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.     

Pinealectomy affects the circannual testicular rhythm in european starlings (Sturnus vulgaris)

Summary Pinealectomized and sham-operated European starlings were maintained for 16 months under a constant 12-h photoperiod and constant temperature conditions. In all birds, testicular width was measured at about monthly intervals and the onset and end of molt was determined. Shortly after the beginning of the experiment, the sham-operated birds went through a cycle of testicular growth and regression which was followed by a complete molt; subsequently most individuals initiated a second testicular cycle. Most of the pinealectomized birds, in contrast, failed to go through a second testicular cycle. Moreover, during the first cycle their testes regressed earlier than in the sham-operated birds and the subsequent molt was relatively advanced. In these respects the pinealectomized birds behaved like intact starlings under a 13-h photoperiod. Since pinealectomy probably changes the phase-relationship between circadian rhythms and the entraining light-dark cycle it is proposed that pinealectomy in the present experiment might have altered the phase-relationship between a circadian rhythm of photosensitivity and the light-dark cycle in such a way that the birds interpreted the 12-h photoperiod as a 13-h photoperiod.This work was supported by the Deutsche Forschungsgemeinschaft SPP Mechanismen biologischer Uhren.     

Endogenous gonadal,LH and molt rhythms in tropical stonechats: effect of pair bond on period,amplitude, and pattern of circannual cycles

To investigate the effects of reproduction and associated stimuli on the circannual cycles of African stonechats Saxicola torquata axillaris birds were held for 29 months in aviaries under a constant equatorial (12.25 h) photoperiod, either singly (10 females and 10 males) or in 10 male/female pairs. The birds of all 3 groups went through circannual cycles in gonadal size, plasma LH and molt, but groups differed with regard to actual reproductive performance. During the second cycle, only one of the singly-held females laid eggs and incubated. In constrast, in the paired females egg-laying and incubation occurred in all but one bird. About 50% of the clutches from paired females contained fertilized eggs confirming the expectation of behavioral differences between the paired and unpaired birds. However, despite differences in reproductive performance there were no differences in either circannual period or duration of reproductive phases. Moreover, there was no correlation between number of broods produced per season and circannual parameters of the paired females. Therefore, the temporal course and, particularly, the period during which reproduction is possible is rigidly determined by an endogenous program that is not influenced by reproductive performance. A rigid program of this kind may be advantageous in the tropics because it prevents prolongation of the breeding season in years with favourable conditions which in turn could jeopardize optimal timing of breeding in the following year and thus reduce lifetime reproductive success.     

Starling circannual systems: are they arrested in long photoperiods?

Summary In the European starling,Sturnus vulgaris, circannual rhythms in gonadal size, molt and other related functions persist only in photoperiods close to 12 h, but are absent in longer or shorter daylengths. To find out whether the arrhythmia seen in long photoperiods results from an arrest of the underlying clock system, three groups of male starlings were held for 10, 14, or 20 months in a 13 h photoperiod and then transferred to a 12 h photoperiod. A control group was held in the 13 h photoperiod throughout the experiment for 28 months. During the initial exposure to the 13 h photoperiod, all birds went through a gonadal cycle, followed by a complete molt. Subsequently, the control birds retained small testes to the end of the experiment and there was no further molt. In contrast, most of the experimental birds re-initiated a testicular cycle, following transfer to the 12 h photoperiod and molted after its completion. The latency between the transfer to the 12 h photoperiod and the onset of testicular growth was not significantly different among the three groups, indicating that the underlying circannual clock had been arrested in the 13 h photoperiod and restarted in the 12 h photoperiod. The pattern of the second testicular cycle did, however, differ among groups. Particularly its amplitude decreased from group 1 to group 3, suggesting that the capacity of the birds to respond to a 12 h photoperiod decreased with increasing duration of exposure to the 13 h photoperiod.Dedicated to Prof. Dr. C.S. Pittendrigh on the occasion of his seventieth birthday.     

Seasonality in a temperate zone bird can be entrained by near equatorial photoperiods

Birds use photoperiod to control the time of breeding and moult. However, it is unclear whether responses are dependent on absolute photoperiod, the direction and rate of change in photoperiod, or if photoperiod entrains a circannual clock. If starlings (Sturnus vulgaris) are kept on a constant photoperiod of 12h light:12h darkness per day (12L:12D), then they can show repeated cycles of gonadal maturation, regression and moult, which is evidence for a circannual clock. In this study, starlings kept on constant 11.5L:12.5D for 4 years or 12.5L:11.5D for 3 years showed no circannual cycles in gonadal maturation or moult. So, if there is a circannual clock, it is overridden by a modest deviation in photoperiod from 12L:12D. The responses to 11.5L:12.5D and 12.5L:11.5D were very different, the former perceived as a short photoperiod (birds were photosensitive for most of the time) and the latter as a long photoperiod (birds remained permanently photorefractory). Starlings were then kept on a schedule which ranged from 11.5L:12.5D in mid-winter to 12.5L:11.5D in mid-summer (simulating the annual cycle at 9 degrees N) for 3 years. These birds entrained precisely to calendar time and changes in testicular size and moult were similar to those of birds under a simulated cycle at 52 degrees N. These data show that birds are very sensitive to changes in photoperiod but that they do not simply respond to absolute photoperiod nor can they rely on a circannual clock. Instead, birds appear to respond to the shape of the annual change in photoperiod. This proximate control could operate from near equatorial latitudes and would account for similar seasonal timing in individuals of a species over a wide range of latitudes.     

Changes in circadian LH-secretion patterns and annual gonadal cycles in the male European starling ( Sturnus vulgaris )

An analysis of the circadian and circannual patterns of luteinizing hormone (LH) secretion in male European starlings (Sturnus vulgaris) held in outdoor aviaries is presented. On the basis of six daily time points, eight calendar dates, and seven individuals per sampling unit, we present a mathematical model developed to compare the two temporal patterns of secretion, the circadian and the circannual one. The superposition of four orthonormal trigonometric base functions, plus a constant, is used to examine the daily fluctuations. The contribution of the second harmonic was not the same at each calendar date. From the subsequent regression functions, hourly estimates of LH concentration were calculated and used to construct circannual cubic splines and an LH(t,T) function over the whole year. The model features indicate strong circannual and circadian components in the data. In addition, there appear to be two daily time regions of LH secretion: morning and afternoon. In particular, the afternoon maxima occur during photorefractoriness and early gonadal development; the morning maxima at the onset of photoinduction and late gonadal development. At the peak of gonadal activity (end of April), the highest LH levels occur during the photic input phase. It appears, then, that the circadian acrophase of LH secretion in male starlings fluctuates over the annual cycle of gonadal activity in a highly predictive and biologically interpretable manner. John Dittami and Hermann Prossinger: both authors contributed equally.     

The non-photic induction of spermatogenic development in European starlings

Detailed studies of the photosexual biology of male European starlings (Sturnus vulgaris) document a non-obligatory involvement of photoperiod in the induction of testicular metamorphosis. Although post-winter solstice increases in daily photophase duration are responsible for the ecologically correct chronology of the annual reproductive cycle, starlings maintained in the absence of daily photostimulation under go testicular metamorphosis with complete spermatogenic development. Present experiments reveal that the rate of testicular growth in starlings held in constant darkness (DD) is affected by previous photoperiodic experience. Birds held under a natural northtemperate zone photoperiod and transferred to DD on 13 September require significantly fewer days to achieve spermatogenic testes than birds pretreated under 12-and 14-h photoperiods or in constant light (LL). Complete spermatogenesis in the 14-h group is achieved only after a greater duration of DD exposure than in all other birds. Variations in the extent of the 12-h pretreatment period do not alter the testis growth rate in starlings subsequently transferred to DD. It is suggested that photoperiodic conditions applied prior to the initiation of DD treatment may affect the characteristics of circadian oscillations that occur in the absence of a photoperiodic zeitgeber, and thus change the reproductive response rate through alterations of hormonal secretions from the hypothalamo-hypophyseal axis.Presented at the Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Circannual rhythms in European starlings: Why do they stop under long photoperiods?

Summary In European starlings (Sturnus vulgaris) exposed in winter to photoperiods of 12 1/2 h or more, testes go through a cycle of growth and regression but then stay inactive for many months. Under a photoperiod of 12 h, in contrast, testes usually go through repeated circannual cycles. We have tested the hypothesis that the failure of starlings held under long photoperiods to initiate a second testicular cycle is a consequence of the fact that photorefractoriness is not broken under photoperiods longer than 12 h. The results of 2 experiments are consistent with this hypothesis: whereas the testes of starlings held continuously in a 13-h photoperiod or in continuous light, remained inactive after an initial testicular cycle, testicular growth was re-initiated after birds were exposed for 4 to 8 weeks to a short 8-h photoperiod.     

A comparison of the annual cycles in testicular size and moult in captive European starlings Sturnus vulgaris during their first and second years

There is considerable evidence that reproductive success improves with age in birds. It is often suggested that improved performance of older birds is because they are more experienced. Less consideration has been given to the possibility that improvements may be a consequence of age-related changes in reproductive physiology. One factor that consistently changes with age is laying date – first year birds lay later than older birds. In European starlings Sturnus vulgaris , older males begin reproductive activity earlier than first year males and dominate available nest sites. I monitored changes in testicular volume in captive starlings exposed to natural changes in day length and temperature, from their first autumn through the next two annual cycles. Testicular maturation was advanced by 3–4 weeks in birds during their second year compared to the first, and testicular regression occurred about 2 weeks later. The period of full sexual maturation was 50% longer during the second year. The timing of the post-nuptial moult was the same. A possible physiological mechanism to explain this is discussed. The results show that earlier reproductive activity in older birds can be explained, at least in part, by intrinsic physiological mechanisms. This does not preclude additional effects of experience of an individual in the improvement in reproductive performance between first and second year birds.     

Seasonal changes in day-light intensity as a potential zeitgeber of circannual rhythms in equatorial Stonechats

Summary The annual rhythms of reproduction and molt of equatorial Stonechats in East Africa (Saxicola torquata axillaris) are controlled by an endogenous circannual rhythmicity. This has been demonstrated in previous experiments in which birds from Africa were kept in a seasonally constant environment: the annual cycles of gonadal size and molt were found to persist for up to 10 years. Since in such constant environmental conditions the period of the rhythms tended to deviate from 12 months, we postulate that seasonally changing factors in the normal environment of these birds normally synchronize the rhythms with the natural year. In the present study we examined the possibility that annual variations in daytime light intensity that occur as a result of changing cloud cover associated with the dry and rainy seasons may provide a circannual zeitgeber. An experimental group consisting of 6 male Stonechats was kept for 25 months in a light-dark cycle in which the light period had a constant duration of 12.5 hours and an intensity alternating between high and low in a 300-day rhythm. Changes in testicular size and molt of these birds were compared with those of a control group of 6 birds that were exposed to the same photoperiod but with bright daylight throughout the experiment. In the control group, only one bird went through 2 testicular cycles and two birds went through one cycle. All these birds showed a molt rhythm, but periods were highly variable among individuals. In contrast, all experimental birds went through two testicular cycles and exhibited rather synchronous molt rhythms (Fig. 1). These findings support the hypothesis that changing light intensity during daytime may provide synchronizing stimuli for circannual rhythms. Nevertheless, premature conclusions should be avoided; possible shortcomings of the experiment are discussed.

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Saisonale Änderungen der Tageslicht-Intensität als potentieller Zeitgeber circannualer Rhythmen bei tropischen Schwarzkehlchen

Zusammenfassung Die Jahresrhythmik der Fortpflanzung und der Mauser äquatorialer Schwarzkehlchen aus Ost-Afrika (Saxicola torquata axillaris) werden von einer endogenen circannualen Rhythmik kontrolliert. Dies zeigten frühere Versuche, in denen nachgewiesen wurde, daß der Rhythmik der Gonadengröße und der Mauser auch bei solchen Vögeln fortbestand, die über mindestens 10 Jahre in einer konstanten Umwelt lebten. Die Periode dieser Rhythmik wich unter solchen konstanten Versuchsbedingungen allerdings von 12 Monaten ab, was impliziert, daß es in der natürlichen Umwelt der Schwarzkehlchen jahresperiodisch schwankende Umweltfaktoren gibt, die die endogenen Rhythmen mit dem astronomischen Jahr synchronisieren. In der vorliegenden Arbeit untersuchten wir die Möglichkeit, daß jahresperiodische Änderungen der Tageslicht-Intensität, wie sie als Folge von Bewölkungsänderungen mit dem Wechsel zwischen Regen — und Trockenzeiten auftreten, einen solchen Zeitgeber darstellen. Eine Versuchsgruppe von 6 männlichen Schwarzkehlchen wurde 25 Monate lang in einer konstanten äquatorialen Photoperiode von 12,25 Stunden gehalten, in der die Tageslicht Intensität in einem 300-tägigen Rhythmus zwischen einer Phase mit hoher Lichtintensität und einer Phase mit niedriger Lichtintensität abwechselte. Die Zyklen der Hodengröße und der Mauser dieser Vögel wurden mit denen von 6 Kontrollvögeln verglichen, die 25 Monate lang unter derselben Photoperiode aber bei gleichbleibend hellem Tageslicht gehalten wurden. In der Kontrollgruppe durchlief nur ein Vogel zwei Gonadenzyklen und zwei durchliefen je einen Zyklus. Alle 6 Vögel gingen durch einen Mauserzyklus mit großen interindividuellen Unterschieden in der Periodenlänge. Im Gegensatz dazu zeigten alle 6 Versuchsvögel zwei Gonaden- und Mauserzyklen (Abb 1). Die Ergebnisse stützen somit die Hypothese, daß Änderungen in der Beleuchtungsstärke am Tage die circannualen Rhythmen dieser Vögel synchronisieren können. Vorsicht vor zu weitgehenden Schlußfolgerungen scheint allerdings angebracht und mögliche Mängel im Experiment werden diskutiert.

     

The seasonal development of photoperiodic responsiveness in an equatorial migrant, the garden warblerSylvia borin

Summary Garden warblers (Sylvia borin) are migrating European passerines that spend the winter in tropical Africa. To investigate how photoperiod controls the annual cycles of reproduction, molt and migration, garden warblers were exposed to photoperiodic changes simulating those experienced by conspecifics with an equatorial wintering area. At three different times groups of birds were moved from a constant equatorial photoperiod of 12.8 h to a 15-h photoperiod: group 1 on November 19, group 2 on February 24, and group 3 on April 12. In all birds the changes in body weight, nocturnal migratory restlessness, testicular width or diameter of the largest oocyte, and plasma LH concentrations were determined. In addition, the birds were examined for molt. In all birds of groups 2 and 3 the transfer to long photoperiods induced a gonadal cycle associated with spring migratory restlessness and, in the group 3 birds, an increase in LH. In group 1 only few birds showed marginal gonadal responses and the gonadal cycles of the group 2 birds were less pronounced than those of group 3. The results suggest that the hypothalamo-pituitary-gonadal axis was refractory to stimulation by long photoperiods in autumn but, later in the season, became responsive to the same long photoperiods. Unlike the situation in other temperate zone birds, the transition from the photorefractory to the photosensitive state seems to be a gradual process which develops spontaneously and with the appropriate temporal characteristics in the absence of photoperiodic change.Abbreviation LH luteinizing hormone     

Neonatal androgenization in ground squirrels: influence on sex differences in body mass and luteinizing hormone levels

Female squirrels were injected at birth with 50 or 1000 micrograms testosterone propionate (TP); control males and females were treated with oil vehicle. Squirrels were gonadectomized at 47 days of age. Body mass was recorded weekly and plasma luteinizing hormone (LH) was determined once monthly over the next year. Marked annual cycles in body mass were manifested by 30 out of 31 squirrels. Peak body mass and peak-to-trough differences were greater for control male and TP-female squirrels than for control female squirrels. Trough body weights did not differ among the groups. Luteinizing hormone was detectable in all male and most androgenized females but not in any control female squirrels during the first 4 mo after gonadectomy. Peak LH values were significantly greater for control male than for control female squirrels and were not influenced by neonatal androgenization in females. Testosterone propionate treatment also did not affect sex differences in timing of LH peaks or the total number of months in which LH was detectable. We conclude that testicular hormones secreted during the early postnatal period induce sex differences in the circannual pattern of weight change and some aspects of LH secretion. Complete masculinization, however, either requires more extensive action of gonadal hormones, perhaps both pre- and postnatally, or occurs through some androgen-independent mechanism.     

Thyroidectomy results in termination of photorefractoriness in starlings (Sturnus vulgaris) kept in long daylengths

Four groups of 10 male starlings were transferred from short daylengths (8 h light/day) to long daylengths (18 h light/day), which caused the tests to develop rapidly to maximum size and then to decrease to minimal size as birds became photorefractory. Birds were surgically thyroidectomized at 8, 16 or 28 weeks. A fourth group was left intact. Testicular volume and plasma FSH and prolactin concentrations were measured. After 42 weeks all birds were castrated and plasma FSH was measured during the next 6 weeks. Testicular growth began in all thyroidectomized birds between 4 and 8 weeks after thyroidectomy. By 42 weeks, the testes of all thyroidectomized birds were large, whereas those of intact birds were still of minimal size. Plasma FSH concentrations remained low in all birds and plasma prolactin values, originally elevated by long daylengths, decreased at a similar rate in thyroidectomized and intact birds. After castration at 42 weeks, plasma FSH values increased rapidly in all thyroidectomized birds but remained low in non-thyroidectomized birds. The results demonstrate that thyroidectomy of photorefractory starlings does not induce immediate testicular growth but may initiate a process which eventually terminates photorefractoriness in a way similar to that caused by return to short daylengths.     

Timing of photorefractoriness in the European starling: significance of photoperiod early and late in the reproductive cycle

In European starlings, as in many other birds inhabiting higher latitudes, gonads develop in response to the increasing daylengths in early spring. Later in the year, however, the hypothalamo-pituitary-gonadal axis becomes refractory to the previously stimulatory long photoperiods and the gonads regress in summer. The present study addresses the question of when during the gonadal growth phase photorefractoriness is determined. A 13-h photoperiod induces testicular development and subsequent testicular regression associated with refractoriness in male starlings. An 11-h photoperiod, in contrast, induces only testicular development, and photorefractoriness never develops. When starlings were transferred to an 11-h photoperiod, either 12 or 25 days following exposure to a 13-h photoperiod, their testes developed to full size, but remained large to the end of the experiment, i.e. refractoriness did not develop. The same was even true of most birds in a third group that were transferred to an 11-h photoperiod after 46 days of the 13-h photoperiod, when gonads had developed to near maximal size. These data show that, in contrast to some other species of passerine birds, the onset of photorefractoriness does not become fixed before the testes have undergone considerable development, and that the photoperiodic conditions experienced at the end of the testicular growth phase are still effective in determining the precise time of onset of photorefractoriness. It is suggested that this peculiarity of the starling is related to the fact that its gonadal development begins rather early in spring and, hence, under much shorter photoperiods than the other species studied.     

Environmental regulation of the reproductive system in a flexibly breeding Sonoran Desert bird, the Rufous-winged Sparrow, Aimophila carpalis

We investigated reproductive regulation in male Rufous-winged Sparrows, Aimophila carpalis, a Sonoran Desert passerine that breeds after irregular summer rains. Field and captive data demonstrate that increased photoperiod stimulates testicular development in March and maintains it until early September. Free-living birds caught in July and placed on captive long days (16L: 8D) maintained developed testes for up to 7 months, and free-living birds caught in September, during testicular regression, redeveloped testes when placed on captive long days, indicating that these birds were still photosensitive. Captive birds on long days maintained testicular development when exposed to temperatures mimicking those occurring during regression in free-living birds. In free-living birds, testicular development was observed during spring and summer, but unless this was associated with rainfall, breeding (indicated by juveniles) did not occur. Large increases in plasma luteinizing hormone (LH) in free-living males were correlated with heavy rainfall in July/August, when the birds bred, and in November, when they did not breed. In captive birds, plasma LH concentrations were unresponsive to photoperiodic changes, but may have responded to social cues. Plasma prolactin concentrations were directly correlated with photoperiod in free-living birds, but an effect of photoperiod on prolactin secretion was not seen in captive birds. It is concluded that male Rufous-winged Sparrows use long photoperiods to stimulate and maintain testicular development, but exposure to long photoperiods does not terminate breeding by inducing absolute photorefractoriness. The specific timing of reproductive behaviors is apparently determined by elevated plasma LH coinciding with long day stimulated gonad development.     

Thyroxine treatment prevents the development of photosensitivity in european starlings (Sturnus vulgaris)

Summary In starlings, the breeding season is terminated by a state of photorefractoriness. Birds remain completely reproductively inactive as long as long days are maintained, and only exposure to short days restores photosensitivity. Two experiments investigated the role of different doses of thyroxine in the development of photosensitivity in castrated starlings. First, photorefractory castrated male starlings were moved from long (18L:6D) to short (8L:16D) days, and received in the drinking water either 1 or 10 mg · 1^-1 thyroxine for the first 7 weeks of a 14-week observation period. Control birds regained photosensitivity after 5 weeks of short days, as signaled by a spontaneous increase in plasma LH, whereas the return to photosensitivity was delayed until weeks 7 and 9 in the 1- and 10-mg · 1^-1 thyroxine-treated birds, respectively. In the second experiment, the effect of different doses of thyroxine was explored at the level of the hypothalamic Gn-RH neurosecretory neurones. The acquisition of photosensitivity in control birds transferred from long to short days was characterized by a marked increase in hypothalamic Gn-RH content (while long-day controls maintained low Gn-RH content). Doses of 10 and 20 mg · 1^-1 of thyroxine completely prevented the return to photosensitivity, as seen through changes in either plasma LH concentrations or hypothalamic Gn-RH content, while a dose of 1 mg · 1^-1 allowed a partial recovery of photosensitivity, as hypothalamic Gn-RH content increased to an intermediate level and the spontaneous rise in plasma LH occurred slowly but steadily.Abbreviations Gn-RH gonadotrophin-releasing hormone - LH luteinizing hormone - LHRH-I luteinizing hormone releasing hormone     

Short days induce premature reproductive maturation in juvenile starlings, Sturnus vulgaris

Castration of juvenile and photorefractory adult starlings caused no immediate increase in circulating concentrations of LH. In castrated juveniles and adults exposed to natural changes in daylength, plasma LH increased between mid-October and mid-November, although the increase was more rapid in adults. In castrated photorefractory adults, plasma LH increased 3-5 weeks after transfer to artificial short days (8L:16D). In castrated juvenile starlings plasma LH increased 4-6 weeks after transfer to 8L:16D, irrespective of the age of the birds. Birds as young as 17 weeks had high LH concentrations. These results suggest that the reproductive system of juvenile starlings is in the same state as that of photorefractory adults, and therefore the activation of the hypothalamo-pituitary axis for the first time in juveniles is analagous to the termination of photorefractoriness in adults.     

Seasonal variations in plasma luteinizing hormone levels of gonadectomized male ground squirrels (Spermophilus lateralis)

Plasma luteinizing hormone (LH) levels were determined at monthly intervals in intact and orchidectomized ground squirrels maintained in a constant 14L:10D photoperiod at a temperature of 23 +/- 2 degrees C. Animals were orchidectomized or sham operated at different ages and at different stages of the annual reproductive cycle. LH levels were elevated in orchidectomized as compared to intact males within 1 month, and often within 48 h, after orchidectomies performed either before or after the normal breeding season. LH levels tended to remain chronically elevated in long-term orchidectomized males. Although negative feedback regulation of LH by the testes was evident at most stages of the reproductive cycle, effects of orchidectomy on LH were most pronounced during the nonbreeding season when plasma LH titers of intact males were at their annual nadirs, and less pronounced or absent during the breeding season when levels were normally elevated in intact males. For most of the year male gonadal hormones exert negative feedback on LH release. We hypothesize that under the influence of a circannual timing mechanism, sensitivity to steroid feedback is reduced as the breeding season approaches and LH secretion increases despite the attendant rise in plasma androgen levels. These results contrast with data for female squirrels in which there is a seasonally restricted and characteristically brief elevation of plasma LH after ovariectomy; in females, but not in males, onset and termination of LH secretion appear to be regulated by a circannual clock that functions independently of gonadal secretions.     

Modulation of gonadotrophin and prolacrin secretion by daylength and breeding behaviour in free-living starlings, Sturnus vulgaris

Plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin were measured by radioimmunoassay in plasma samples collected from free-living starlings, Sturnus vulgaris , trapped in nest-boxes. By leaving some nest-boxes undisturbed, and repeatedly destroying nests in others, birds from a single-brooded population were trapped whilst nest-building, incubating or feeding nestlings, at different times throughout the normal breeding season. In both males and females trapped whilst nest-building, plasma LH and prolactin levels increased progressively from mid March until late May. In females sampled during incubation, LH and FSH levels were high throughout May but decreased in early June. Prolactin levels were highest in late May. In both males and females trapped during mid May, LH levels were highest in these birds which were nest-building at this time and lowest in those feeding nestlings, FSH did not change significantly, and prolactin was low in those birds which were nest-building and high in those incubating or feeding nestlings. In female starlings from a double-brooded population, levels of LH and FSH were much lower whilst feeding the second brood than when feeding the first brood, whereas prolactin levels were similar. The results suggest that incubation and feeding behaviour inhibit the increase in LH secretion caused by increasing daylength, but stimulate prolactin secretion in excess of that caused by increasing daylength.