Testosterone-induced depression of male parental behavior in the barn swallow: female compensation and effects on seasonal fitness

Longitudinal covariation of testosterone and sperm quality across reproductive stages in the zebra finch

Birds that breed opportunistically maintain partial activation of reproductive systems to rapidly exploit environmental conditions when they become suitable for breeding. Maintaining reproductive systems outside of a breeding context is costly. For males, these costs are thought to include continual exposure to testosterone. Males of seasonally breeding birds minimise these costs by downregulating testosterone production outside of a breeding context. Opportunistically breeding birds trade off the need to rapidly initiate reproduction with the costs of elevated testosterone production. One way opportunistically breeding males could minimise these costs is through fine scale changes in testosterone production across discrete reproductive stages which have a greater or lesser requirement for active sperm production. Although spermatogenesis broadly depends on testosterone production, whether changes in testosterone levels across the reproductive stages affect sperm quality and production is unknown. Here, we measured testosterone, sperm quality, and body condition in male zebra finches at discrete stages within reproductive bouts (egg laying, incubation, nestling provisioning, and fledging) and across two consecutive reproductive events in captive male zebra finches (Taeniopygia castanotis). We also examined associations between male testosterone, sperm quality/production, body condition, and nestling body condition. We found that testosterone levels varied across discrete reproductive stages with the lowest levels during incubation and the highest following chick fledging. Testosterone levels were positively associated with sperm velocity and the proportion of motile sperm but were not associated with male body condition. We found no associations between paternal body condition, testosterone levels, or sperm traits with nestling body condition (a proxy for the reproductive quality of a male and his partner). This study is the first to show that opportunistically breeding males vary testosterone synthesis and sperm traits at discrete stages within a reproductive event.

Convict cichlid parents that stay with the same mate develop unique and consistent divisions of roles

Previous studies, largely on avian species, have suggested that pairs that are permanently monogamous and have biparental care develop a coordination over time that enhances offspring survival. If this is the case, we predicted that a parent involved in biparental care would develop a pattern of biparental care specific to a particular mate and remain consistent in that pattern over time but would lose this pattern if it were to change mates. We tested this prediction with the convict cichlid fish (Amatitlania nigrofasciata) which has biparental care that is both complex and flexible. In this species, each parent can perform all parental roles but typically shows a division of labor in which males typically defend against offspring predators while the female typically provides direct care to the offspring. At various times, the parents briefly switch roles. Our experiments revealed that pairs that remained together for two consecutive broods were more consistent in their parental behaviors, including time they spent near the intruder and in the nest compared to pairs that were comprised of individuals that had previously mated with other partners. Also individuals that remained with the same partner were also more consistent as a parental unit, maintaining their sex-specific roles of males defending aggressively against an intruder and females spending more time directly caring for young. While our experiment clearly support our prediction that individuals do develop unique coordination with specific individuals, convict cichlids in nature appear to be largely serially monogamous in which they mate only once before changing partners. Thus, it is likely this coordination may be available in many species that have biparental care but become adaptive when repeated matings become common.

Androgen and prolactin manipulation induces changes in aggressive and nurturing behavior in a fish with male parental care

Parental care can include two general types of behavior: (1) aggressive behavior, which is used to defend offspring from predators; and (2) nurturing behavior, which is used to provide offspring with environmental conditions or resources necessary for survival. Many studies have implicated androgens in promoting aggressive behavior and prolactin in promoting nurturing behavior. We experimentally manipulated these hormones to investigate their effects on parental care behavior in bluegill (Lepomis macrochirus). Parental males, which provide sole care to the developing eggs and larvae, received an implant with an androgen (11-ketotestosterone [11-KT]), an androgen antagonist (flutamide), prolactin, a prolactin-release inhibitor (bromocriptine), or castor oil (placebo). We found that 11-KT implants led to a significant increase in the frequency of aggressive behavior directed towards a simulated brood predator, and were associated with a nearly significant decrease in the frequency of nurturing behavior directed towards the developing eggs. In contrast, prolactin implants were associated with a significant increase in the frequency of nurturing behavior, but also reduced the frequency of aggressive behavior directed towards the simulated brood predator. These results suggest a hormone-mediated mechanistic trade-off between nurturing and aggressive behavior, whereby parental males are unable to be both highly nurturing and highly aggressive.

Determination of testosterone and corticosterone in feathers using liquid chromatography-mass spectrometry

Feathers gradually accumulate hormones and reflect long-term average plasma steroid levels during their growth. Feather hormone levels thus provide for the measurement of plasma hormones concentrations integrated over a period of several days or weeks. In this study, we focused on the development of a method to determine testosterone (TEST) and corticosterone (CORT) levels in extracts from feathers of small bodied birds with a limited amount of feathers available per individual. For this purpose, the method had to be verified for a small weight of samples. The present study describes the effect of the conditions of sample preparation and keto-derivatisation on the sensitivity of the LC-ESI-MS/MS analysis of TEST and CORT. Generally, chemical derivatization improves the sensitivity and selectivity of LC-MS/MS analysis. It can be used particularly in situations when the total amount of collected sample is limited (such as in our studies). Both the conditions of feather sample preparation (the selection of the extraction solvent, the time of extraction, and the conditions of solid phase extraction) and the reaction conditions affecting the formation of keto-derivatives (such as reaction temperature and reaction time) were tested. Methanol as the extraction solvent, 8 h as the extraction time, 50 °C as the reaction temperature of derivatization, and 90 min as the reaction time of derivatization are the most suitable conditions in terms of achieving a high sensitivity of analyses. Calibration curves are linear, at least in the range 25-2500 pg mL^-1, which is usually found in feather extracts. The limit of detection (LOD) for TEST and CORT was 1.0 and 0.3 pg per mL, respectively. The limit of quantification (LOQ) for TEST and CORT was 3.3 and 1.0 pg per mL, respectively. The optimized procedure was successfully applied for the analysis of TEST and CORT in real feather samples. The method could be used in a variety of research direction including wildlife, agricultural or veterinary studies.

Reproduction impairments in metal-polluted environments and parental hormones: No evidence for a causal association in an experimental study in breeding feral pigeons exposed to lead and zinc

Humans are responsible for land-cover changes resulting in the emission of hazardous chemical elements including metallic trace elements i.e. MTEs. As a consequence, urban wildlife is exposed to high concentrations of MTEs, which exposure is linked to reproductive impairments. MTE effects on reproduction outputs might result from MTE exposure disrupting the endocrine pathways involved in reproductive behaviours. In birds, there is strong evidence that prolactin, corticosterone and testosterone are all involved in the regulation of parental effort during incubation and chick rearing. Endocrine-disrupting chemicals might stimulate or inhibit the production of those hormones and consequently alter parental investment and reproductive success outcomes. We measured baseline corticosterone, prolactin and testosterone plasma levels, and the corticosterone stress response of breeding feral pigeons (Columba livia) experimentally exposed to ecologically relevant lead and/or zinc concentrations. Independently of lead and/or zinc exposure, male and female plasma levels of corticosterone and prolactin (but not testosterone) showed temporal variations along the reproduction stages (i.e. incubation, early rearing and late rearing). In addition, both hatching and fledging success were slightly correlated with corticosterone, prolactin and testosterone levels. However, our study did not find any influence of lead or zinc exposure on hormone levels, suggesting that MTE effects on reproductive outputs might not be explained by MTE-induced modifications of corticosterone, prolactin and testosterone-linked behaviours during incubation and rearing. Alternatively, MTE-induced reproductive impairments might result from MTE exposure having direct effects on offspring phenotypes or prenatal indirect effects on the embryo (e.g. maternal transfer of MTEs, hormones or immune compounds).

Acute peaks of testosterone suppress paternal care: evidence from individual hormonal reaction norms

A long-standing tenet of evolutionary endocrinology states that testosterone mediates the life-history trade-off between mating and paternal care. However, the support for a role of testosterone in suppressing paternal care is mixed: implantation studies in birds suggest that high-level testosterone implants suppress paternal care, but circulating levels of testosterone and paternal care are typically not correlated. Because any trade-off in real life must be realized with hormone levels that are within an individual's reaction norm, it is crucial to show that natural changes in the hormone can modulate behaviour. Here, we used GnRH-injections to alter testosterone levels of free-living male black redstarts within each individual's hormonal reaction norm: individuals experiencing a short-term peak in testosterone resumed feeding their offspring later and showed a stronger suppression of offspring-feeding behaviour than control males. For the first time, this study demonstrated that short-term peaks in testosterone within the hormonal reaction norm of individuals can suppress paternal behaviour. Our findings reconcile previous seemingly contradictive effects that testosterone implants had on paternal care and the absence of correlations between circulating testosterone levels and paternal care, and demonstrate that the differential production of testosterone within the hormonal reaction norm of single individuals can indeed function as a mechanism to mediate a potential trade-off between mating and parenting. On a broader note, our results suggest that natural and short peaks in testosterone can elicit adaptive behavioural changes.

Plasma Testosterone Levels Increase with Expression of Male Ornaments During Mating, but not Incubation, in Japanese Barn Swallows

Recent experimental studies involving the manipulation of sexual traits have demonstrated that sexual trait expression feeds back to testosterone levels, perhaps via social interactions, reinforcing the linkage between sexual trait expression and testosterone levels during the mating period. However, information on this reinforcement under the natural variation of sexual traits remains limited. Using Japanese barn swallows, Hirundo rustica gutturalis, in which extra-pair paternity is quite rare (< 3%), we studied the relationship between plasma testosterone level and a male sexual trait, throat patch size, during the mating and incubation periods. Given the importance of social interaction, we predicted that this relationship should be intense during the mating period, but not the incubation period, due to reduced social interaction during the latter. We found low plasma testosterone levels during the incubation period compared with those in the mating period, and plasma testosterone levels were significantly positively related to throat patch area during the mating period, but not the incubation period. Similar relationships were found in another sexual trait, the size of white tail spots. During the incubation period, body condition, instead of male sexual trait expression, was negatively related to plasma testosterone level, indicating that an intrinsic link, rather than reinforcement, is important during this period. These relationships are consistent with the hypothesis that social interaction reinforces the relationship between sexual traits and plasma testosterone levels. The current study provides evidence for a highly variable relationship between testosterone and ornamentation across breeding periods in the natural variation of sexual traits.

Testosterone secretion in a socially monogamous but sexually promiscuous migratory passerine

The steroid hormone testosterone (T) influences a multitude of traits critical to reproduction in vertebrates. In birds, high male T supports territory establishment and mate attraction, but is thought to interfere with parental care. Interspecific comparisons indicate that migratory species with short, synchronous breeding seasons have the highest peak T, and that the seasonal profile of T exhibits a rapid decline with the onset of incubation by females. We describe the T profile of the migratory, socially monogamous, and biparental Eastern Kingbird (Tyrannus tyrannus) from the high desert of eastern Oregon, USA, where breeding occurs within a short 2-3 month period. Eastern Kingbirds are socially monogamous but exhibit high rates of extra-pair paternity as ∼60% of broods contain extra-pair young. We therefore evaluate whether Eastern Kingbirds exhibit the "typical" T profile expected for a synchronously breeding migratory species, or whether T is maintained at a more constant level as would be predicted for a species with opportunities for mating that extend over a majority of the breeding season. Our samples were divided into six periods of the reproductive cycle from territory establishment to the feeding of fledglings. T did not change across stages of the nest cycle. Instead, T declined with sampling date and nest density, and increased with the number of fertile females in the population. Male kingbirds advertise their presence through song for most of the breeding season, and we suggest that T is maintained throughout most of the breeding season because male fitness is equally dependent on within- and extra-pair reproductive success.

Exposure to the androgenic brominated flame retardant 1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane alters reproductive and aggressive behaviors in birds

Detected in environmental samples, 1,2-dibromo-4-(1,2-dibromoethyl) cyclohexane (DBE-DBCH) is a bioaccumulative isomer of a current-use brominated flame retardant. All 4 structural isomers are androgen agonists; however, little toxicological information exists for this compound. The objective of the present study was to determine if β-DBE-DBCH, the isomer found most prominently in animal tissue, affects androgen-dependent behavior of breeding American kestrels (Falco sparverius). The authors hypothesized that if β-DBE-DBCH acts as an androgen agonist in kestrels, androgen-dependent behaviors (i.e., copulation, courtship, aggression) would increase and behaviors inhibited by androgens (i.e., parental care behaviors) would decrease. Sixteen captive experimental kestrel pairs were exposed to 0.239 ng β-DBE-DBCH/g kestrel/d by diet from 4 wk prior to pairing until their nestlings hatched (mean 82 d) and compared with vehicle only-exposed control pairs (n = 15). Androgen-dependent behaviors were significantly increased in β-DBE-DBCH-exposed birds, consistent with the authors' hypothesis. These behavioral changes included copulation and other sexual behaviors in males and females and aggression in males, suggesting that β-DBE-DBCH may have acted like an androgen agonist in these birds. Parental behaviors were not reduced in exposed birds as predicted, although dietary exposure had ceased before chicks hatched. Further assessment of β-DBE-DBCH is recommended given these behavioral changes and the previously reported reproductive changes in the same birds.

State-dependent cooperation in burying beetles: parents adjust their contribution towards care based on both their own and their partner's size

Handicapping experiments on species with biparental care show that a focal parent increases its contribution when its partner is handicapped. Such results are interpreted as evidence for negotiation, whereby each parent adjusts its amount of care to that of its partner. However, it is currently unclear whether the focal parent responds to a change in its handicapped partner's behaviour or state. To address this gap, we conducted an experiment on the burying beetle Nicrophorus vespilloides where we first generated different-sized males and females by varying the duration of larval development. We then used a 2 × 2 factorial design in which a small or large male was paired with a small or large female. Small females provided less direct care (food provisioning and interactions with larvae) than large females, and both males and females provided less direct care when paired with a small partner. Thus, the focal parent adjusted its contribution towards care based on both its own state and that of its partner. There was also evidence for negotiation between the two parents as the focal parent adjusted its contribution based on the amount of care by its partner. However, there was no evidence that negotiation accounted for how the focal parent responded to its partner's size. Our results have important implications for our understanding of biparental cooperation as they show that each parent adjusts its contribution not only based on the amount of care provided by its partner but also based on its own state and its partner's state.

Testosterone elevation and response to gonadotropin-releasing hormone challenge by male northern cardinals (Cardinalis cardinalis) following aggressive behavior

There is much discrepancy about the relationship between testosterone (T) and male aggressive behavior. For example, in birds, males of many species significantly elevate T levels during inter-male conflict. However, this is not universal, and in species where males typically do not elevate T during aggressive interactions, concentrations of the hormone are often assumed to be circulating at maximum levels. We examined if male northern cardinals (Cardinalis cardinalis) significantly elevated T during simulated territorial intrusions (STIs). We also examined if individuals had the capacity to further elevate T levels in response to gonadotropin-releasing hormone (GnRH) injections immediately after an aggressive encounter. Our results indicate that male cardinals do not significantly elevate T levels in response to STIs, but have the physiological capacity to significantly elevate T in response to GnRH injections following aggressive interactions. This implies that T levels of individuals captured during STIs were not at maximum concentrations. However, additional findings in this study also suggest the possibility that prolonged social instability could elicit significant elevations in T in males of this species, warranting further investigation.

Early elevation of testosterone advances migratory preparation in a songbird

The timing of events associated with spring migration can be an important determinant of fitness for migratory birds. The need to prepare and maintain physical condition for migration is one demand that must be met for early arrival on breeding areas, and this demand is compounded by the energetic demands of the physiological transition to breeding. Here, we examined whether migratory birds, specifically males, can adequately meet both of these demands by elevating the sex steroid testosterone early during migratory preparation. To test this, we used a captive experiment in which we photostimulated male dark-eyed juncos (Junco hyemalis) and manipulated circulating testosterone. We predicted males with elevated testosterone would prepare to migrate and breed faster than control males or males where the ability of testosterone to bind with receptors, or be converted to estradiol, was inhibited (testosterone inhibited). We measured migratory preparation using mass, food intake, fat deposits and nocturnal activity (Zugunruhe). To estimate breeding preparation, we measured the diameter of the cloacal protuberance (CP). We found that males in the testosterone treatment group began migratory preparation approximately 10 days before controls. There was no difference in the magnitude of peak migratory condition between testosterone-implanted birds and controls. Males implanted with testosterone also reached the onset of breeding preparation faster and had larger peak CP diameter compared with controls. Testosterone-inhibited birds showed no signs of migratory preparation and only a weak increase in CP diameter. These results demonstrate that early elevation of testosterone during migratory preparation could incur benefits for males in terms of the ability to depart earlier from non-breeding areas and arrive in a more advanced breeding condition. This experiment demonstrates that there may be important physiological underpinnings to known winter to breeding season carry-over effects in migratory birds.

An experimental test of the information model for negotiation of biparental care

BACKGROUND

Theoretical modelling of biparental care suggests that it can be a stable strategy if parents partially compensate for changes in behaviour by their partners. In empirical studies, however, parents occasionally match rather than compensate for the actions of their partners. The recently proposed "information model" adds to the earlier theory by factoring in information on brood value and/or need into parental decision-making. This leads to a variety of predicted parental responses following a change in partner work-rate depending on the information available to parents.

METHODOLOGY/PRINCIPAL FINDINGS

We experimentally test predictions of the information model using a population of long-tailed tits. We show that parental information on brood need varies systematically through the nestling period and use this variation to predict parental responses to an experimental increase in partner work-rate via playback of extra chick begging calls. When parental information is relatively high, partial compensation is predicted, whereas when parental information is low, a matching response is predicted.

CONCLUSIONS/SIGNIFICANCE

We find that although some responses are consistent with predictions, parents match a change in their partner's work-rate more often than expected and we discuss possible explanations for our findings.

Seasonal and population variation in male testosterone levels in breeding orange-crowned warblers (Vermivora celata)

Comparative hormone studies can reveal how physiology underlies life history variation. Here, we examined seasonal variation in plasma testosterone concentration between populations of male orange-crowned warblers (Vermivora celata) breeding in Fairbanks, Alaska (V. c. celata) and on Santa Catalina Island, California (V. c. sordida). These populations face different ecological constraints and exhibit different life histories. Alaska birds have a short breeding season, low annual adult survival, and high reproductive rates. In contrast, Catalina Island birds exhibit high adult survival and low reproductive rates despite having a long breeding season. We examined seasonal variation in male testosterone concentrations as a potential mechanism underlying differences in male reproductive strategies between populations. From 2006 to 2008, we sampled males during the pre-incubation, incubation, and nestling stages. Alaska males exhibited a seasonal testosterone pattern typical of northern passerines: testosterone levels were high during pre-incubation and declined during incubation to low levels during nestling provisioning. Testosterone concentrations in Catalina Island males, however, did not vary consistently with breeding stage, remained elevated throughout the breeding season, and were higher than in Alaska males during the nestling stage. We hypothesize that in Alaska, where short seasons and high adult mortality limit breeding opportunities, the seasonal testosterone pattern facilitates high mating effort prior to incubation, but high parental investment during the nestling stage. On Catalina Island, elevated testosterone levels may reflect the extended mating opportunities and high population density facing males in this population. Our results suggest that population variation in seasonal testosterone patterns in orange-crowned warblers may be a function of differences in life history strategy and the social environment.

Impact of season and social challenge on testosterone and corticosterone levels in a year-round territorial bird

Plasma testosterone increases during breeding in many male vertebrates and has long been implicated in the promotion of aggressive behaviors relating to territory and mate defense. Males of some species also defend territories outside of the breeding period. For example, the European nuthatch (Sitta europaea) defends an all-purpose territory throughout the year. To contribute to the growing literature regarding the hormonal correlates of non-breeding territoriality, we investigated the seasonal testosterone and corticosterone profile of male (and female) nuthatches and determined how observed hormone patterns relate to expression of territorial aggression. Given that non-breeding territoriality in the nuthatch relates to the reproductive context (i.e., defense of a future breeding site), we predicted that males would exhibit surges in plasma testosterone throughout the year. However, we found that males showed elevated testosterone levels only during breeding. Thus, testosterone of gonadal origin does not appear to be involved in the expression of non-breeding territoriality. Interestingly, territorial behaviors of male nuthatches were stronger in spring than in autumn, suggesting that in year-round territorial species, breeding-related testosterone elevations may upregulate male-male aggression above non-breeding levels. In females, plasma testosterone was largely undetectable. We also examined effects of simulated territorial intrusions (STIs) on testosterone and corticosterone levels of breeding males. We found that STIs did not elicit a testosterone response, but caused a dramatic increase in plasma corticosterone. These data support the hypothesis that corticosterone rather than testosterone may play a role in the support of behavior and/or physiology during acute territorial encounters in single-brooded species.

Behavioral insensitivity to testosterone: why and how does testosterone alter paternal and aggressive behavior in some avian species but not others?

Considerable research has been conducted on the interrelationships of the steroid hormone testosterone and reproductive behavior in seasonally breeding birds. In species in which males provide paternal care, males experience a peak in testosterone secretion that coincides with territory establishment and pair bonding, and then drops to a breeding baseline as nests are initiated. A large body of evidence suggests that a male's seasonal profile of testosterone reflects a trade-off between expression of sexual and territorial behavior and expression of paternal behavior. For example, studies utilizing testosterone implants to extend the early season peak in plasma testosterone have demonstrated that testosterone tends to increase sexual behavior as well as intensity and persistence of male-male aggression, but also decreases paternal care of offspring. However, recent studies demonstrate that in some species, males do not respond to experimentally elevated testosterone with alterations in aggression and/or paternal care. This phenomenon of "behavioral insensitivity" to testosterone may relate to a variety of environmental factors, particularly those that necessitate high levels of paternal investment. This review explores both ultimate and proximate explanations for behavioral insensitivity to testosterone, and explores hypotheses to explain how this phenomenon may relate to androgen responses to social interactions during breeding. Further research into behavioral insensitivity to testosterone in a variety of vertebrates may provide additional insights into the complex patterns of sex steroid secretion and its behavioral consequences.

Distinguishing seasonal androgen responses from male-male androgen responsiveness-revisiting the Challenge Hypothesis

Androgen levels show strong patterns throughout the year in male vertebrates and play an important role in the seasonal modulation of the frequency, intensity and persistence of aggression. The Challenge Hypothesis (Wingfield, J.C., Hegner, R.E., Dufty, A.M., Ball, G.F., 1990. The "Challenge Hypothesis": Theoretical implications for patterns of testosterone secretion, mating systems, and breeding strategies. Am. Nat. 136, 829-846) predicts that seasonal patterns in androgen levels vary as a function of mating system, male-male aggression and paternal care. Although many studies have addressed these predictions, investigators have often assumed that the ratio of the breeding season maximum and breeding baseline concentrations (termed "androgen responsiveness") reflects hormonal responses due to social stimulation. However, increasing evidence suggests that seasonal androgen elevations are not necessarily caused by social interactions between males. Here, we separate the seasonal androgen response (R(seasonal)) and the androgen responsiveness to male-male competition (R(male-male)) to begin to distinguish between different kinds of hormonal responses. We demonstrate that R(seasonal) and R(male-male) are fundamentally different and should be treated as separate variables. Differences are particularly evident in single-brooded male birds that show no increase in plasma androgen levels during simulated territorial intrusions (STIs), even though R(seasonal) is elevated. In multiple-brooded species, STIs typically elicit a rise in androgens. We relate these findings to the natural history of single- and multiple-brooded species and suggest a research approach that could be utilized to increase our understanding of the factors that determine different types of androgen responses. This approach does not only include R(seasonal) and R(male-male), but also the androgen responsiveness to receptive females (R(male-female)) and to non-social environmental cues (R(environmental)), as well as the physiological capacity to produce and secrete androgens (R(potential)). Through such studies, we can begin to better understand how social and environmental factors may lead to differences in androgen responses.

Physiological Effects on Demography: A Long‐Term Experimental Study of Testosterone’s Effects on Fitness

Understanding physiological and behavioral mechanisms underlying the diversity of observed life-history strategies is challenging because of difficulties in obtaining long-term measures of fitness and in relating fitness to these mechanisms. We evaluated effects of experimentally elevated testosterone on male fitness in a population of dark-eyed juncos studied over nine breeding seasons using a demographic modeling approach. Elevated levels of testosterone decreased survival rates but increased success of producing extra-pair offspring. Higher overall fitness for testosterone-treated males was unexpected and led us to consider indirect effects of testosterone on offspring and females. Nest success was similar for testosterone-treated and control males, but testosterone-treated males produced smaller offspring, and smaller offspring had lower postfledging survival. Older, more experienced females preferred to mate with older males and realized higher reproductive success when they did so. Treatment of young males increased their ability to attract older females yet resulted in poor reproductive performance. The higher fitness of testosterone-treated males in the absence of a comparable natural phenotype suggests that the natural phenotype may be constrained. If this phenotype were to arise, the negative social effects on offspring and mates suggest that these effects might prevent high-testosterone phenotypes from spreading in the population.

Exogenous testosterone inhibits several forms of male parental behavior and stimulates song in a monogamous songbird: the blue-headed vireo (Vireo solitarius)

Natural variation in circulating testosterone is thought to play a role in creating inter- and intraspecific variation in paternal care in birds. Experimental elevation of plasma testosterone in males has reduced rates of male care in many species. However, a small number of studies suggest that male care is not inhibited by testosterone when there appears to be strong selection for male care. Both genetic monogamy and male incubation are thought to be the result of selection for biparental care, yet the sensitivity of male care to inhibition by T had not been examined in a species with both characteristics. Male blue-headed vireos have been shown to be strictly monogamous and provide extensive parental care, including incubation. Males received subcutaneous implants of testosterone, flutamide (an anti-androgen), or empty controls. Consistent with previous studies, testosterone treatment decreased the relative contribution to incubation by males and increased the average duration that the nest was left unattended during incubation, relative to control and flutamide treatments. The relative and absolute rates of feeding offspring and nest sanitation by T-treated males also decreased relative to controls. Flutamide-treated males incubated eggs and brooded young more than testosterone-treated males, with control males performing at intermediate values. Testosterone-treated males sang more often and at higher rates, and flutamide-treated males sang less often, than controls. These results demonstrate that even in a strictly monogamous songbird with biparental incubation, all forms of paternal care remain sensitive to elevated plasma testosterone, suggesting a shift from parental effort to mating effort.

Testosterone, cuckoldry risk and extra-pair opportunities in the Seychelles warbler

In male birds, testosterone (T) plays an important role in aggressive and mate-attraction behaviour. In the cooperatively breeding Seychelles warbler, Acrocephalus sechellensis, extra-group copulations (EGCs) occur frequently, but are not accompanied by sexual courtship displays as in within-pair copulations. Paternity is nearly always gained by primary males. We investigated whether T levels and sperm storage capability (cloacal protuberance (CP)) in adult primary and subordinate males were related to timing of egg laying, levels of cuckoldry and extra-group paternity (EGP) opportunities. During the sexually active period before egg laying, T levels and CP were only elevated or enlarged (respectively) in primary males, and some suggestion was found that subordinate males do not invest in elevated T levels. The peak in T occurred during the fertile period of the female partner and corresponded to the peak period of male sexual displays and mate guarding, but was independent of cuckoldry risk (density of neighbouring primary males). CP was also enhanced during this period; however, CP but not T remained elevated after egg laying by their mates, and CP but not T was positively related to EGP opportunities (density of neighbouring fertile females). We conclude that T is involved in sexual courtship displays and mate guarding, but not in gaining EGCs. These findings contrast with those in other species where EGP involves elaborate sexual displays.

Genetic variation in infestation with a directly transmitted ectoparasite

Genetic variation in levels of parasitism of hosts is an underlying assumption of studies of coevolution, but few such estimates are available from the field. We studied genetic variation in the abundance of the chewing louse Hirundoecus malleus on its barn swallow host Hirundo rustica. These parasites are directly transmitted and a test of genetic variation of parasite abundance would thus provide a particularly strong test. The prevalence and the abundance of the chewing lice did not differ significantly between adult male and female hosts. The resemblance in parasite intensity of H. malleus of offspring and their parents was positive and highly significant, and an analysis of extra-pair paternity in the host allowed partitioning of this resemblance between genetic and common environment effects. There was no significant resemblance in parasite intensity between extra-pair offspring and their foster parents, although the resemblance remained for within-pair offspring. This provides evidence for the abundance of directly transmitted parasites having an additive genetic component. We found no evidence of common environment effects as parents did not resemble each other with respect to lice abundance.

Androgen and behavior in the male three-spined stickleback, Gasterosteus aculeatus. II. Castration and 11-ketoandrostenedione effects on courtship and parental care during the nesting cycle

Courtship declines and ceases while parental care increases in the presence of developing eggs during the nesting cycle of the male three-spined stickleback, Gasterosteus aculeatus. Furthermore, circulating 11-ketotestosterone (11KT) levels are higher during the initial "courtship phase" than during the later "parental phase," similar to that found in other paternal fishes. This study aimed to investigate a possible functional relationship between changes in 11KT levels and changes in reproductive behavior during the nesting cycle. To this end, groups of nonspawned and spawned male sticklebacks were sham-operated, castrated, or castrated and treated with 11-ketoandrostenedione (11KA), and the effects of the treatments on courtship and parental care were studied. Castration removed circulating 11KT, while 11KA replacement prevented the natural decline in 11KT during the parental phase (11KA converts to 11KT extratesticularly), as assessed by radioimmunoassay. Regardless of treatment, parental care remained low and courtship was present in all nonspawned males, even at the end of the experiment. However, courtship did eventually decline in castrated nonspawned males compared to the other two nonspawned groups. In all treatments of spawned males there was a drastic decline in courtship and an increase in parental care. In castrated spawned males, however, the decline in courtship came earlier than in the other two spawned groups. 11KA treatment did not prevent the natural decline in courtship/increase in parental care in spawned males, indicating that the natural decline in 11KT is not responsible for the main portion of the rapid changes in these behaviors over the stickleback's nesting cycle. The limited effects of castration also exclude other gonadal hormones from being responsible for most of these changes.

Effects of testosterone on song, aggression, and nestling feeding behavior in male great tits, Parus major

As outlined in the trade-off hypothesis of testosterone (T) secretion, fluctuations in T during the breeding season might reflect how males allocate their time and energy to competitive behaviors for mates and territories, associated with high T levels, and parental activities, associated with low T levels. In the present study, great tit, Parus major, males were implanted with T-filled or empty silastic capsules at the start of the breeding season and the behavior of these two male categories was compared during the entire breeding season. As a measure of competitive behavior we looked at song behavior and territorial responsiveness to a male decoy, during the three main stages of the breeding period (the egg-laying, incubation, and nestling stages). As a measure of parental care we looked at feeding behavior during the nestling stage. Our results only partly supported the trade-off hypothesis. T implants increased plasma androgen levels and enhanced spontaneous song activity and the production of aggressive vocalizations in response to a decoy. However, our results suggest that the degree of physical aggression might be less than fully coupled with T. First, approach to the decoy was not affected by the treatment. Second, although T levels are known to vary from high during egg laying to low while feeding young, control and T-treated males spent similar amounts of time close to the decoy in the three breeding stages. Our results thus suggest that vocal and physical aggression might be regulated differently in the great tit. Furthermore, in contrast with most other studies on temperate bird species but in agreement with a previous study on the great tit, T treatment did not affect male feeding rates. As the dose of T we used was lower than that typically used in other studies, we cannot at present completely exclude the possibility that the latter result reflects this lower dose of T rather than the species used.

The effects of testosterone manipulation on the body condition of captive male yellow-legged gulls

Persistently high testosterone levels are believed to be costly to males due to their negative effect on body condition. However, this assumption could not be validated when we analysed birds isolated from all social interactions. The hypothesis was tested on birds kept in isolation in order to analyse the effects of testosterone per se, and thereby exclude the influence of social interactions. Adult male yellow-legged gulls (Larus cachinnans) were captured, and after a period of adjustment, some individuals were subcutaneously implanted with testosterone, while the rest were used as controls. The gulls received ad libitum food for 10 days and were then fasted for 4 days. Thyroid hormones, body-mass change, daily food intake, hematocrit and several plasma biochemical parameters were analysed. Treated (T)-males maintained constant levels of plasma total protein throughout the experiment, whilst control (C)-males showed a decrease. We did not find any other differences between groups for the other variables analysed. Since the implanted birds sustained high testosterone levels for a number of days, any cost to body condition would have been revealed if these costs levels were actually important. Our results do not support the hypothesis that a reduction in body condition can be directly produced by plasma testosterone, although total protein changes do suggest different anabolic patterns in testosterone-treated gulls.

Age-related changes in paternal responses of gerbils parallel changes in their testosterone concentrations

Results of previous studies in our laboratory have shown that testosterone (T) inhibits parental response in adult male Mongolian gerbils. Here, we examined developmental changes in parental responses of male gerbils before, during, and after a naturally occurring surge in T that peaks on Day 75 postpartum. On the hypothesis that T inhibits parental response in male gerbils, we predicted that (a) 75-day-old male gerbils would be less responsive to neonates than would either younger or older male gerbils, and (b) young male gerbils whose T titers were rising as the litters that they were helping to rear matured would show a decrease over days in parental effort relative to older male gerbils whose T titers were falling as the litters that they were helping to rear matured. Both predictions were confirmed, providing evidence consistent with the view that naturally occurring, developmental changes in circulating concentrations of T play a role in age-related changes in the level of parental response of male Mongolian gerbils.

Interactions among paternal behavior, steroid hormones, and parental experience in male marmosets (Callithrix kuhlii)

Male black tufted-ear marmosets (Callithrix kuhlii) contribute to the rearing of their offspring. Here we evaluated predictions of hypotheses suggesting that (1) T and E2 influence infant-care behavior in male marmosets, (2) levels of T and E2 are modulated by paternal experience, and (3) paternal behavior and levels of T and E2 in male marmosets covary with stress. We observed the behavior of marmosets in their family groups following the birth of infants and evaluated urinary concentrations of T, E2, and the stress hormone cortisol (CORT) among fathers before and after the birth of young. Urinary levels of T, E2, and CORT were lower among males who carried infants at high rates than males who carried at low rates, and T and CORT levels were negatively correlated with carrying rates across all males. Males had significantly lower T levels while carrying the second compared to the first litter and slightly lower rates of infant-carrying, possibly due to assistance provided by offspring of the first litter. There were increases in CORT levels of fathers after the birth of the first litter, but decreases in CORT after the birth of the second. Our results suggest a relationship in C. kuhlii between paternal behavior, hormones, and paternal experience. Rates of infant-carrying appear to be linked to hormone levels, and hormone levels in turn are affected by experience caring for young. Our data also suggest that T, E2, and CORT have synergistic influences on infant-carrying behavior or alternatively that associations between T and E2 and rates of infant-carrying are influenced by stress or other glucocorticoid-related variables. Finally, we propose a hypothesis suggesting that experience-related changes in hormones reinforce the commitment of males to successful breeding partnerships.

Variation in steroid hormones associated with infant care behaviour and experience in male marmosets (Callithrix kuhlii)

We describe temporal patterns of change in paternal behaviour and urinary concentrations of the steroid hormones testosterone (T) and oestradiol (E(2)) in male black tufted-ear marmosets, Callithrix kuhlii, relative to the birth of their young, and test predictions of the hypotheses that (1) high levels of T are incompatible with paternal care and (2) levels of T and E(2)vary with a father's prior experience in his family group. After young were born, levels of urinary T and E(2)remained near prepartum concentrations and rates at which fathers carried infants were below peak levels until the approximate time that postpartum mating ordinarily occurs, suggesting a possible trade-off between readiness to mate and paternal behaviour in C. kuhlii. Infant-carrying behaviour of fathers occurred at its highest rate 3-4 weeks after parturition and coincided with significant declines in urinary levels of T and E(2), providing preliminary support for the hypothesis that these hormones are antagonistic to paternal behaviour. Urinary T and E(2)declined among fathers regardless of whether their young survived to weaning or died at birth, indicating that variation in these hormones after parturition occurs even in the absence of continued stimuli from infants. When adjusted for declines ordinarily associated with aging, urinary T tended to be lower among fathers with a great deal of prior experience caring for young compared with fathers having little or no experience, suggesting that either experience affects T levels of fathers, or that T levels influence fathers' chances of successfully rearing infants. Overall, our results suggest that in male C. kuhlii, T, and possibly E(2), play an important role in balancing the expression of paternal care with that of other reproductive behaviours. Copyright 2000 The Association for the Study of Animal Behaviour.

Better red than dead: carotenoid-based mouth coloration reveals infection in barn swallow nestlings

Nestling birds solicit food from their parents by displaying their open brightly coloured gapes. Carotenoids affect gape colour, but also play a central role in immunostimulation. Therefore, we hypothesize that, by differentially allocating resources to nestlings with more brightly coloured gapes, parents favour healthy offspring which are able to allocate carotenoids to gape coloration without compromising their immune defence. We demonstrated that, in the barn swallow Hirundo rustica, (i) parents differentially allocate food to nestlings with an experimentally brighter red gape, (ii) nestlings challenged with a novel antigen (sheep red blood cells, SRBCs) have less bright gape colour than their control siblings, (iii) nestlings challenged with SRBCs but also provided with the principal circulating carotenoid (lutein) have more brightly coloured red gapes than their challenged but unsupplemented siblings and (iv) the gape colour of nestlings challenged with SRBCs and provisioned with lutein exceeds that of siblings that were unchallenged. This suggests that parents may favour nestlings with superior health by preferentially feeding offspring with the brightest gapes.