Relative photorefractoriness, prolactin, and reproductive regression in a flexibly breeding sonoran desert passerine, the rufous-winged sparrow, Aimophila carpalis

We tested the hypothesis that adult male rufous-winged sparrows, Aimophila carpalis, exhibit relative photorefractoriness. This condition results in partial loss of sensitivity to photoperiod as a reproductive stimulus after prolonged exposure to long photoperiods and is similar to the mammalian condition called photoperiodic memory. Captive birds were exposed either to 8 h of light/16 h of dark per day (8L) or to 16L for 11 weeks and were then exposed either to 8L, 13L, 14L, or 16L. Testicular diameter, plasma luteinizing hormone (LH), and plasma prolactin (PRL) were measured to assess reproductive system activity in response to photostimulation. In free-living birds, testicular diameter, plasma LH, and PRL were compared in birds caught in September in a year when birds were breeding and in a year when birds were not breeding to further evaluate the role of PRL in the termination of seasonal breeding. Testes completely developed after transfer from 8L to 14L or to 16L and partially developed after transfer from 8L to 13L. However, after 11 weeks of 16L exposure, transfer to 14L caused partial regression and transfer to 13L caused complete regression of the testes. Plasma LH increased in all birds that were transferred from 8L to a longer photoperiod. PRL showed a weak response to longer photoperiod treatment and was elevated in birds after chronic 16L exposure in comparison to birds exposed to chronic 8L. These data indicate that male rufous-winged sparrows lose sensitivity to photoperiod after long photoperiod exposure consistent with the relative photorefractoriness and photoperiodic memory models. Lower PRL in birds that developed testes on 13L and 14L compared to birds that regressed testes on 13L and 14L are consistent with the hypothesis that PRL regulates relative photorefractoriness. However, PRL does not appear to regulate interannual differences in the timing of testicular regression.     

Photoperiod affects reproductive responsiveness to 6-methoxy-2-benzoxazolinone in house mice

House mice (Mus musculus) have traditionally been characterized as nonphotoperiodic because reproductive function is unaffected by day length in the laboratory. In the present study, the reproductive responsiveness of CF1 mice to a naturally occurring plant metabolite was assessed in animals that were maintained in either long (16L:8D) or short (8L:16D) photoperiods from birth until the end of the study. Males and females were implanted i.p. with either an empty Silastic capsule or one containing 6-methoxy-2-benzoxazolinone (6-MBOA) at either 21 or 60 days of age for 3 days. Other 31-day-old mice were implanted with capsules for 8 wk. Three-day exposure to 6-MBOA stimulated uterine growth in short-day, adult females, but did not affect adult females housed in long photoperiods. Short-term treatment with 6-MBOA did not significantly affect reproductive parameters in either long- or short-day peripubertal house mice, or in adult males regardless of photoperiod, nor did exposure to 6-MBOA for 8 wk influence reproduction in males in either photoperiodic condition. However, short-day female mice had significantly reduced ovarian and uterine masses after 8 wk chronic 6-MBOA treatment as compared to long-day animals or mice unexposed to 6-MBOA. Short-day females exposed to 6-MBOA for 8 wk developed a denser pelage compared to long-day mice treated with this compound. Photoperiod-mediated differential responsiveness to 6-MBOA indicates that female house mice can discriminate long from short days, and these results suggest that the physiological mechanisms for photoperiodic responsiveness remain extant in this species previously characterized as nonphotperiodic.     

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.     

Effect of ambient temperature on serum prolactin and luteinizing hormone levels during the reproductive life cycle of the female turkey (Meleagris gallopavo)

An ambient temperature of 30 degrees C compared to 18 degrees C accelerated the increase in serum prolactin (Prl) level induced by photostimulation of female turkeys. The contribution of reproductive stage and nesting behavior to this serum Prl elevation was assessed by housing adult female turkeys in individual wire cages while allowing other females free access to nests on the floor. Birds of both groups were exposed to 10 degrees C, 24 degrees C or 30 degrees C beginning 4 wk prior to photostimulation and continuing throughout the reproductive phase. Lapsed time between the onset of photostimulation and onset of sexual maturity, and between the onset of sexual maturity and onset of incubation behavior was shorter in birds housed at 30 degrees C with access to nests than in corresponding birds housed at 24 degrees C and 10 degrees C. The increases in serum Prl associated with sexual maturity or incubation behavior occurred at a greater rate in the birds maintained at 30 degrees C. Cage-reared birds had the same lapsed time between onset of photostimulation and onset of sexual maturity and the same sustained low Prl level regardless of ambient temperature exposure. All groups exhibited similar luteinizing hormone profiles. These findings indicate that the accelerated increase in Prl under elevated temperature in floor-reared turkeys is related to accelerated development of reproductive function, and not the direct effect of ambient temperature on mechanisms controlling Prl.     

Plasma prolactin, thyroxine, triiodothyronine, testosterone, and luteinizing hormone during a photoinduced reproductive cycle in mallard drakes

The temporal relationships between plasma concentrations of prolactin, thyroxine (T4) and triiodothyronine (T3) were determined in a group of six wild mallard drakes during the development and maintenance of long-day refractoriness after transfer from 6 h light: 18 h darkness (6L:18D) to 20L:4D for 24 weeks. As shown by changes in the plasma concentrations of luteinizing hormone (LH) and testosterone, the birds came into breeding condition and then became long-day refractory within 5 weeks of photostimulation. Long-day refractoriness was maintained for the remainder of the study. Plasma prolactin began to increase immediately after photostimulation, although not as fast as the increases in plasma LH and testosterone. The concentration of plasma T4 also increased after photostimulation but, as shown by decreased plasma LH and testosterone levels, only after the birds had become long-day refractory. The development of long-day refractoriness was thus directly correlated with an increased plasma prolactin and not with a change in plasma concentration of T4. Plasma T3 decreased after photostimulation but returned to prestimulation values as the birds became long-day refractory and remained stable for the remainder of the study. Concentrations of plasma T4 and prolactin returned to baseline values after about 15 weeks photostimulation showing that the long-term maintenance of long-day refractoriness is not directly related to continuously high plasma concentrations of either hormone.     

Effect of intranasal irrigation with zinc sulphate on male-induced oestrus in the wild mouse, Mus musculus domesticus

Adult males of the subtropical house sparrow (Passer domesticus) were subjected to continuous darkness (DD) or short days (8L:16D) after photostimulation (15L:9D) for 30 days. One group was retained on the long day (15L:9D) schedule. Another two groups of photosensitive birds were placed under DD or 8L:16D. The photostimulated birds under DD or 8L:16D showed testicular regression and the rate of regression was similar in both the treatments. The photosensitive birds, on the other hand, did not show any initiation in testicular growth and development in either of the treatments. Long day birds (15L:9D) maintained enlarged testes without regression (the onset of photorefractoriness) during the treatment period. These results conform with the external coincidence model, but could also be explained by a modified internal coincidence model.     

Maternal transfer of photoperiodic information in Siberian hamsters. IV. Peripubertal reproductive development in the absence of maternal photoperiodic signals during gestation

Peripubertal reproductive development of Siberian hamsters is influenced by photoperiodic information received during gestation; the maternal pineal is important for this process. We observed that in the absence of the maternal pineal, the fetus appears to receive no information about gestational photoperiods. This is not the equivalent of receipt of a long-day signal by the fetus. Pinealectomized and sham-operated pregnant females were exposed to photoperiods of 12L:12D, 14L:10D, 16L:8D, or constant light (LL); young were reared from birth to 28 days of age in LL or 14L:10D. Regardless of the gestational photoperiod, LL-reared male young born to pinealectomized dams had smaller testes than LL-reared young of pineal-intact dams exposed to 16L:8D while pregnant. Thus, pinealectomy did not result in transmission of a long-day signal, nor did young born of pinealectomized dams receive short- or intermediate-day signals. Unlike young of pineal-intact females exposed to 12L:12D or 14L:10D while pregnant, young born of pinealectomized dams had small testes when reared in 14L:10D, irrespective of gestational photoperiod. Uterine weights of female young presented similar patterns of responses. In a second experiment, adult females were entrained to 12L:12D, 14L:10D, or 16L:8D for 3 wk prior to pinealectomy to determine if the effect of maternal pinealectomy would be altered. Entrainment to the new photoperiod prior to surgery did not alter the effects of maternal pinealectomy.     

Influence of a new slow-release GnRH analogue implant on reproduction in the Budgerigar (Melopsittacus undulatus, Shaw 1805)

The neuroendocrine conditioning of reproduction in birds could perform a very important role in captive breeding, especially in endangered species. Whereas in domestic and wild mammals pharmacological reproductive conditioning is well developed, in birds an effective method is not available. The aim of this study was to test the influence of a new slow-release GnRH analogue (buserelin acetate) implant on the reproductive activity of the Budgerigar (Melopsittacus undulatus), used as model species for captive-bred endangered birds. The effects were assessed by looking at reproductive parameters (egg-laying rate, egg fertility rate) and measuring excreted sex steroid metabolite concentrations in male and female birds. Modification of reproductive parameters and steroid metabolites excretion patterns were observed among birds administered with a GnRH analogue implant and maintained under artificial photoperiod (group I; 16L:8D). Implanted birds showed higher rates of egg-laying, potentially a higher proportion of fertile eggs and higher excreted steroid metabolite concentrations than birds maintained under natural photoperiod (group II; 10L:14D) and birds maintained under artificial photoperiod (group III; 16L:8D). Thus, it is concluded that the new slow-release GnRH analogue implant may represent an innovative and practicable treatment to rapidly induce reproductive activity in the Budgerigar, and that excreted sex hormone metabolites detection permits to monitor male and female gonadal activity.     

Auditory stimulation of reproductive function in male Rufous-winged Sparrows, Aimophila carpalis

Prolonged exposure to conspecific song stimulates gonadal function and reproductive hormone secretion in female birds but few studies have investigated the physiological effects of conspecific song exposure on males outside of short-term, aggressive interactions. We exposed male Rufous-winged Sparrows, Aimophila carpalis, either to conspecific song (CS Song), to heterospecific song (Black-throated Sparrow, Amphispiza bilineata; HS Song), or to no recorded song (No Song) for 59 consecutive days (two h per day). Birds were exposed to short days (8L:16D) for the first 21 days of treatment and were then transferred to long days (13L:11D) for the remaining 38 days. During long day exposure, CS Song birds experienced faster growth of testes than HS Song and No Song birds. HS Song birds also grew their testes faster than No Song birds. Plasma luteinizing hormone (LH) and testosterone did not differ between CS Song and No Song birds. However, plasma LH was higher in HS Song birds compared to other groups. There were no differences in hypothalamic immunocytochemical labeling for gonadotropin-releasing hormone, its precursor proGnRH, or gonadotropin-inhibitory hormone, nor were there differences in two song control nuclei volumes (HVC and RA) between CS Song and No Song treatment groups. Furthermore, we found no effect of heterospecific song on free-living Rufous-winged Sparrow aggressive behaviors. These data indicate that long-term exposure to auditory stimuli, such as song, can influence the reproductive system of male songbirds and different types of auditory stimuli can have differential effects on reproductive function.     

Influence of helping and breeding experience on reproductive performance in the Seychelles warbler: a translocation experiment

Reproductive success of the cooperative breeding Seychelleswarbler (Acrocephalus sechellensis) increases with age. Thisage effect is not due to differential survival or increasedreproductive effort, but to accumulated helping and breedingexperience. In their first year of breeding, reproductive performanceof inexperienced warblers with neither helping nor breedingexperience was significandy lower than that of warblers of thesame age with either previous helping or breeding experience.Reproductive performance was the same for primiparae with helpingexperience and for birds with breeding experience. Female primiparaewith helping experience or breeding experience built betternests and spent more time incubating than inexperienced females,which led to increased hatching success. Male primiparae withhelping experience or males with breeding experience guardedthe clutch better than inexperienced males, which led to reducedegg predation. Even-aged warblers with different previous experienceswere transferred to unoccupied islands, where birds startedbreeding immediately in high-quality territories. The experimentshowed that birds with helping experience produced their firstfledgling as fast as experienced breeders, and significandyfaster than inexperienced birds. Breeding performance did notimprove further with experience after the first successful breedingattempt. Only birds with previous breeding experience who pairedwith inexperienced birds, were likely to change mate. The otherpair combinations remained stable. Thus, primiparous birds withhelping experience have greater lifetime reproductive successthan inexperienced primiparae of the same age. This experimentshows that helping behavior has not only been selected for inthe context of promoting an individual's indirect fitness, butalso in the context of gaining helping experience which translatesinto improved reproductive success when a helper becomes a breeder.[Behav Ecol 7: 326-333 (1996)]     

Inhibition of reproductive maturation and somatic growth of Fischer 344 rats by photoperiods shorter than L14:D10 and by gradually decreasing photoperiod

Photoperiod is the major regulator of reproduction in temperate-zone mammals. Laboratory rats are generally considered to be nonphotoresponsive, but young male Fischer 344 (F344) rats have a uniquely robust response to short photoperiods of 8 h of light. Rats transferred at weaning from a photoperiod of 16 h to photoperiods of < 14 h of light slowed in both reproductive development and somatic growth rate. Those in photoperiods < 13 h of light underwent the strongest responses. The critical photoperiod of F344 rats can be defined as 13.5 h of light, but photoperiods of 相似文献   

Senescence and carryover effects of reproductive performance influence migration,condition, and breeding propensity in a small shorebird

Breeding propensity, the probability that an animal will attempt to breed each year, is perhaps the least understood demographic process influencing annual fecundity. Breeding propensity is ecologically complex, as associations among a variety of intrinsic and extrinsic factors may interact to affect an animal's breeding decisions. Individuals that opt not to breed can be more difficult to detect than breeders, which can (1) lead to difficulty in estimation of breeding propensity, and (2) bias other demographic parameters. We studied the effects of sex, age, and population reproductive success on the survival and breeding propensity of a migratory shorebird, the piping plover (Charadrius melodus ), nesting on the Missouri River. We used a robust design Barker model to estimate true survival and breeding propensity and found survival decreased as birds aged and did so more quickly for males than females. Monthly survival during the breeding season was lower than during migration or the nonbreeding season. Males were less likely to skip breeding (range: 1–17%) than females (range: 3–26%; β_sex = ?0.21, 95% CI: ?0.38 to ?0.21), and both sexes were less likely to return to the breeding grounds following a year of high reproductive success. Birds that returned in a year following relatively high population‐wide reproductive output were in poorer condition than following a year with lower reproductive output. Younger adult birds and females were more likely to migrate from the breeding area earlier than older birds and males; however, all birds stayed on the breeding grounds longer when nest survival was low, presumably because of renesting attempts. Piping plovers used a variety of environmental and demographic cues to inform their reproduction, employing strategies that could maximize fitness on average. Our results support the “disposable soma” theory of aging and follow with predictions from life history theory, exhibiting the intimate connections among the core ecological concepts of senescence, carryover effects, and life history.     

Lack of reproductive photoresponsiveness and correlative failure to respond to melatonin in a tropical rodent, the cane mouse.

Cane mice (Zygodontomys brevicauda) are year-round breeders in Venezuela. As shown previously, these animals are not reproductively responsive to variation in photoperiod. In the present experiments, male cane mice were maintained on long or short day lengths (16L:8D or 8L:16D, respectively) and challenged with each of three experimental treatments known to "unmask" reproductive photoresponsiveness in laboratory rats: olfactory bulbectomy, prolonged food restriction, and exposure as neonates to a single injection of testosterone. Variation in photoperiod had no inhibitory effect on the responses of cane mice to any of these three treatments, as assessed by the weight of their testes and seminal vesicles. A fourth experiment demonstrated that cane mice are insensitive to 10 wk of continuous exposure to pharmacological levels of melatonin, again as assessed by reproductive organ weight. Likewise, a fifth experiment documented a lack of response to 10 wk of late-afternoon injections of massive amounts of melatonin. The cane mouse apparently is unique among the animals challenged so far in these ways in that it seems to have no vestige of reproductive photoresponsiveness.     

Short day lengths delay reproductive aging

Caloric restriction and hormone treatment delay reproductive senescence in female mammals, but a natural model of decelerated reproductive aging does not presently exist. In addition to describing such a model, this study shows that an abiotic signal (photoperiod) can induce physiological changes that slow senescence. Relative to animals born in April, rodents born in September delay their first reproductive effort by up to 7 mo, at which age reduced fertility is expected. We tested the hypothesis that the shorter day lengths experienced by late-born Siberian hamsters ameliorate the reproductive decline associated with advancing age. Short-day females (10L:14D) achieved puberty at a much later age than long-day animals (14L:10D) and had twice as many ovarian primordial follicles. At 10 mo of age, 86% of females previously maintained in short day lengths produced litters, compared with 58% of their long day counterparts. Changes in pineal gland production of melatonin appear to mediate the effects of day length on reproductive aging; only 30% of pinealectomized females housed in short days produced litters. Exposure to short days induces substantial decreases in voluntary food intake and body mass, reduced ovarian estradiol secretion, and enhanced production of melatonin. One or more of these changes may account for the protective effect of short day lengths on female reproduction. In delaying reproductive senescence, the decrease in day length after the summer solstice is of presumed adaptive significance for offspring born late in the breeding season that first breed at an advanced chronological age.     

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.     

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.     

Effect of various photoperiods on testicular weight, weekly sperm output and plasma levels of LH and testosterone over the reproductive season in male turkeys

The effects of duration and variation in photoperiod on testis weight, testicular sperm production, semen output, and hormone status over the reproductive season in male turkeys were investigated. In Experiment 1, four groups of males raised from 17 to 23 wk of age under a constant short photoperiod were subjected to a constant short (Group 1: 7L:17D; Group 2: 10.5L:13.5D), constant long (Group 3: 14L:10D) or progressively increasing photoperiod (Group 4: 7L:17D to 14L:10D) up to 60 wk of age. In Experiment 2, four groups of males first raised as in Experiment 1 up to 23 wk of age were placed under a constant short (Group 5: 10.5L:13.5D), constant long (Group 6: 14L:10D), or night-interrupted photoperiod (Group 7: 6L:2.5D:1L:14.5D, referred to as subjective 9.5L:14.5D; Group 8: 6L:3.5D:1L:13.5D), referred to as subjective 10.5L:13.5D) up to 60 wk of age. Males in Groups 2-4 had similar reproductive characteristics, whereas sexual maturity was delayed from 29 to 49 wk in males from Group 1. In Experiment 2, males in Groups 5 and 8 had similar reproductive characteristics, whereas sexual maturity was delayed in males in Group 7 in a manner similar to that observed in Group 1. In both experiments, plasma LH and testosterone concentrations were poor indicators of testis development and semen production, irrespective of age and photoperiod. We conclude that a moderately short photoperiod such as 10.5L:13.5D or subjective 10.5L:13.5D may stimulate reproductive characteristics of male turkeys in a manner comparable to constant long or increasing photoperiods. We inferred the existence of a threshold of photosensitivity in male turkeys for photoperiods longer than 9.5L:14.5D, but shorter than or equal to 10.5L:13.5D.     

Cue hierarchies and testicular development: is food a more potent stimulus than day length in an opportunistic breeder (Taeniopygia g. guttata)?

Opportunistic breeders inhabit areas with unpredictable changes in environmental conditions. In such places favorable breeding conditions can occur during any time of year, and one prediction is that individuals should attend to photoperiod less than to more immediate cues to time reproduction. This study tests whether zebra finches utilize photoperiod independently of other proximate cues, specifically food availability. We transferred semi-domesticated male Lesser Sundas zebra finches (Taeniopygia guttata guttata) from 8 h light, 16 h dark per day (8L:16D) with ad libitum food availability to 20L:4D with ad libitum food (LD ad lib group) or food restriction (LD restricted group). A third group remained on 8L:16D with ad libitum food availability (SD ad lib group). Testis volume in LD ad lib males increased and was larger than other groups within 30 and 60 days of photostimulation. By contrast, LD restricted males and SD ad lib males did not exhibit significant gonadal growth. However, both LD groups increased mass irrespective of food availability. Surprisingly, at the end of the experiment the SD ad lib group sang the most undirected song. Our data demonstrate that long days alone are not sufficient to drive reproductive development in this opportunistically breeding species. Rather, it appears that reproductive development is stimulated by extended feeding times or increased food abundance during long days, and not by changes in day length per se. These data lend support to the proposition that photoperiod acts as a supplementary cue or permissive factor in this system, and thus represents the possibility of a reversal in the hierarchy of cue sensitivity.     

Social cues attenuate photoresponsiveness of the male reproductive system in Siberian hamsters (Phodopus sungorus)

Transfer of adult Siberian hamsters (Phodopus sungorus) from long (16 h light and 8 h dark, 16L:8D) to short (8L:16D) daily photoperiods induces an involution of the gonads and a cessation of reproductive behavior 8 to 10 weeks later. However, when male and female long-day hamsters were paired on transfer to short photoperiods, the males' gonads did not undergo the typical short-day response. Similarly, when male long-day hamsters were paired with refractory females (i.e., females housed in short photoperiods for at least 28 weeks so that they became unresponsive to short photoperiods), the response of the males' reproductive system to short photoperiods also was attenuated. Thus, social cues can override or delay the effects of photoperiod on the testes of this species. These results suggest that the inhibitory effects of long durations of melatonin secretion (in response to short photoperiods) on the male hypothalamic-pituitary-gonadal axis may be attenuated by social cues such as contact with the opposite sex.     

Breeding Propensity of Female Harlequin Ducks

Abstract: Breeding propensity, the proportion of sexually mature females that initiate egg production, can be an important demographic trait when considering reproductive performance and, subsequently, population dynamics in birds. We measured egg production using yolk precursor (vitellogenin and very-low-density lipoprotein) analyses and we measured nesting using radiotelemetry to quantify breeding propensity of adult female harlequin ducks (Histrionicus histrionicus) in British Columbia, Canada, in 2003 and 2004. Using both methods combined, and accounting for error rates of each, we estimated that breeding propensity of adult females that migrated to breeding streams was 92%. These data suggest that, despite speculation that harlequin ducks have low breeding propensity, almost all adult females on our study site were not constrained in their ability to produce eggs and that influences on reproductive performance at later stages likely have much stronger effects on population dynamics.