Gonadal Cycle Research Articles

Annual cycles of urinary reproductive steroid concentrations in wild and captive endangered Fijian ground frogs ( Platymantis vitiana)

Annual cycles of reproductive steroid metabolites were measured in urine collected from free-living and captive tropical endangered Fijian ground frogs ( Platymantis vitiana) a terrestrial breeding. Free-living frogs were sampled on Viwa Island, Fiji and captive frogs were maintained in an outdoor enclosure in Suva, Fiji. Urinary estrone, progesterone and testosterone metabolite concentrations increased in male and female frogs after hCG challenges, with clear peaks in steroid concentrations 2 or 3 days after the challenges. There were annual cycles of testosterone metabolites in wild and captive males, and of estrone and progesterone metabolites in wild and captive females. Peaks of steroid concentrations in the wet season corresponded with periods of mating and egg laying in females in December and January. Steroid concentrations declined in January and February when maximum egg sizes in females were also declining. Body weights of wild male and vitellogenic female frogs showed annual cycles. Body weights of non-vitellogenic female frogs varied significantly between months, although there was no clear pattern of annual changes. Body weights of the 3 captive male frogs and 4 captive female frogs were similar to those of the wild frogs. Estrone metabolites were 80% successful in identifying non-vitellogenic females from males. The results suggest that the Fijian ground frog is a seasonal breeder with an annual gonadal cycle, and this species is likely to be photoperiodic. Urinary steroid measurements can provide useful information on reproductive cycles in endangered amphibians.

The timing of gonadal development and moult in three raptors with different breeding seasons: effects of gender, age and body condition

Differences between species in breeding seasons are thought to be mediated through differences in their reproductive physiology. Little is known about how the timing and duration of gonadal maturation varies between raptor species, how the timing of moult relates to the gonadal cycle, whether the timing and degree of sexual maturation varies between juveniles and adults or whether body condition has a significant effect. To address these questions, data on gonadal size and moult for adults and juveniles of both sexes of three raptor species were extracted from the Predatory Bird Monitoring Scheme (based on birds found dead by members of the public). The three species, Sparrowhawk Accipiter nisus, Kestrel Falco tinnunculus and Barn Owl Tyto alba, have different ecologies – diurnal bird predator, diurnal mammal predator and nocturnal mammal predator, respectively. All are single‐brooded but have different breeding seasons. The duration of gonadal maturation was markedly different between the species. Barn Owls showed the earliest maturation and the latest gonad regression, and Sparrowhawks the latest maturation and earliest gonad regression. Kestrels were intermediate. In males of all species, the testes remained fully mature throughout their respective breeding seasons. In females, the ovaries remained partially mature throughout the breeding season. Moult started slightly earlier in Sparrowhawks than in Kestrels and coincided with gonadal regression in the two species. Although females of the two species started to moult earlier than males, moult duration was similar between the sexes. Barn Owls showed no distinct annual pattern of moult. In juveniles of all three species, the gonads were smaller than in adults throughout spring and started to mature later. Gonad size in birds that had starved tended to be smaller than in birds dying from other causes, but did not influence the difference in gonad mass between adults and juveniles and between seasons. Body condition had no effect on moult. Whilst ecology has led to the evolution of different breeding seasons, differences between species, and between adults and juveniles, are mediated through adaptive differences in their reproductive physiology.

Gonadal Cycle ofTegula eiseni(Jordan1936) (Mollusca: Gastropoda) in Bahía Asunción, Baja California Sur, Mexico

ABSTRACT The gonadal cycle of Tegula eiseni (Jordan, 1936) was determined on monthly samples (25–30 organisms) collected from a population in Bahia Asuncion, B.C.S., from January 2006 to December 2006. The results of the histological study indicate that both males and females show stages of gametogenesis and gonad maturation throughout the year and only two major spawning events: one in spring and another in fall. The species behaved as a partial spawner, and only at certain times was total spawning observed in females. Maturation and release or spawning times seem to be closely related to seasonal temperature rises and abrupt changes in surface sea water temperature, which occur at brief intervals for several days at certain times of year.

Expression and gonadotropin regulation of membrane progestin receptor alpha in Atlantic croaker ( Micropogonias undulatus) gonads: Role in gamete maturation

Recent results suggest that membrane progestin receptor alpha (mPRα) mediates nongenomic actions of progestin hormones to induce oocyte maturation and sperm hypermotility in several teleost species. The role of mPRα in gamete and gonadal physiology was further evaluated in the present study by examining gonadal expression of mPRα during gamete maturation in Atlantic croaker ( Micropogonias undulatus), a well-characterized teleost model of oocyte maturation and sperm motility. Sequencing of the croaker mPRα gene isolated from croaker ovaries showed it is 98% homologous at the nucleotide level to spotted seatrout mPRα. The mPRα mRNA and protein were detected in both somatic and gonadal tissues. In croaker ovaries, the mPRα protein was present throughout the gonadal cycle and was upregulated by gonadotropin in vitro, coincident with the acquisition of oocyte maturational competence (i.e., ability to respond to progestin hormones and complete oocyte maturation). Both mPRα mRNA and protein were also expressed in croaker testes throughout the gonadal cycle. Expression of mPRα protein was weakly upregulated in testes after 18 h of in vitro gonadotropin treatment. Immunocytochemical staining showed mPRα was localized to both germ and interstitial cells. Finally, elevated levels of mPRα protein in croaker sperm were associated with high sperm motility. Taken together, these data strongly support the hypothesis that mPRα mediates progestin induction of oocyte maturation and upregulation of sperm motility in teleosts.

Reproductive maturity in the edible Chilean crabCancer edwardsii:methodological and management considerations

Size-at-maturity, often used by fisheries managers to specify reference points, was estimated forCancer edwardsii, the most intensely exploited brachyuran crab in Chile, using morphometric data and observation of the gonadal cycle. Sampling was conducted in Chiloé Island, the principal landing region. Several morphometric measurements of secondary sexual characters were recorded for both sexes. Their size-dependence was investigated in search of ontogenetic allometric changes. Six stages of maturity were established for females, and four for males, on the basis of macroscopic and histological observation of the reproductive system, allowing distinction between adults and juveniles. Females and males, on average, attained full development of secondary sexual characters at 106 and 118 mm carapace width (CW) respectively. The CW at which 50% of females and males have gonads of adult appearance was around 101 mm. Current minimum legal size is 120 mm CW, seemingly high enough for conservation purposes. This management control, however, is unlikely to provide effective protection due to the high proportion of crab of sub-legal size in the landings. Traps with escape vents could be introduced in this fishery in order to ensure its sustainability.

Cloning and expression of StAR during gonadal cycle and hCG-induced oocyte maturation of air-breathing catfish, Clarias gariepinus

Complementary DNAs encoding steroidogenic acute regulatory protein (StAR) have been isolated from different fish species, yet the relevance of StAR during gonadal cycle and more importantly in final oocyte maturation has not been assessed so far. A cDNA encoding StAR was isolated from the ovarian follicles of air-breathing catfish, Clarias gariepinus. Catfish StAR exhibited 55 to 72% identity at nucleotide level with other vertebrate orthologs. RT-PCR analysis of tissue distribution pattern demonstrated the presence of StAR mRNA in various tissues including gonads, kidney, liver, brain and intestine of catfish. Real-time RT-PCR analysis revealed high expression of StAR mRNA in the pre-spawning phase of ovary while it was low in preparatory, spawning and regressed phases. In testis, maximum expression was noticed during the preparatory phase. During human chorionic gonadotropin (hCG)-induced oocyte maturation, both in vitro and in vivo, StAR mRNA levels were augmented by 2 h and then declined gradually to reach basal levels by 12 h as that of saline-treated controls. Taken together, high level of expression during hCG-induced oocyte maturation vis-à-vis in spawning suggests a role for StAR, in addition to the steroidogenic enzyme genes in final oocyte maturation.

Brooding of the sub-Antarctic heart urchin, Abatus cavernosus (Spatangoida: Schizasteridae), in southern Patagonia

The reproductive cycle of the sub-Antarctic spatangoid sea urchin, Abatus cavernosus, was examined during a 2-year period in southern Patagonia, Argentina. The population studied is the northernmost known coastal population in the austral oceans, and is influenced by a greater annual range of seawater temperature than other Abatus species. The sex ratio of the overall A. cavernosus population did not differ significantly from 1:1, but was not homogeneous across size classes. A clearly defined annual reproductive cycle was found. Spawning occurred from May to July and was synchronous between sexes. Females were observed to be brooding within a 9-month period, from May to February. Juveniles were released during the austral summer, from January to February. The length at which 50% of the females were brooding occurred at a test length of 25.9 mm. A. cavernosus had a large egg size (mean diameter = 1.4 mm) and low fecundity (maximum = 57 eggs per female) compared to closely related species. The number of eggs within each brood pouch was highest in larger anterior brood pouch, which is close to two gonopores, as opposed to the other anterior and two smaller posterior pouches. Significant interannual variation was observed in gonad cycles, fecundity, and embryo development such as: brood size decreased during 2001; adjusted gonad dry weight and fecundity were higher during 2003. Hypotheses concerning the gonadal and brooding cycles and fecundity of sub-Antarctic and Antarctic Schizasteridae are discussed.

Life-history traits of non-native black bullheadAmeiurus melaswith comments on its invasive potential

Summary Life-history traits of non-native black bullhead Ameiurus melas from an oxbow of the River Vah (near the city of Komarno), located in the middle Danube reaches (Slovakia) were investigated during 2006 and 2007, including samples from the pre-spawning (March-May), spawning (May-June), post- spawning (late June-August) and resting (September) periods. In total, 600 specimens were examined. The sex ratio oscillated close to 1 : 1. Females were found to mature at 98.6-120 mm SL (118-145.1 mm TL, compared to 171-254 mm TL in the native North American populations). Spawning took place in May and June, although the spawning period in 2007 was more protracted. Based on the analyses of annual gonadal cycle and oocyte size distribution, the black bullhead is considered a multiple spawner with asynchronous oocyte development and indeterminate fecundity. Absolute fecundity ranged from 1238 to 4755 (mean 2610), and relative fecundity from 36.2 to 154.3 (mean 56.1) oocytes g )1 . The high flexibility in the life history of the black bullhead as demonstrated by its non-native populations, as well as its extreme tolerance and capability to live in systems with poor water quality, suggests that this species has a high potential to invade new areas and establish viable populations.

Time’s arrow flies like a bird: Two paradoxes for avian circadian biology

Biological timekeeping in birds is a fundamental feature of avian physiology, behavior and ecology. The physiological basis for avian circadian rhythmicity has pointed to a multi-oscillator system of mutually coupled pacemakers in the pineal gland, eyes and hypothalamic suprachiasmatic nuclei (SCN). In passerines, the role of the pineal gland and its hormone melatonin is particularly important. More recent molecular biological studies have pointed to a highly conserved mechanism involving rhythmic transcription and translation of “clock genes”. However, studies attempting to reconcile the physiological role of pineal melatonin with molecular studies have largely failed. Recent work in our laboratory has suggested that melatonin-sensitive physiological processes are only loosely coupled to transcriptional oscillations. Similarly, although the pineal gland has been shown to be critical for overt circadian behaviors, its role in annual cycles of reproductive function appears to be minimal. Recent work on the seasonal control of birdsong, however, suggests that, although the pineal gland does not directly affect gonadal cycles, it is important for seasonal changes in song. Experimental analyses that address these paradoxes will shed light on the roles the biological clock play in birds and in vertebrates in general.

Reproductive biology of freshwater fishes from the Venezuelan floodplains

This review describes the endocrine changes that occur during the annual reproductive cycle of Pygocentrus cariba, Pimelodus blochii, and Oxydoras sifontesi and their relationships with the environmental characteristics of Venezuelan floodplains. Most reproductive studies of teleosts have focused on changes that occur during annual cycles in temperate species but, in tropical fish, this has been examined less frequently. P. cariba, P. blochii, and O. sifontesi are seasonal breeders widely distributed along the Orinoco River. Under natural conditions they have an annual gonadal cycle closely related to changes in the annual hydrology cycle of the Orinoco River which defines two seasons on the floodplain: inundation and isolation. The reproductive cycle of these species seems to be controlled by cues from the external environment. Relevant data about gonadal maturation, for example gonadosomatic index and sexual hormones secretion, are contrasted. The role of catecholamines in neuroendocrine control of the reproductive axis is also considered in this work.

Gonadal Cycle of Northern Quahogs, Mercenaria mercenaria (Linne, 1758), from Fished and Non-fished Subpopulations in Narragansett Bay

Abstract To determine if population density in areas closed to fishing in Narragansett Bay is causing differences in the reproductive potential of the organisms, this study used two approaches to determine the reproductive condition of the animals. The first approach consisted in employing a gravimetric condition index (CI) to evaluate the general condition of quahogs from nine different sites, 3 sites open conditionally for fishing (conditional areas) and 6 sites closed to fishing. The second approach was a determination of gonadal index (GI) of a subset of the sample sites, by histological observation of gonadal tissue sections. Initial sampling included determination of CI only, and lasted from March 25 to Sep. 22, 2005. The subset of six of the sites was sampled the next year (2006), every three weeks from April 15 until Sep. 28 to determine CI and the gonadal index (GI). Results show that there is a significant difference between the CI of northern quahogs from conditional areas and quahogs from clos...

Immunolocalization of cholesterol side chain cleavage enzyme (P450scc) in Mytilus galloprovincialis and its induction by nutritional levels

We have examined the localization of P450scc in different tissues of the mollusc Mytilus galloprovincialis along a gonadal cycle by using a polyclonal antibody against rat cholesterol side-chain cleavage enzyme (cytochrome P450scc). Immunoreactivity specific for P450scc was found only in the cytoplasm of basophilic cells from digestive gland; gonad, gills and kidney appeared to be devoid of P450scc immunoreactivity. SDS-gel electrophoresis and western blot analysis of different subcellular fractions revealed that this protein is mainly located in microsomes, has a molecular weight of 51.4 kDa and is recognized by the antibody against rat P450scc. These results suggest that the digestive gland of M. galloprovincialis could be considered a steroidogenic tissue where the first step in biosynthesis of steroid hormones occurs. Moreover, seasonal slot blot analysis of post-mitochondrial fractions showed a P450scc induction by available nutrients and phytoplankton blooms.

Ambient temperature effects on photo induced gonadal cycles and hormonal secretion patterns in Great Tits from three different breeding latitudes

The present study determines how populations of Great Tits ( Parus major) breeding in southern, mid and northern European latitudes have adjusted their reproductive endocrinology to differences in the ambient temperature during the gonadal cycle. A study based on long-term breeding data, using the Colwell predictability model, showed that the start of the breeding season has a high predictability (≈ 0.8–0.9) at all latitudes, and that the environmental information factor ( I e) progressively decreased from mid Italy ( I e > 4) to northern Finland ( I e < 1). The results indicate that integration of supplementary information, such as ambient temperature, with photoperiodic initial predictive information (day length), becomes progressively more important in maintaining the predictability of the breeding season with decreasing latitude. This hypothesis was verified by exposing photosensitive Great Tits from northern Norway, southern Sweden and northern Italy to sub-maximal photo-stimulatory day lengths (13L:11D) under two different ambient temperature regimes (+ 4 °C and + 20 °C). Changes in testicular size, plasma levels of LH and testosterone were measured. The main results were: (1) Initial testicular growth rate, as well as LH secretion, was affected by temperature in the Italian, but not in birds from the two Scandinavian populations. (2) Maximum testicular size, maximum LH and testosterone levels were maintained for a progressively shorter period of time with increasing latitude, regardless of whether the birds were kept on a low or a high ambient temperature. (3) In birds from all latitudes, the development of photorefractoriness, as indicated by testicular regression and a decrease in plasma levels of LH and testosterone, started much earlier (with the exception for LH Great Tits from northern Scandinavia) when kept on + 20 °C than when kept on + 4 °C. The prolonging effects of a low temperature was more pronounced in Mediterranean birds, than in birds from Scandinavia, and more pronounced in Great Tits from southern Scandinavia than in Great Tits from northern Scandinavia. Ecological implications of the results are discussed, as well as possible impact of global warming on the breeding success of European Great Tits from different breeding latitudes.

Changes in vasotocin immunoreactivity of paraventricular nuclei and adrenal function of Japanese quail in relation to different phases of sexual development

In relevance to osmoregulatory and reproductive functions, activity of hypothalamic neurosecretory neurons may also vary seasonally. The current study was performed to determine annual changes in ir-AVT neurons of hypothalamus and adrenal gland function. We examined changes in ir-AVT neuron by immunohistochemical method and plasma testosterone was measured by enzyme immunoassay. The steroidogenic interrenal activity was studied by histological and biochemical methods. Birds were sampled in February (quiescent), April (recrudescent), June (breeding) and November (regressive). A significant and gradual increase in the number of ir-AVT neurons was observed from quiescent to breeding phase which decreased during regressive phase of annual gonadal cycle. The gradual increase in ir-AVT neurons along with annual gonadal activity of quail were accompanied by increase in plasma levels of testosterone. These results indicate a functional interaction between sex steroid and AVT synthesizing neurons. Adrenal activity (as judged by weight, ascorbic acid content, cortical cord width and cortico-medullary ratio) was also maximum during breeding phase. It is thus postulated that domesticated quail when exposed to natural day length (NDL), exhibits seasonal/annual cyclicity in vastocinergic activity and adrenal function which may be due to difference in sex steroid hormone.

Delayed arm regeneration in the Antarctic brittle star Ophionotus victoriae

We describe the levels of natural arm damage in the Antarctic brittle star Ophionotus victoriae and the rate of arm regeneration over the period of 1 yr, as measured in our aquarium facil- ities at Rothera Point, West Antarctic Peninsula. The natural incidence of arm damage in O. victoriae is high (97% of individuals examined). The rate of regeneration was 0.44 mm wk -1 , which is slow, but within the range of data from temperate brittle stars and also in line with data from the literature showing a general trend of decreasing arm regeneration rate with temperature. The Q10 for arm regeneration rate of 2.6 across brittle stars is within the expected biological range. However, O. victo- riae arm regeneration experiences a lag phase of up to 5 mo before reproducible amounts of new tissue are produced. Such a long lag phase has not been documented for any other brittle star species and produces a range of Q10 values from 3.6 (when compared to the slowest documented lag phase for a temperate brittle star) to exceptionally high Q10 values of 14.9 and 15.4 (when either fastest or average regeneration times are considered for temperate brittle stars and this Antarctic species). This indicates that the initial process of arm regeneration in O. victoriae is either different to that of other brittle stars or it is dependent on factors other than the effects of temperature on enzyme-mediated biochemical reactions, such as gonadal cycles and seasonality.

Uncommon mechanism of egg incubation in the endemic Southern hermit crab Loxopagurus loxochelis: how is this phenomenon related to egg production?

AbstractThe aim of the present work was to characterize the egg production of Loxopagurus loxochelis. A total of 71 ovigerous females were obtained, 28% of which were simultaneously incubating eggs at different developmental stages. This phenomenon can be the result of incomplete fertilization, or may represent a rapid gonadal cycle by this species in this area, which is, to our understanding, the best explanation of this phenomenon. Egg volume decreased 25.6% during the incubation period. The reproductive output based on dry and wet weight was 6.8 and 19.3%, respectively. Water was the prevailing component of the eggs, representing 86.0% of the total weight at initial stage, increasing to 95.1% at the final stage. Ash content increased at the same time as a decrease in the organic content occurred, indicating the consumption of yolk and absorption of salts from the water medium. In conclusion, we hypothesized that this population has the capacity to copulate and deposit another brood even before the release of the larvae from the previous one, intensifying the reproductive effort of these hermit crabs as a strategy of adaptation in a region considered the northern limit of the geographical distribution of this species.

Distribution, abundance and reproduction of the Indo-Pacific acorn barnacle Balanus amphitrite (Crustacea: Cirripedia)

The patterns of distribution, abundance and reproduction of the Indo-Pacific barnacle Balanus amphitrite were surveyed at Costa do Sol mangrove, Maputo Bay, southern Mozambique using stratified sampling design. Balanus amphitrite exhibited a spatial variation in which high numbers of individuals colonized the upper stratum in comparison to the lower stratum. Seasonal abundance of B. amphitrite was greater in the upper stratum and declined in June and September. In contrast, very high densities of individuals were recorded in winter in the lower stratum. Gonad cycle showed continuous trend and release of embryos mainly took place in winter (May–July). There was a strong quadratic effect of temperature on breeding activity of B. amphitrite, with females continuing to brood at lower temperatures. Egg numbers increased linearly with female size. The success of the reproductive activity may be synchronized within the species, enhancing the establishment and colonization of B. amphitrite in the study area.

Gametogenesis correlated with steroid levels during the gonadal cycle of the sea urchin Paracentrotus lividus (Echinodermata: Echinoidea)

The specific mechanism regulating reproduction in invertebrates is a field of topical interest which needs to be explored in detail considering also the intriguing possible comparison with vertebrates. In this paper levels of Testosterone (T) and Estradiol (E2) and their reciprocal ratios were determined in ovaries and testis of the echinoid model species Paracentrotus lividus during the year 2004 by taking into account a putative relationship between steroid levels and reproductive cycle. T levels appeared to significantly vary during male reproductive cycle, thus suggesting a possible role of this hormone in regulation of spermatogenesis as demonstrated for other echinoderms. E2 levels were lower in males with respect to females; consequently E2 involvement in oogenesis is hypothesized. In parallel with steroid levels evaluation, variations in P450-aromatase activity and its possible role on regulation of gametogenesis were also considered. Clear correlations between steroid levels and gonad index (GI), as well as between GI and reproductive cycle were not detected, suggesting that GI alone is not a reliable parameter in describing the reproductive status of the gonads. Altogether the results obtained so far confirm the presence of a relationship between steroid levels and reproductive cycle as suggested by previous results on different echinoderm species.

Morphological and Morphometric Aspects of Crassostrea rhizophorae (Guilding, 1828) Oocytes in Three Stages of the Gonadal Cycle

Crassostrea rhizophorae (Guilding, 1828) es una de las especies de ostras tropicales cultivadas en la costa brasilera, que presenta alto valor comercial, no existiendo informacion sobre las caracteristicas gonadales, durante las fases reproductivas de esta especie. El objetivo de este trabajo fue analizar la morfologia de los ovocitos y de los foliculos gonadales de Crassostrea rhizophorae, en tres fases del ciclo gonadal. Analizando los cortes histologicos de las gonadas y considerando los ovocitos que presentaban nucleo y nucleolo evidentes, fueron determinadas tres fases del ciclo gonadal: gametogenesis inicial, crecimiento y maduracion. En la fase de gametogenesis inicial, el diametro de los foliculos y de los ovocitos era de 180,29 (± 41,91) e 18,66 (± 6,85) µm, respectivamente; tambien identificamos gran cantidad de tejido conjuntivo interfolicular y un numero mayor de ovocitos previtelogenicos. En la fase de crecimiento, el diametro de los foliculos y de los ovocitos era de 218,02 (± 43,19) y 25,92 (± 9,94) µm, respectivamente. Este fase fue caracterizada por una pequena cantidad de tejido conjuntivo interfolicular y predominio de ovocitos vitelogenicos. En la fase de maduracion, el diametro de los foliculos y de los ovocitos era de 298,16 (± 99,24) y 35,27 (± 6,2) µm, respectivamente, existiendo gran numero de ovocitos maduros. De esta manera, concluimos que durante la implantacion del cultivo, Crassostrea rhizophorae tolera la influencia de los factores intrinsecos y extrinsecos y no presenta alteraciones significativas en su actividad reproductiva

Control of molt in birds: Association with prolactin and gonadal regression in starlings

Despite the importance of molt to birds, very little is known about its environmental or physiological control. In starlings Sturnus vulgaris, and other species, under both natural conditions and experimental regimes, gonadal regression coincides with peak prolactin secretion. The prebasic molt starts at the same time. The aim of this series of experiments was to keep starlings on photo-schedules that would challenge the normally close relationship between gonadal regression and molt, to determine how closely the start of molt is associated with gonadal regression and/or associated changes in prolactin concentrations. In one series of experiments, photosensitive starlings were moved from a short photoperiod, 8 h light per day (8L), to 13 or 18L, and from 13 to 18L or 13 to 8L during testicular maturation. Later, photorefractory birds under 13L that had finished molting were moved to 18L. In another series of experiments, photorefractory starlings were moved from 18 to 8L for 7 weeks, 4 weeks, 2 weeks, 1 week, 3 days, 1 day, or 0 days, before being returned to 18L. There was no consistent relationship between photoperiod, or the increase in photoperiod, and the timing of the start of molt. Nor was there a consistent relationship with gonadal regression and the start of molt–molt could be triggered in the absence of a gonadal cycle. However, there was always an association between the start of molt and prolactin. In all cases where molt was induced, there had been an earlier increase in prolactin. However, the timing of molt was related to the time of peak prolactin, not the magnitude of that peak. This relationship between peak prolactin and the start of molt could explain the normally close relationship between the end of breeding activity and the start of molt.