Glucocorticoids, male sexual signals, and mate choice by females: Implications for sexual selection

In your CORT: Corticosterone and its receptors in the brain underlie mate choosiness in female Cope's gray treefrogs (Hyla chrysoscelis)

Selecting an attractive mate can involve trade-offs related to investment in sampling effort. Glucocorticoids like corticosterone (CORT) are involved in resolving energetic trade-offs. However, CORT is rarely studied in the context of mate choice, despite its elevated levels during reproductive readiness and the energetic transitions that characterize reproduction. Few systems are as well suited as anuran amphibians to evaluate how females resolve energetic trade-offs during mate choice. Phonotaxis tests provide a robust bioassay of mate choice that permit the precise measurement of inter-individual variation in traits such as choosiness-the willingness to pursue the most attractive mate despite costs. In Cope's gray treefrogs (Hyla chrysoscelis), females exhibit remarkable variation in circulating CORT as well as choosiness during mate choice, and a moderate dose of exogenous CORT rapidly (<1 h) and reliably induce large increases in choosiness. Here we measured the expression of glucocorticoid (GR) and mineralocorticoid (MR) receptors in the brains of females previously treated with exogenous CORT and tested for mate choosiness. We report a large decrease in GR expression in the hindbrain and midbrain of females that were treated with the moderate dosage of CORT-the same treatment group that exhibited a dramatic increase in choosiness following CORT treatment. This association, however, does not appear to be causal, as only forebrain GR levels, which are not affected by CORT injection, are positively associated with variation in choosiness. No strong effects were found for MR. We discuss these findings and suggest future studies to test the influence of glucocorticoids on mate choice.

Male Cope's gray treefrogs (Hyla chrysoscelis) in amplexus have elevated and correlated steroid hormones compared to solitary males

Gonadal steroid hormones are typically elevated during the breeding season-a finding known as an associated reproductive pattern. Though less studied, there is also evidence, in both sexes, for elevated adrenal/interrenal steroids, including acute elevations on the day of mating. I investigated gonadal and interrenal steroids in wild male Cope's gray treefrogs at breeding aggregations. I collected blood from males found in amplexus with female mates (amplexed males) and males sampled at the same time and location that were actively advertising vocally and without a mate (solo males). Concentrations of plasma corticosterone, testosterone, and 17β-estradiol (CORT, T and E2, respectively) were validated and measured. These two categories of males differed in four ways: (1) amplexed males exhibited significantly elevated concentrations of all three steroids compared to solo males (CORT: +347 %; T: +60 %; and E2: +43 %); (2) these hormone profiles alone accurately predicted male mating category with ca. 83 % accuracy using a discriminant function analysis; (3) amplexed males exhibited significant between-hormone correlations (T and E2 were positively correlated and CORT and E2 were negatively correlated) whereas no correlations were found in solo males; (4) amplexed males showed a negative correlation with CORT concentration and the time of night, whereas no such pattern was present in solo males. These findings suggest an acute and strong coactivation of the interrenal and gonadal axes that could drive phenotypic integration during this fitness-determining moment. I discuss these findings and suggest experiments to determine causation, including the role of motor behavior driving endocrine states and the role of female selection on endocrine profiles.

Noise and light pollution elicit endocrine responses in urban but not forest frogs

Urban areas are characterised by the presence of sensory pollutants, such as anthropogenic noise and artificial light at night (ALAN). Animals can quickly adapt to novel environmental conditions by adjusting their behaviour, which is proximately regulated by endocrine systems. While endocrine responses to sensory pollution have been widely reported, this has not often been linked to changes in behaviour, hampering the understanding of adaptiveness of endocrine responses. Our aim was, therefore, to investigate the effects of urbanisation, specifically urban noise and light pollution, on hormone levels in male urban and forest túngara frogs (Engystomops pustulosus), a species with reported population divergence in behaviour in response to urbanisation. We quantified testosterone and corticosterone release rates in the field and in the lab before and after exposure to urban noise and/or light. We show that urban and forest frogs differ in their endocrine phenotypes under field as well as lab conditions. Moreover, in urban frogs exposure to urban noise and light led, respectively, to an increase in testosterone and decrease in corticosterone, whereas in forest frogs sensory pollutants did not elicit any endocrine response. Our results show that urbanisation, specifically noise and light pollution, can modulate hormone levels in urban and forest populations differentially. The observed endocrine responses are consistent with the observed behavioural changes in urban frogs, providing a proximate explanation for the presumably adaptive behavioural changes in response to urbanisation.

Mitochondria as the powerhouses of sexual selection: Testing mechanistic links between development, cellular respiration, and bird song

The developmental environment can affect the expression of sexually selected traits in adulthood. The physiological mechanisms that modulate such effects remain a matter of intense debate. Here, we test the role of the developmental environment in shaping adult mitochondrial function and link mitochondrial function to expression of a sexually selected trait in males (bird song). We exposed male zebra finches (Taeniopygia guttata) to corticosterone (CORT) treatment during development. After males reached adulthood, we quantified mitochondrial function from whole red blood cells and measured baseline CORT and testosterone levels, body condition/composition, and song structure. CORT-treated males had mitochondria that were less efficient (FCR_L/R) and used a lower proportion of maximum capacity (FCR_R/ETS) than control males. Additionally, CORT-treated males had higher baseline levels of CORT as adults compared to control males. Using structural equation modelling, we found that the effects of CORT treatment during development on adult mitochondrial function were indirect and modulated by baseline CORT levels, which are programmed by CORT treatment during development. Developmental treatment also had an indirect effect on song peak frequency. Males treated with CORT during development sang songs with higher peak frequency than control males, but this effect was modulated through increased CORT levels and by a decrease in FCR_R/ETS. CORT-treated males had smaller tarsi compared to control males; however, there were no associations between body size and measures of song frequency. Here, we provide the first evidence supporting links between the developmental environment, mitochondrial function, and the expression of a sexually selected trait (bird song).

A comprehensive overview of the effects of urbanisation on sexual selection and sexual traits

Urbanisation can affect mating opportunities and thereby alter inter- and intra-sexual selection pressures on sexual traits. Biotic and abiotic urban conditions can influence an individual's success in pre- and post-copulatory mating, for example through impacts on mate attraction and mate preference, fertilisation success, resource competition or rival interactions. Divergent sexual selection pressures can lead to differences in behavioural, physiological, morphological or life-history traits between urban and non-urban populations, ultimately driving adaptation and speciation. Most studies on urban sexual selection and mating interactions report differences between urban and non-urban populations or correlations between sexual traits and factors associated with increased urbanisation, such as pollution, food availability and risk of predation and parasitism. Here we review the literature on sexual selection and sexual traits in relation to urbanisation or urban-associated conditions. We provide an extensive list of abiotic and biotic factors that can influence processes involved in mating interactions, such as signal production and transmission, mate choice and mating opportunities. We discuss all relevant data through the lens of two, non-mutually exclusive theories on sexual selection, namely indicator and sensory models. Where possible, we indicate whether these models provide the same or different predictions regarding urban-adapted sexual signals and describe different experimental designs that can be useful for the different models as well as to investigate the drivers of sexual selection. We argue that we lack a good understanding of: (i) the factors driving urban sexual selection; (ii) whether reported changes in traits result in adaptive benefits; and (iii) whether these changes reflect a short-term ecological, or long-term evolutionary response. We highlight that urbanisation provides a unique opportunity to study the process and outcomes of sexual selection, but that this requires a highly integrative approach combining experimental and observational work.

Elevated corticosterone levels are associated with increased immunocompetence in male toads, both when calling and under experimental conditions

Circulating androgens can influence immune responses and sexual traits in male vertebrates. However, in the last 20 years, glucocorticoids have also been implicated as a possible source of variation in male sexual traits and immunocompetence. In this context, we studied the relations between male vocal mating display, immunity, androgens, and glucocorticoids in the explosive breeding toad Rhinella granulosa. In the field, males with high calling effort display either high- or low-corticosterone (CORT) plasma levels, but only males with both high calling effort and high CORT plasma levels showed high bacterial killing ability (BKA), suggesting that the acute CORT elevation can be immunostimulatory. CORT treatments increased BKA in laboratory experiments, confirming the functional relationship observed in the field. However, toads treated with a low dose of CORT increased BKA for 10 h after the treatment, while toads that received a high dose increased BKA for only 1 h after the treatment. These results indicate that different CORT doses can result in temporal differences in the immune response. We did not find any relationship between calling effort, immune response (BKA and PHA swelling response), and testosterone plasma levels in the field, or any effects of testosterone treatment on immunocompetence. Our results suggest a complex relationship between calling effort and immunity, mediated by CORT plasma levels.

Leptinergic Regulation of Vertebrate Communication Signals

Animal communication signals are regulated by multiple hormonal axes that ensure appropriate signal targeting, timing, and information content. The regulatory roles of steroid hormones and many peptide hormones are well understood and documented across a wide range of vertebrate taxa. Two recent studies have reported a novel function for leptin, a peptide hormone central to energy balance regulation: regulating communication signals of weakly electric fish and singing mice. With only limited evidence available at this time, a key question is just how widespread leptinergic regulation of communication signals is within and across taxa. A second important question is what features of communication signals are subject to leptinergic regulation. Here we consider the functional significance of leptinergic regulation of animal communication signals in the context of both direct and indirect signal metabolic costs. Direct costs arise from metabolic investment in signal production, while indirect costs arise from the predation and social conflict consequences of the signal's information content. We propose a preliminary conceptual framework for predicting which species will exhibit leptinergic regulation of their communication signals and which signal features leptin will regulate. This framework suggests a number of directly testable predictions within and across taxa. Accounting for additional factors such as life history and the potential co-regulation of communication signals by leptin and glucocorticoids will likely require modification or elaboration of this model.

Glucocorticoid-Mediated Changes in Male Green Treefrog Vocalizations Alter Attractiveness to Females

Adrenal glucocorticoids (GCs) are increasingly recognized as important modulators of male courtship signals, suggesting that circulating levels of these steroids can play a central role in sexual selection. However, few studies have examined whether GC-mediated effects on male sexual signals actually impact mate choice by females. Here, we examine how corticosterone (CORT)-mediated changes in the vocalizations of male green treefrogs, Dryophytes cinereus, influence attractiveness to females. In this species, agonistic acoustic signaling between rival males competing for mates increases circulating CORT levels in contest losers. Acute elevations in CORT, in turn, decrease the duration of male advertisement calls and increase the latency between successive calls, resulting in a net reduction in vocal effort (the amount of signaling per unit time) that occurs independently of changes in circulating androgens. Based on known preferences for acoustic features in D. cinereus, and other anuran species, the direction of CORT-mediated effects on temporal call characteristics is expected to compromise attractiveness to females, but whether they are of sufficient magnitude to impact female mate choice decisions is unclear. To examine whether CORT-mediated effects on male advertisement calls reduce attractiveness to females, we broadcast vocalizations in dual speaker playback experiments approximating the mean and 1 SD above and below the mean call duration and vocal effort values (the two primary vocal features impacted by elevated CORT) of males with low and high CORT levels. Results revealed strong preferences by females for the calls characteristic of males with low CORT in tests using the approximate mean and 1 SD above the mean call duration and vocal effort values, but females did not show a preference for calls of males with low CORT in trials using call values approximating 1 SD below the mean. Overall, females preferred males with signal traits predictive of low CORT, however this effect was nonlinear with attenuated preferences when signal alternatives differed only marginally indicating a possible thresholding effect. Specifically, females appeared to discriminate between males with low versus high CORT based primarily on differences in call rates associated with CORT-mediated changes in call duration and vocal effort. Our results highlight that changes in circulating CORT during male-male vocal interactions can decrease attractiveness to females, suggesting that circulating levels of CORT can play a critical role in both intra- and intersexual selection.

Male competition reverses female preference for male chemical cues

Females must choose among potential mates with different phenotypes in a variety of social contexts. Many male traits are inherent and unchanging, but others are labile to social context. Competition, for example, can cause physiological changes that reflect recent wins and losses that fluctuate throughout time. We may expect females to respond differently to males depending on the outcome of their most recent fight. In Bolitotherus cornutus (forked fungus beetles), males compete for access to females, but copulation requires female cooperation. In this study, we use behavioral trials to determine whether females use chemical cues to differentiate between males and whether the outcome of recent male competition alters female preference. We measured female association time with chemical cues of two size-matched males both before and after male-male competition. Females in our study preferred to associate with future losers before males interacted, but changed their preference for realized winners following male competitive interactions. Our study provides the first evidence of change in female preference based solely on the outcome of male-male competition.

Moderately elevated glucocorticoids increase mate choosiness but do not affect sexual proceptivity or preferences in female gray treefrogs

Glucocorticoids (GCs) are rarely studied in the context of female mate choice, despite the expression of receptors for these products in sexual, sensory and decision-making brain areas. Here we investigated the effects of GC concentrations on three aspects of female sexual behavior in breeding Cope's gray treefrogs (Hyla chrysoscelis): proceptivity-a measure of sexual motivation, intraspecific mate preferences, and mate choosiness. To our knowledge this is the first experimental study on the endocrine basis of mate choosiness. We predicted that mate choosiness-forfeiting an initial mate preference to pursue a suddenly more attractive mate-would be particularly impacted by elevated GCs with moderate GC levels associated with greater choosiness. We found support for this predicted inverted-U relationship. Females in the control group (no injection) showed no change in choosiness across timepoints. In contrast, females in the vehicle, Low (20 ng g^-1) and High (180 ng g^-1) corticosterone groups exhibited a nominal decline in choosiness after injection, suggesting that the experience of injection has little or perhaps slightly suppressive effects on female choosiness. Females in the moderate dose group (60 ng g^-1), however, exhibited a significant increase (>100%) in choosiness. Further, we found no effect of elevated GCs on sexual proceptivity or the species-typical preference for longer calls. These findings may reflect a buffering of primary sensory areas in the brain against elevated GCs. The recruitment of other cognitive processes during active decision-making, however, may facilitate GC modulation of mate choosiness, thereby promoting tactical plasticity at this critical life history juncture.

Excreted testosterone and male sexual proceptivity: A hormone validation and proof-of-concept experiment in túngara frogs

Conventional methods for sampling hormones often preclude strong inference experimental designs, including repeated measures of both hormones and behavior and balanced or simultaneous designs for hormone-behavior sampling. In amphibians there is an opportunity to non-invasively and repeatedly sample excreted steroids in the water. We examined testosterone (T) in túngara frogs (Physalaemus (=Engystomops) pustulosus) using minimally invasive water-borne methods. First, we validated procedures for the collection, extraction and measurement of T in adult males and females using pharmacological challenges coupled with estimates of parallelism and recovery determination. Next, we extended the timeline of sampling over 9 days in order to evaluate the kinetics of excretion (baseline phase, challenge phase, recovery phase), including the estimation of individual differences during baseline sampling. We also estimated concentrations of creatinine (Cr) in each water sample and evaluated whether correcting for this proxy of urine concentration significantly decreased error variance in T estimates. Lastly, we incorporated a standardized and repeated measures assay of male sexual proceptivity (phonotaxis) during the predicted peak T and recovery T timepoints. We found strong evidence supporting the utility of these methods for precise, biologically informative estimates of T in both sexes. Males had higher T than females and responded to pharmacological challenges by elevating T substantially within 48 h of challenge (hCG, GnRH). Males exhibited repeatability in baseline T and phonotaxis frequencies were positively associated with higher T. Adjusting T levels for the simultaneous measure of Cr significantly improved model fit, which in conjunction with marked variation in urine concentration, suggests that urine likely serves as the major source of excreted T. In summary, this proof-of-concept and methods study demonstrates the utility and accuracy of measuring water-borne T and behavior in amphibians.