Interacting effect of thermoperiod and photoperiod on the eclosion rhythm in the onion fly, Delia antiqua supports the two-oscillator model

Daily light and temperature cycles entrain adult eclosion rhythms in many insect species, but little is known about their interaction. We studied this problem in the onion fly, Delia antiqua. Pupae were subjected to various combinations of a photoperiod of 12L:12D and thermoperiods. The thermoperiods consisted of 12 h warm phase (W) and 12 h cool phase (C), giving a mean temperature of 25 °C with different temperature steps of 8, 4 and 1 °C. As the phase relation of the two Zeitgebers was varied, the phase of eclosion rhythm was shifted, depending on the phase angle with the light cycle and the amplitude of the temperature cycle. When the temperature step in the thermoperiod was 8 °C (WC 29:21 °C), the eclosion rhythm was entrained mainly to thermoperiod rather than photoperiod. In the regime with a 4 °C temperature step (WC 27:23 °C), both thermoperiod and photoperiod affected eclosion rhythm, and a phase jump of the eclosion rhythm occurred when the warm phase of thermoperiod was delayed 15-18 h from light-on. In regimes with a 1 °C temperature step (WC 25.5:24.5 °C), the eclosion rhythm was completely entrained to photoperiod. The observed interacting effect of light and temperature cycle on the eclosion rhythm in D. antiqua can be explained by the two-oscillator model proposed by Pittendrigh and Bruce (1959).     

Photoperiodic induction of the pupal diapause in the tobacco hornworm, Manduca sexta

A study was made of photoperiodic induction of the facultative pupal diapause in the tobacco hornworm, Manduca sexta, reared on artificial diet in the laboratory. The species entered a prolonged diapause when the egg and larval feeding stages were reared in daily photoperiods of 13·5 hr or less. Diapause was induced in all insects at photoperiods ranging from 1 to 13 hr, and part of the population entered diapause at only 15 to 30 min of light per day. Photoperiods of 14 hr or more and continous darkness prevented diapause. Duration of diapause varied with the inductive photoperiod in which the hornworms were reared during the sensitive period. Insects reared in longer diapause-inducing photoperiods within a range of 12 to 13·25 hr remained in diapause longer than those reared in shorter photoperiods. There was no difference in the rate of larval development of hornworms reared in diapause-inducing vs diapause-preventing photoperiods. Temperatures of 26 to 30°C were most favourable for the photoperiodic induction of diapause; at 21°C, the critical photoperiod and incidence of diapause were decreased. Diapause induction was suppressed by low (18°C) and higher (33°C) temperatures. The number of inductive 12L:12D (light = 12 hr; dark = 12 hr) cycles required to induce diapause ranged from as few as 5 for some insects to as many as 12 for others when the post-inductive régimen was continuous light, but with insects previously held in continuous dark, as few as 2 12L:12D cycles during the last 2 days of larval feeding induced diapause in 38 per cent of the population. Only 3 to 4 cycles of 15L:9D during the final larval instar reversed inductive effects of 14 to 15 12L:12D cycles. Photoperiodic sensitivity extended from the late embryo to the end of larval feeding but showed considerable fluctuation during development with maximum sensitivity occurring just before egg hatch and during larval growth.Light breaks applied at different times during the dark period of 12L:12D cycles generated different response curves, depending on the number of cycles in which light breaks were repeated. When repeated for 6 cycles, a unimodal response curve was obtained; 10 cycles produced a bimodal curve and light breaks given for 18 cycles throughout the sensitive period averted diapause regardless of time of night applied. It is suggested that diapause is regulated by a photo- and thermolabile substance that accumulates during long nights (11 hr or more) and acts during the early pupal stage to inhibit the translocation and release of development-promoting neurosecretion from the brain.     

Interval between preovulatory surges of luteinizing hormone increases late in the reproductive period in turkey hens

In turkey hens, the egg production rate is relatively high early during a reproductive period, but declines as the period progresses. Among lines with different egg production potential, the interval between preovulatory surges of LH is the primary determinant of the egg production rate. The main objective of this study was to determine whether the decline in egg production rate late during an egg production period is also associated with a difference in the interval between LH preovulatory surges. A group of photosensitive turkey hens (Early) were photostimulated with continuous light (24L:0D) at 40 wk of age to induce egg laying, and serial blood samples were collected after about 3 wk of egg production. A second group of hens (Late) were housed in floor pens and photostimulated with 14L:10D at 40 wk of age for a normal 36-wk reproduction period and were then switched to 24L:0D lighting for 2 wk before collection of serial blood samples. Continuous light photostimulation was used for at least 2 wk before and during serial blood sampling to avoid potential masking effects of diurnal lighting on the interval between LH surges. The Early (n = 12) and Late (n = 16) hens were cannulated 3 days before being serially bled hourly for 10 days. The mean interval between preovulatory surges of LH was shorter in the Early hens than in the Late hens (26.1 +/- 2.5 h and 34.7 +/- 3.9 h, respectively). The intra-hen LH surge interval coefficient of variation was lower in the Early hens than in the Late hens (7.2% and 18.6%, respectively). The inter-hen LH surge interval coefficient of variation was similar in the Early and Late hens (9.5% and 11.2%, respectively). The incidence of blind surges of LH (those not retrospectively associated with ovipositions) was not different between Early and Late laying hens (8.4% +/- 15.2% and 7.3% +/- 14.6%, respectively). In conclusion, in turkey hens, longer intervals and greater intra-hen variation between LH surges were associated with a poorer rate of egg production late in the reproductive period relative to early in the reproductive period.