Reproductive endocrinology of free living Pied flycatchers (Ficedula hypoleuca): prolactin and FSH secretion in relation to incubation and clutch size

Parenting in a changing environment: A long-term study of prolactin, parental effort and reproductive success in common eiders

Parental care is regulated by multiple endocrine mechanisms. Among these hormones, prolactin (PRL) is involved in the expression of parental behaviors. Despite the consensus that PRL mediates variation in parental effort with age and body condition, its role in the adjustment of parental effort to fluctuating environmental conditions, including changing predation pressure, still awaits further investigation. To shed light on this knowledge gap, we relied on a long-term monitoring of female common eiders Somateria mollissima (n = 1277 breeding attempts, 2012-2022) incubating under fluctuating predation risk to investigate the link between baseline PRL levels and female minimum age, body condition, clutch size, environmental parameters (predation pressure, climate, nest microhabitat) and hatching success. We predicted that PRL would be higher in older females, those in better condition or incubating larger clutches. We also predicted that females would reduce parental effort when nesting under challenging environmental conditions (high predation pressure or poor climatic conditions), translated into reduced baseline PRL levels. We also explored how variation in PRL levels, female characteristics and environmental parameters were related to hatching success. Following our predictions, PRL levels were positively associated with body condition and female age (before showing a senescent decline in the oldest breeders). However, we did not observe any population-level or individual-level reduction in PRL levels in response to increasing predation pressure. Population-level baseline PRL levels instead increased over the study period, coincident with rising predation threat, but also increasing female body condition and age. While we did not provide evidence for a direct association between baseline PRL levels and predation risk, our results support the idea that elevated baseline PRL levels promote hatching success under internal constraints (in young, inexperienced, breeders or those incubating a large clutch) or constraining environmental conditions (during years of high predation pressure or poor climatic and foraging conditions). Finally, the low repeatability of baseline PRL levels and high interannual variability highlight considerable within-individual flexibility in baseline PRL levels. Further research should explore flexibility in parental effort to changing environmental conditions, focusing on both baseline and stress-induced PRL levels.

Endocrine and neuroendocrine regulation of fathering behavior in birds

This article is part of a Special Issue "Parental Care". Although paternal care is generally rare among vertebrates, care of eggs and young by male birds is extremely common and may take on a variety of forms across species. Thus, birds provide ample opportunities for investigating both the evolution of and the proximate mechanisms underpinning diverse aspects of fathering behavior. However, significant gaps remain in our understanding of the endocrine and neuroendocrine influences on paternal care in this vertebrate group. In this review, I focus on proximate mechanisms of paternal care in birds. I place an emphasis on specific hormones that vary predictably and/or unpredictably during the parental phase in both captive and wild birds: prolactin and progesterone are generally assumed to enhance paternal care, whereas testosterone and corticosterone are commonly-though not always correctly-assumed to inhibit paternal care. In addition, because endocrine secretions are not the sole mechanistic influence on paternal behavior, I also explore potential roles for certain neuropeptide systems (specifically the oxytocin-vasopressin nonapeptides and gonadotropin inhibitory hormone) and social and experiential factors in influencing paternal behavior in birds. Ultimately, mechanistic control of fathering behavior in birds is complex, and I suggest specific avenues for future research with the goal of narrowing gaps in our understanding of this complexity. Such avenues include (1) experimental studies that carefully consider not only endocrine and neuroendocrine mechanisms of paternal behavior, but also the ecology, phylogenetic history, and social context of focal species; (2) investigations that focus on individual variation in both hormonal and behavioral responses during the parental phase; (3) studies that investigate mechanisms of maternal and paternal care independently, rather than assuming that the mechanistic foundations of care are similar between the sexes; (4) expansion of work on interactions of the neuroendocrine system and fathering behavior to a wider array of paternal behaviors and taxa (e.g., currently, studies of the interactions of testosterone and paternal care largely focus on songbirds, whereas studies of the interactions of corticosterone, prolactin, and paternal care in times of stress focus primarily on seabirds); and (5) more deliberate study of exceptions to commonly held assumptions about hormone-paternal behavior interactions (such as the prevailing assumptions that elevations in androgens and glucocorticoids are universally disruptive to paternal care). Ultimately, investigations that take an intentionally integrative approach to understanding the social, evolutionary, and physiological influences on fathering behavior will make great strides toward refining our understanding of the complex nature by which paternal behavior in birds is regulated.

Environmental and hormonal correlates of immune activity in a cooperatively breeding tropical bird

Because climatic patterns in temperate regions are generally predictable, species can allocate resources adaptively among competing physiological processes before environmental conditions change. In the semi-arid tropics where environments are seasonal, but highly unpredictable, allocation decisions may be more sensitive to short-term fluctuations in conditions. We asked (i) whether investments in immune function were affected by inter-annual variation in rainfall and (ii) whether corticosterone and prolactin, two hormones that modulate immune activity in other vertebrates, predict environmentally induced alterations in immune activity in cooperatively breeding superb starlings (Lamprotornis superbus). Superb starlings inhabit African savannas characterized by high among-year variation in rainfall, which influences their breeding life histories and hormone levels. We quantified bactericidal capacity of plasma, or bacterial killing, and prolactin and corticosterone concentrations in blood samples collected over a four year period during the dry season prior to breeding, as this is the period when reproductive roles are determined in this species and when rainfall is most variable. We found that bacterial killing was weakest in the driest year of the study, and we detected a positive relationship between bacterial killing and prolactin, but not a negative relationship with corticosterone. Together these results suggest that prolactin may mediate rainfall-induced changes in immune activity in superb starlings. This study is the first to examine relationships between prolactin and an index of constitutive, innate immunity in birds, and suggests that even species inhabiting unpredictable environments adjust their physiological priorities to environmental conditions, perhaps via prolactin.

Factors affecting plasma concentrations of prolactin in the common eider Somateria mollissima

In the common eider only the females incubate while they fast for 25 days. Thus, since they rely entirely on their body reserves for successful incubation, they can be defined as capital incubators. To assess the potential effects of their initial body mass, the incubation duration, and depletion in body reserves on prolactinemia, blood samples of eiders were analyzed during the breeding cycle and an experimental manipulation of the duration of incubation. Levels of circulating prolactin increased at the onset of incubation and then reached a high and stable level during incubation before increasing sharply before hatching. The prolactin level decreased significantly upon hatching. Captive females deprived from their eggs exhibited a rapid decrease in prolactinemia, suggesting that egg stimuli are necessary to prolactin secretion. Aunts, i.e., helper females caring for conspecific young, presented prolactin levels higher than nonbreeding captive females but not significantly different from those of females at hatching. Plasma prolactin at hatch was directly related to body mass loss. Birds with shortened incubation have higher body masses and showed higher levels of prolactinemia at hatching than the control group, in accordance with the idea that circulant prolactin at hatching is linked to body condition. Females which underwent an extended incubation (and started to eat again) displayed a low body mass and a high prolactinemia. These data therefore suggest that refeeding, albeit increasing the risk of predation, enhances prolactin secretion and allows the bird to continue incubation despite that it has reached a poor body condition.

The role of prolactin in the regulation of clutch size and onset of incubation behavior in the American kestrel

In most bird species, the timing of incubation onset may influence the degree of hatching asynchrony, which, together with variation in clutch size, affects reproductive success. In some domesticated species that usually show no hatching asynchrony, plasma prolactin concentrations in females rise with the onset of incubation and the end of laying, and this rise enhances incubation behavior and may terminate laying. To investigate whether a rise in prolactin during laying is involved in the regulation of clutch size and incubation onset in a species with hatching asynchrony, we measured plasma concentrations of immunoreactive prolactin (ir-prolactin) in laying American kestrels, Falco sparverius, and quantified clutch size and incubation behavior. In a separate study, we administered one of three concentrations of ovine prolactin (o-prolactin) via osmotic pumps implanted in females when egg 2 of a clutch was laid. ir-Prolactin concentrations during laying were higher in small than in large clutches and increased in parallel with the development of incubation behavior. o-Prolactin treatment enhanced incubation behavior, but did not affect clutch size, possibly because the manipulation was performed after clutch size had already been determined. Consistent with studies on domesticated species that show synchronous hatching, our results indicate that rising prolactin during laying enhances the expression of incubation behavior in a species that shows hatching asynchrony. Further studies are necessary to determine whether the relationship between prolactin and clutch size in the American kestrel is one of causation or of mere association.

Annual changes in plasma levels of LH, and prolactin in free-living female great tits (Parus major)

Plasma levels of LH and prolactin were measured in free-living female great tits throughout the year. Plasma concentrations of LH increased at the onset of winter (January), onset of spring (March), nest-building period, (for both the first and the second clutch), last part of the incubation period, (for both the first and the second clutch), and early autumn (August). Plasma prolactin increased between March and April. A further increase occurred during the nest-building period, which continued during the early part of the egg-laying period. Prolactin levels increased further once incubation had started. During nest-building and egg-laying of the second clutch, prolactin levels were low, but not basal. Prolactin levels increased again during incubation of the second clutch. Between July and March prolactin levels usually were below the limit of detection of the assay.

Hormonal correlates of migration and territorial behavior in juvenile willow tits during autumn

This study compared plasma levels of dihydrotestosterone, testosterone, corticosterone, luteinizing hormone, growth hormone, and prolactin in migrating juvenile willow tits with those in territorial juveniles. Both categories of birds were caught in late September. Migrating juveniles had higher plasma levels of corticosterone than territorial juveniles. Only corticosterone secretion was affected by "handling stress" in both migrating and territorial juveniles. However, territorial birds showed a much stronger relationship between these two variables. It is suggested that high corticosterone levels are involved in the emigration of juveniles out of the coniferous forest. Only juveniles were found among the migrating willow tits, and these birds were not well adapted for migration. Migrating juvenile males had less fat stored than did territorial ones. Furthermore, migrating juvenile males had higher liver/somatic index and higher plasma levels of growth hormone than did territorial males. These results indicate that migrating males had been, or were, exposed to food restrictions. The same proportion of migrating and territorial juveniles, males as well as females, had high plasma levels of testosterone. We suggest that these high levels were caused by recent aggressive interactions. To test the hypothesis that high plasma levels of testosterone are important for a juvenile to become a member of a territorial winter group, we performed a field experiment in which juveniles were given testosterone implants (controls were given empty silastic tubes) at the beginning of the territorial establishment period. The same proportion of testosterone-implanted birds and control birds succeeded in becoming members of territorial winter groups. Thus, testosterone does not seem to play an essential role in autumn territoriality, and it does not prevent autumn migration.

Hormonal patterns in breeding and nonbreeding kestrels, Falco tinnunculus: field and laboratory studies

Plasma levels of luteinizing hormone (LH), androgens, and corticosterone (B) were measured in breeding and nonbreeding kestrels, both in the field and in captivity under different food conditions. LH levels in breeding males were elevated from courtship through incubation and androgen concentration peaked during courtship and laying. Seasonal changes in LH and androgens were similar in breeding and nonbreeding males, although androgens declined sharply after laying in breeding males. Egg laying was characterized by marked increases in both female body mass and plasma concentrations of LH and B. In both breeding and nonbreeding females LH and B increased during pair formation and courtship (March-April), although maximum levels were lower in nonbreeders. Two marked differences were found between free-living and captive (paired) birds. First, during winter plasma levels of LH were basal in free-living birds, while significantly elevated in both captive males and females. Second, during courtship androgen levels in breeding males were three-fold higher in the field than in captivity, probably as a result of intermale aggression under natural conditions. Females breeding early, late, or not at all had similar LH concentrations on arrival and during early courtship. Before breeding there were also no differences in LH (males and females) or in androgen levels (males) between pairs fed ad libitum and pairs temporary food rationed. However, captive females with experimentally reduced food intake showed low levels of B until their rations were increased. Nonbreeders showed hormonal changes similar to breeders, except for those changes in females that were associated with laying. These results suggest that at the start of the breeding season both nonbreeding and breeding kestrels have functional reproductive systems. That some breed early, late, or not at all is primarily an effect of food availability and is not due to hormonal modulation of the reproductive cycle.

Plasma LH and prolactin levels during the reproductive cycle of the cockatiel (Nymphicus hollandicus)

Plasma-luteinizing hormone (LH) and prolactin (Prl) levels were determined using radioimmunoassay during two reproductive cycles in captive cockatiels (Nymphicus hollandicus)--an altricial species in which both parents share incubation and care of young. Birds were stimulated to breed by increasing daylength, light intensity, ambient temperature, and presenting nest boxes. LH levels were elevated during the time of nest inspection in females and peaked during egg laying. In contrast, LH levels were highest in males during nest inspection but were lower during egg laying. In both sexes, LH continued to decline during incubation and care of the young but rose in pairs laying a second clutch. Female and male Prl levels increased during egg laying, peaked during incubation, then declined to egg-laying levels during the nestling stage. Prl continued to decline during the fledgling stage and reached prelaying levels unless a second clutch was begun. In conclusion, in cockatiels, nest inspection and laying are characterized by high LH levels while high Prl levels occur during incubation and feeding of nestlings in both males and females.

Plasma concentrations of prolactin during the breeding cycle in the Cape gannet (Sula capensis): a foot incubator

Plasma prolactin was measured in the Cape gannet (Sula capensis) which differs from most other avian species in that although both sexes share equally in incubation duties, neither sex possesses an incubation patch but rather incubates a single egg with its foot webbing. Five main categories of birds were studied: nonbreeders, unmated birds which were nevertheless defending a nest site, recently mated birds, incubators, and brooders with nestlings. Prolactin titers in the plasma were 8-9 ng/ml in nonbreeding gannets, increasing to 17.7 +/- 1.6 and 16.7 +/- 1.2 ng/ml in incubating females and males, respectively. Plasma concentrations of prolactin progressively increased in nonbreeding gannets from 5.5 +/- 0.4 ng/ml in 1-year-old birds to 8.7 +/- 1.6 ng/ml in birds defending a nest site but without an egg. Blood samples taken from both incubators and nonincubators every 3 hr over 27 hr did not reveal any significant daily cycling in prolactin titers. Plasma osmolarity did not vary significantly between nonincubators and incubators, being 300.1 +/- 3.2 and 298.2 +/- 3.0 mOsm/kg, respectively. No positive correlation was found between possible osmotic stress during incubating and plasma concentrations of prolactin. Plasma concentrations of luteinizing hormone and testosterone increased while prolactin decreased, significantly, 24 hr after clutch loss. The putative inhibition of the pituitary-gonadal axis by prolactin is discussed together with other possible roles of prolactin during the breeding season in gannets.

The endocrine control of reproduction and molt in male and female emperor (Aptenodytes forsteri) and adelie (Pygoscelis adeliae) penguins. I. Annual changes in plasma levels of gonadal steroids and LH

Changes in plasma LH, testosterone, and estrogens were investigated throughout reproduction and molt in free-living male and female emperor (Aptenodytes forsteri) and adelie (Pygoscelis adeliae) penguins. In both sexes and species, plasma LH and gonadal steroids were severalfold above basal level at the time of arrival on the breeding grounds, suggesting that environmental cues (especially decreasing daylength in emperors) rather than mating and courting primarily stimulate gonadal development and reproduction. In both species a marked increase in plasma LH (both sexes), testosterone (males), and estrogens (females) corresponded with the time of maximum gonadal size, and peak values were obtained at the time of copulation, i.e., in emperors about 10-15 days prior to egg laying. In females, plasma LH and estrogens dropped to low levels between copulation and egg laying. Similarly, following copulation in males plasma testosterone fell to lower levels that in emperors were maintained during incubation and brooding of the non-thermally emancipated chick. Plasma LH levels followed the same trend as testosterone, falling after copulation and falling further prior to molt. Though lower than at copulation, plasma LH was higher in incubating (males) and brooding (males and females) emperors than during rearing of the thermally emancipated chicks, suggesting that plasma LH might be related to incubating, brooding, and territorial behavior. In male and female emperors and in male adelies, plasma gonadal steroids and LH were at basal levels throughout molt.

The effects of modifying incubation on prolactin secretion in free-living pied flycatchers

Plasma prolactin concentrations in female pied flycatchers are high during incubation, but fall from 156 +/- 27 to 67 +/- 6 ng/ml (P less than 0.01) between 2 and 4 days after the young hatch. The timing of the fall in prolactin could be determined by factors associated with the hatching of the eggs and growth of the offspring; alternatively, prolactin may remain high for a fixed period (16 days) after the onset of incubation. To distinguish between these possibilities, eggs were exchanged between nest boxes to ensure incubation periods of either 10, 14, or 19 days by different females. The results showed that the period of high prolactin secretion was shortened by 2-4 days where the eggs hatched early, but, conversely, was not lengthened in those females with extended incubation periods, the levels actually falling in these birds before the eggs hatched. There was, however, a secondary rise in prolactin 3 days after hatching, when the level of 114 +/- 11 ng/ml 22 days after the onset of incubation was much higher than that (30 +/- 4 ng/ml) at a comparable stage in the control females. Thus, high prolactin secretion cannot be maintained for longer than 16-17 days in incubating birds, but newly hatched, naked young can stimulate prolactin release in female flycatchers over a flexible period.