Individual variation in testosterone and parental care in a female songbird; the dark-eyed junco (Junco hyemalis)

Fathers and sons, mothers and daughters: Sex-specific genetic architecture for fetal testosterone in a wild mammal

Testosterone plays a critical role in mediating fitness-related traits in many species. Although it is highly responsive to environmental and social conditions, evidence from several species show a heritable component to its individual variation. Despite the known effects that in utero testosterone exposure have on adult fitness, the heritable component of individual testosterone variation in fetuses is mostly unexplored. Furthermore, testosterone has sex-differential effects on fetal development, i.e., a specific level may be beneficial for male fetuses but detrimental for females, producing sexual conflict. Such sexual conflict may be resolved by the evolution of a sex-specific genetic architecture of the trait. Here, we quantified fetal testosterone levels in a wild species, free-ranging nutrias (Myocastor coypus) using hair-testing and estimated testosterone heritability between parent and offspring from the same and opposite sex. We found that in utero accumulated hair testosterone levels were heritable between parents and offspring of the same sex. Moreover, there was a low additive genetic covariance between the sexes, and a low cross-sex genetic correlation, suggesting a potential for sex-specific trait evolution, expressed early on, in utero.

The insemination status and social context influences quail laying and social behavior: A novel experimental set up

Japanese quails in wild life live in small groups with females being even solitary during the laying period. Although it is a poultry species widely used for egg production, information regarding laying behavior motivations or influencing variables is scarce. Our study focuses on evaluating along 7 d the quail laying behavior in a novel environmental set up. This set up allows the female to choose between remaining separated from a conspecific in one side of the apparatus or to voluntarily enter their space (box-mate side) and interact with it. We evaluated whether the female insemination status prior to enter the environmental set up, and the presence of a female or a male partner in the box-mate side can influence their laying and social behavior. Thus, 4 experimental groups were established. Females spent a higher (P < 0.05) percentage of time in the box-mate side than in their separated sector in all groups. In 3 of the 4 experimental groups (non-inseminated females interacting with a female or a male box-mate, and inseminated females interacting with a male box-mate) females also laid a greater percentage (≥65%, P < 0.05, in all cases) of eggs in the box-mate sector than in their separated sector. However, the group of inseminated females that interacted with a female box-mate shifted their egg distribution and laid equally between both sides of the apparatus. Aggressive social interactions were reduced (P < 0.05) throughout the testing days but this was depending upon the female insemination status and the sex of their box-mate. Findings suggest that females can change their laying side choice when they are inseminated but depending on the sex identity of their box-mate partners. Thus, providing quail female breeders with the option of laying their eggs in separated enclosures from conspecifics could be key to favor their well-being.

Testing the role of testosterone versus estrogens in mediating reproductive transitions in female rhesus macaques

In male vertebrates, testosterone is generally known to coordinate reproductive trade-offs, in part by promoting the transition to the next reproduction at the expense of current parental care. The role of testosterone in reproductive transitions has been little tested in female vertebrates, especially in mammals. The present study sought to fill this gap, by first undertaking an experimental study, in which we identified DHT, androstenediol, and in particular etiocholanolone, as fecal androgen metabolites which reflect serum testosterone concentration in female rhesus macaques (Macaca mulatta). Using concentrations of fecal etiocholanolone as proxy for circulating testosterone, we then conducted a field study on 46 free-ranging rhesus macaques of Cayo Santiago, Puerto Rico, to test if testosterone mediates the trade-off between reproductive transition (a higher chance of reproducing in the next year) and current reproduction (providing more care to current offspring). While the evidence for testosterone was weak, the testing of fecal immunoreactive estrogen metabolites suggested a potential role of estrogen in reproductive trade-offs. We found large individual differences in fecal etiocholanolone concentrations during the early postpartum period that were unexplained even after accounting for sociodemographic factors such as age and dominance rank. Further investigation is needed to understand this variation. Our study suggests that the actions of testosterone in females may not have evolved to fulfil the same role in primate reproductive transitions as it does in males, and we encourage more studies to consider the function of testosterone in reproductive behaviors and life history transitions in females of mammalian taxa.

How research on female vertebrates contributes to an expanded challenge hypothesis

The bi-directional links between hormones and behavior have been a rich area of research for decades. Theory on the evolution of testosterone (T) was greatly advanced by the challenge hypothesis, which presented a framework for understanding interspecific, seasonal, and social variation in T levels in males, and how they are shaped by the competing demands of parental care and male-male competition. Female competition is also widespread in nature, although it is less clear whether or how the challenge hypothesis applies to females. Here, we evaluate this issue in four parts: (1) We summarize and update prior analyses of seasonal plasticity and interspecific variation in T in females. (2) We evaluate experimental links between T and female aggression on shorter timescales, asking how T manipulations affect aggression and conversely, how social manipulations affect T levels in female mammals, birds, lizards, and fishes. (3) We examine alternative mechanisms that may link aggression to the social environment independently of T levels in circulation. (4) We present a case study, including new data analyses, in an aggressive female bird (the tree swallow, Tachycineta bicolor) to explore how variation in tissue-level processing of T may bridge the gap between circulating T and variation in behavior that is visible to natural selection. We close by connecting these multivariate levels of sex steroid signaling systems alongside different temporal scales (social, seasonal, and evolutionary) to generate broadly applicable insights into how animals respond to their social environment, regardless of whether they are male or female.

Testosterone as a mediator of the tradeoff between cooperation and competition in the context of cooperative reproductive behaviors

Behavioral tradeoffs occur when the expression of one behavior detracts from the expression of another. Understanding the proximate mediators of behavioral tradeoffs is important as these tradeoffs can act as potential constraints on evolutionary responses to selection. Here, we describe the tradeoff between cooperation and competition faced by species that exhibit cooperative reproductive behaviors and propose that testosterone is a key hormonal mediator of the tradeoff. Cooperative reproductive behaviors occur when multiple individuals coordinate their efforts to gain a reproductive advantage over other individuals and/or those individuals attempting to reproduce in absence of cooperation. We propose that testosterone, a sex steroid known to mediate a number of physiological and behavioral actions associated with reproductive competition, is involved in mediating the tradeoff between cooperation and competition. To support this proposition, we first describe the importance of individual variation in behavior to the evolution of cooperative behaviors. We then describe how proximate mechanisms represent a prominent source of individual variation in social behaviors and highlight evidence suggesting testosterone mediates variation in cooperative behaviors. Two case studies in which the relationship between testosterone and cooperative behaviors have been investigated in detail are then summarized. Throughout we highlight the importance of studying individual variation to understand the mechanistic basis of behaviors, behavioral tradeoffs, and the evolution of cooperative reproductive behaviors more broadly.

Sexually opposite effects of testosterone on mating success in wild rock hyrax

Although males and females share traits, their motivations and needs may be different, due to life-history disparities that lead to divergent selection pressures. Proximate mechanisms underlying differences between the sexes include hormones that mediate the development and activation of suites of traits. Testosterone is associated with morphological features, physiological processes, and social behaviors in both sexes. However, even if present in similar concentrations in the circulation, testosterone often affects males and females differently. We combined behavioral mating observations of the wild polygynandrous rock hyrax (Procavia capensis) with hair testosterone that represents long-term integrated levels. We found that whereas copulation success increases with the rise in testosterone in males it decreases in females. We did not find an association between testosterone and choosiness in either sex. However, we found that males with higher testosterone mate-guarded females with lower testosterone. Our findings show disassortative mating and mate-guarding in respect to testosterone and provide clues to the cost of testosterone for females, in terms of copulation success. These results open up intriguing questions relating to the role of testosterone in mediating a similar trade-off in male and female reproductive success.

Experimentally induced increases in fecundity lead to greater nestling care in blue tits

Models on the evolution of bi-parental care typically assume that maternal investment in offspring production is fixed and predict subsequent contributions to offspring care by the pair are stabilized by partial compensation. While experimental tests of this prediction are supportive, exceptions are commonplace. Using wild blue tits (Cyanistes caeruleus), we provide, to our knowledge, the first investigation into the effects of increasing maternal investment in offspring production for subsequent contributions to nestling provisioning by mothers and male partners. Females that were induced to lay two extra eggs provisioned nestlings 43% more frequently than controls, despite clutch size being made comparable between treatment groups at the onset of incubation. Further, experimental males did not significantly reduce provisioning rates as expected by partial compensation, and if anything contributed slightly (9%) more than controls. Finally, nestlings were significantly heavier in experimental nests compared with controls, suggesting that the 22% average increase in provisioning rates by experimental pairs was beneficial. Our results have potential implications for our understanding of provisioning rules, the maintenance of bi-parental care and the timescale over which current–future life-history trade-offs operate. We recommend greater consideration of female investment at the egg stage to more fully understand the evolutionary dynamics of bi-parental care.

The quantity-quality trade-off: differential effects of daily food times on reproductive performance and offspring quality in diurnal zebra finches

Abundant food supply is crucial to reproductive performance, as shown by restricted food availability experiments, in small-sized vertebrates including birds. However, whether daily times of feeding would affect the reproduction is largely unknown. Present study investigated the effects of daily food availability times on reproductive performance and quality of eggs and offspring survivors in zebra finches (Taeniopygia guttata). To randomly paired birds kept under 12 h light per day for about 52 weeks, food availability period was restricted to 4 h in morning (FA-M) or evening (FA-E), with controls on food ad libitum; thus, daily food deprivation period began after 4 h of food in FA-M and was continuous with nighttime starvation in FA-E. Both food restrictions adversely affected reproductive health as shown by reduced sex steroids and mesotocin levels, but not general metabolism as indicated by no-difference in thyroxin and triiodothyronine levels. Food for 4 h negatively affected the reproductive performance, although with differences between FA-M and FA-E pairs. Particularly, there was delayed onset of reproduction and compromised reproductive success in FA-E, but not in FA-M pairs; conversely, the offspring health was severely compromised in FA-M, but not in FA-E pairs. Furthermore, FA-M females were in better health, implicating sex-biasness in parental food provisioning. Overall, we demonstrate trade-off of ‘quantity’ (offspring produced and/ or survived) for ‘quality’ (how good offspring were in health) in response to daily food availability times in zebra finches that much like humans are diurnal and retain the ability to reproduce throughout the year.

Testosterone production and social environment vary with breeding stage in a competitive female songbird

In many vertebrates, males increase circulating testosterone (T) levels in response to seasonal and social changes in competition. Females are also capable of producing and responding to T, but the full extent to which they can elevate T across life history stages remains unclear. Here we investigated T production during various breeding stages in female tree swallows (Tachycineta bicolor), which face intense competition for nesting sites. We performed GnRH and saline injections and compared changes in T levels 30 min before and after injection. We found that GnRH-injected females showed the greatest increases in T during territory establishment and pre-laying stages, whereas saline controls dramatically decreased T production during this time. We also observed elevated rates of conspecific aggression during these early stages of breeding. During incubation and provisioning, however, T levels and T production capabilities declined. Given that high T can disrupt maternal care, an inability to elevate T levels in later breeding stages may be adaptive. Our results highlight the importance of saline controls for contextualizing T production capabilities, and they also suggest that social modulation of T is a potential mechanism by which females may respond to competition, but only during the period of time when competition is most intense. These findings have broad implications for understanding how females can respond to their social environment and how selection may have shaped these hormone-behavior interactions.

Social environment during egg laying: Changes in plasma hormones with no consequences for yolk hormones or fecundity in female Japanese quail, Coturnix japonica

The social environment can have profound effects on an individual's physiology and behaviour and on the transfer of resources to the next generation, with potential consequences for fecundity and reproduction. However, few studies investigate all of these aspects at once. The present study housed female Japanese quail (Coturnix japonica) in pairs or groups to examine the effects on hormone concentrations in plasma and yolk and on reproductive performance. Circulating levels of androgens (testosterone and 5-α-dihydrotestosterone) and corticosterone were measured in baseline samples and after standardised challenges to assess the responsiveness of the females' endocrine axes. Effects of the social environment on female fecundity were analysed by measuring egg production, egg mass, fertilization rates, and number of hatched offspring. Counter to expectation, females housed in pairs had higher plasma androgen concentrations and slightly higher corticosterone concentrations than females housed in groups, although the latter was not statistically significant. Pair vs. group housing did not affect the females' hormonal response to standardised challenges or yolk testosterone levels. In contrast to previous studies, the females' androgen response to a gonadotropin-releasing hormone challenge was not related to yolk testosterone levels. Non-significant trends emerged for pair-housed females to have higher egg-laying rates and higher fertility, but no differences arose in egg weight or in the number, weight or size of hatchlings. We propose that our unexpected findings are due to differences in the adult sex ratio in our social treatments. In pairs, the male may stimulate female circulating hormone levels more strongly than in groups where effects are diluted due to the presence of several females. Future studies should vary both group size and sex composition to disentangle the significance of sexual, competitive and affiliative social interactions for circulating and yolk hormone levels, and their consequences for subsequent generations.

Testosterone production in response to exogenous gonadotropin releasing hormone (GnRH challenge) depends on social environment and color polymorphism

Testosterone is an important mediator of behavior, morphology and physiology. A cascade of signals regulates the amount of testosterone (T) circulating in the plasma; in response to stimulus the hypothalamus releases gonadotropin-releasing hormone (GnRH), which triggers secretion of gonadotropins from the pituitary, stimulating the synthesis and release of T from the gonads. Previous work has shown that changes to the social environment can alter circulating T-levels, which may have important fitness consequences, but it is currently unclear whether these changes are due to alterations in the signal from the brain, or changes in the ability of the pituitary and gonads to respond to this signal. Further, the strength and direction of response to a changing environment may differ according to life-history strategy. Species with genetically determined alternative strategies offer a pathway for examining these differences. Here we use a finch with a genetically determined polymorphism, the Gouldian finch (Erythrura gouldiae), to determine whether T-levels change in response to social environment. We also use injections of GnRH to determine whether these changes are due to alterations in the ability of the pituitary and gonads to respond to this signal. We found that social environment (presence of females) had a rapid effect on male circulating T-levels, and that this difference was reflected in responsiveness to GnRH. We observed no overall morph differences in T-levels, but we did observe morph differences in the pattern of T secretion across environments, and morph differences in the repeatability of T-levels across time and environment.

Testosterone production ability predicts breeding success and tracks breeding stage in male finches

Testosterone (T) is an important mediator of reproductive behaviours and potential target for selection. However, there are few data relating natural variation in T to fitness estimates. Here, we used the GnRH challenge (an injection of gonadotropin-releasing hormone which stimulates maximal T release), to examine how individual differences in T relate to reproductive success and how T changes across date and breeding stage. We measured pre- and post-challenge T, in captive male Gouldian finches (Erythrura gouldiae), before and after introducing females, and across breeding stage. Post-challenge T before introducing females positively predicted breeding success. Post-challenge T levels were unrelated to date, but strongly related to stage; T production ability was strongly attenuated in incubating males. Prechallenge T levels related only to date. Our results suggest that T production ability is an important target for selection and that when males invest heavily in parental care they reduce their sensitivity to GnRH.

Effects of experimentally sustained elevated testosterone on incubation behaviour and reproductive success in female great tits (Parus major)

In many seasonally breeding birds, female and male testosterone (T) levels peak at the start of the breeding season, coinciding with pair bonding and nesting activities. Shortly after the onset of egg laying, T levels slowly decline to baseline levels in both sexes, but more rapidly so in females. During this period, T in males may still function to facilitate territorial behaviour, mate guarding and extra pair copulations, either via short lasting peaks or elevated basal levels of the hormone. In some species, however, males become insensitive to increased T after the onset of egg laying. It has been postulated that in these species bi-parental care is essential for offspring survival, as T is known to inhibit paternal care. However, only very few studies have analysed this for females. As females are heavily involved in parental care, they too might become insensitive to T after egg laying. Alternatively, because territorial defence, mate guarding and extra pair copulations are expected to be less important for females than for males, they may not have had the need to evolve a mechanism to become insensitive to T during the period of maternal care, because their natural T levels are never elevated during this part of the breeding season anyway. We tested these alternative hypotheses in female great tits (Parus major). Male great tits have previously been shown to be insensitive to T after egg laying with regard to nestling feeding behaviour (but not song rate). When females had started nest building, we experimentally elevated their T levels up to the nestling feeding phase, and measured incubation behaviour (only females incubate) and reproductive success. T did not significantly affect nest building or egg laying behaviour, although egg laying tended to be delayed in T females. Females with experimentally enhanced T maintained lower temperature during incubation but did not spend less time incubating. This might explain the reduced hatching success of their eggs, smaller brood size and lower number of fledglings we found in this study. As in this species T-dependent behaviour by females during the phase of parental care is not needed, the results support the hypothesis that in this species the need for selection in favour of T-insensitivity did not occur.

Costs and benefits of competitive traits in females: aggression, maternal care and reproductive success

Recent research has shown that female expression of competitive traits can be advantageous, providing greater access to limited reproductive resources. In males increased competitive trait expression often comes at a cost, e.g. trading off with parental effort. However, it is currently unclear whether, and to what extent, females also face such tradeoffs, whether the costs associated with that tradeoff overwhelm the potential benefits of resource acquisition, and how environmental factors might alter those relationships. To address this gap, we examine the relationships between aggression, maternal effort, offspring quality and reproductive success in a common songbird, the dark-eyed junco (Junco hyemalis), over two breeding seasons. We found that compared to less aggressive females, more aggressive females spent less time brooding nestlings, but fed nestlings more frequently. In the year with better breeding conditions, more aggressive females produced smaller eggs and lighter hatchlings, but in the year with poorer breeding conditions they produced larger eggs and achieved greater nest success. There was no relationship between aggression and nestling mass after hatch day in either year. These findings suggest that though females appear to tradeoff competitive ability with some forms of maternal care, the costs may be less than previously thought. Further, the observed year effects suggest that costs and benefits vary according to environmental variables, which may help to account for variation in the level of trait expression.