Androgens modulate song effort and aggression in Neotropical singing mice

Differential expression of steroid-related genes across electrosensory brain regions in two sexually dimorphic species of electric knifefish

The production of communication signals can be modulated by hormones acting on the brain regions that regulate these signals. However, less is known about how signal perception is regulated by hormones. The electrocommunication signals of weakly electric fishes are sexually dimorphic, sensitive to hormones, and vary across species. The neural circuits that regulate the production and perception of these signals are also well-characterized, and electric fishes are thus an excellent model to examine the neuroendocrine regulation of sensorimotor mechanisms of communication. We investigated (1) whether steroid-related genes are expressed in sensory brain regions that process communication signals; and (2) whether this expression differs across sexes and species that have different patterns of sexual dimorphism in their signals. Apteronotus leptorhynchus and Apteronotus albifrons produce continuous electric organ discharges (EODs) that are used for communication. Two brain regions, the electrosensory lateral line lobe (ELL) and the dorsal torus semicircularis (TSd), process inputs from electroreceptors to allow fish to detect and discriminate electrocommunication signals. We used qPCR to quantify the expression of genes for two androgen receptors (ar1, ar2), two estrogen receptors (esr1, esr2b), and aromatase (cyp19a1b). Four out of five steroid-related genes were expressed in both sensory brain regions, and their expression often varied between sexes and species. These results suggest that expression of steroid-related genes in the brain may differentially influence how EOD signals are encoded across species and sexes, and that gonadal steroids may coordinately regulate central circuits that control both the production and perception of EODs.

Anatomy and mechanisms of vocal production in harvest mice

Characterizing mechanisms of vocal production provides important insight into the ecology of acoustic divergence. In this study, we characterized production mechanisms of two types of vocalizations emitted by western harvest mice (Reithrodontomys megalotis), a species uniquely positioned to inform trait evolution because it is a sister taxon to peromyscines (Peromyscus and Onychomys spp.), which use vocal fold vibrations to produce long-distance calls, but more ecologically and acoustically similar to baiomyines (Baiomys and Scotinomys spp.), which employ a whistle mechanism. We found that long-distance calls (∼10 kHz) were produced by airflow-induced vocal fold vibrations, whereas high-frequency quavers used in close-distance social interactions (∼80 kHz) were generated by a whistle mechanism. Both production mechanisms were facilitated by a characteristic laryngeal morphology. Our findings indicate that the use of vocal fold vibrations for long-distance communication is widespread in reithrodontomyines (Onychomys, Peromyscus, Reithrodontomys spp.) despite overlap in frequency content that characterizes baiomyine whistled vocalizations. The results illustrate how different production mechanisms shape acoustic variation in rodents and contribute to ecologically relevant communication distances.

Comparative analysis of gonadal hormone receptor expression in the postnatal house mouse, meadow vole, and prairie vole brain

The socially monogamous prairie vole (Microtus ochrogaster) and promiscuous meadow vole (Microtus pennsylvanicus) are closely related, but only prairie voles display long-lasting pair bonds, biparental care, and selective aggression towards unfamiliar individuals after pair bonding. These social behaviors in mammals are largely mediated by steroid hormone signaling in the social behavior network (SBN) of the brain. Hormone receptors are reproducible markers of sex differences that can provide more information than anatomy alone and can even be at odds with anatomical dimorphisms. We reasoned that behaviors associated with social monogamy in prairie voles may emerge in part from unique expression patterns of steroid hormone receptors in this species, and that these expression patterns would be more similar across males and females in prairie than in meadow voles or the laboratory mouse. To obtain insight into steroid hormone signaling in the developing prairie vole brain, we assessed expression of estrogen receptor alpha (Esr1), estrogen receptor beta (Esr2), and androgen receptor (Ar) within the SBN, using in situ hybridization at postnatal day 14 in mice, meadow, and prairie voles. We found species-specific patterns of hormone receptor expression in the hippocampus and ventromedial hypothalamus, as well as species differences in the sex bias of these markers in the principal nucleus of the bed nucleus of the stria terminalis. These findings suggest the observed differences in gonadal hormone receptor expression may underlie species differences in the display of social behaviors.

Females counter-sing, but response to male song differs by sex in Alston's singing mouse

Vocal display behaviours are common throughout the animal kingdom, play important roles in both courtship and aggression, and are frequent subjects of behavioural research. Although females of many species vocalize, an overwhelming fraction of behavioural research has focused on male display. We investigated vocal display behaviours in female singing mice (Scotinomys teguina), small muroid rodents in which both sexes produce songs consisting of trills of rapid, downward frequency sweeps. Previous research established that male singing mice increase song production and engage in precisely timed counter-singing behaviour in response to playback of conspecific male song. We tested whether female singing mice also increased their rate of singing in response to playback of male song, whether they counter-sing, and whether there are sexual dimorphisms in song effort. Our results demonstrate that much like males, female singing mice increase their song effort and counter-sing in response to playback of male song; however, females sing fewer and shorter songs compared to males. This study further informs the understanding of female vocal behaviour and establishes the singing mouse as a valuable model for investigating female vocal display.

De novo assembly and annotation of the singing mouse genome

BACKGROUND

Developing genomic resources for a diverse range of species is an important step towards understanding the mechanisms underlying complex traits. Specifically, organisms that exhibit unique and accessible phenotypes-of-interest allow researchers to address questions that may be ill-suited to traditional model organisms. We sequenced the genome and transcriptome of Alston's singing mouse (Scotinomys teguina), an emerging model for social cognition and vocal communication. In addition to producing advertisement songs used for mate attraction and male-male competition, these rodents are diurnal, live at high-altitudes, and are obligate insectivores, providing opportunities to explore diverse physiological, ecological, and evolutionary questions.

RESULTS

Using PromethION, Illumina, and PacBio sequencing, we produced an annotated genome and transcriptome, which were validated using gene expression and functional enrichment analyses. To assess the usefulness of our assemblies, we performed single nuclei sequencing on cells of the orofacial motor cortex, a brain region implicated in song coordination, identifying 12 cell types.

CONCLUSIONS

These resources will provide the opportunity to identify the molecular basis of complex traits in singing mice as well as to contribute data that can be used for large-scale comparative analyses.

Breaking Through the Bottleneck: Krogh's Principle in Behavioral Neuroendocrinology and the Potential of Gene Editing

In 1929, August Krogh wrote that for every question in biology, there is a species or collection of species in which pursuing such questions is the most appropriate for achieving the deepest insights. Referred to as "Krogh's Principle," these words are a guiding force for many biologists. In practice, Krogh's principle might guide a biologist interested in studying bi-parental care to choose not to use lab mice, in which the female does most of the parenting, but instead study species in which bi-parental care is present and clearly observable, such as in certain poison dart frogs. This approach to pursuing biological questions has been fruitful, with more in-depth insights achievable with new technologies. However, up until recently, an important limitation of Krogh's principle for biologists interested in the functions of certain genes, was certain techniques were only available for a few traditional model organisms such as lab mice, fruit flies (Drosophila melanogaster), zebrafish (Danio rerio) and C. elegans (Caenorhabditis elegans), in which testing the functions of molecular systems on biological processes can be achieved using genetic knockout (KO) and transgenic technology. These methods are typically more precise than other approaches (e.g., pharmacology) commonly used in nontraditional model organisms to address similar questions. Therefore, some of the most in-depth insights into our understanding of the molecular control of these mechanisms have come from a small number of genetically tractable species. Recent advances in gene editing technology such as CRISPR (Clustered Regularly Interspersed Short Palindromic Repeats)/Cas9 gene editing as a laboratory tool has changed the insights achievable for biologists applying Krogh's principle. In this review, we will provide a brief summary on how some researchers of nontraditional model organisms have been able to achieve different levels of experimental precision with limited genetic tractability in their non-traditional model organism in the field of behavioral neuroendocrinology, a field in which understanding tissue and brain-region specific actions of molecules of interest has been a major goal. Then, we will highlight the exciting potential of Krogh's principle using discoveries made in a popular model species of social behavior, the African cichlid fish Astatotilapia burtoni. Specifically, we will focus on insights gained from studies of the control of social status by sex steroid hormones (androgens and estrogens) in A. burtoni that originated during field observations during the 1970s, and have recently culminated in novel insights from CRISPR/Cas9 gene editing in laboratory studies. Our review highlighting discoveries in A. burtoni may function as a roadmap for others using Krogh's principle aiming to incorporate gene editing into their research program. Gene editing is thus a powerful complimentary laboratory tool researchers can use to yield novel insights into understanding the molecular mechanisms of physiology and behavior in non-traditional model organisms.

Neuroendocrine mechanisms contributing to the coevolution of sociality and communication

Communication is inherently social, so signaling systems should evolve with social systems. The 'social complexity hypothesis' posits that social complexity necessitates communicative complexity and is generally supported in vocalizing mammals. This hypothesis, however, has seldom been tested outside the acoustic modality, and comparisons across studies are confounded by varying definitions of complexity. Moreover, proximate mechanisms underlying coevolution of sociality and communication remain largely unexamined. In this review, we argue that to uncover how sociality and communication coevolve, we need to examine variation in the neuroendocrine mechanisms that coregulate social behavior and signal production and perception. Specifically, we focus on steroid hormones, monoamines, and nonapeptides, which modulate both social behavior and sensorimotor circuits and are likely targets of selection during social evolution. Lastly, we highlight weakly electric fishes as an ideal system in which to comparatively address the proximate mechanisms underlying relationships between social and signal diversity in a novel modality.

Genetic dissection of steroid-hormone modulated social behavior: Novel paralogous genes are a boon for discovery

Research across species has led to important discoveries on the functions of steroid hormones in the regulation of behavior. However, like in many fields, advancements in transgenic and mutagenic technology allowed mice to become the premier genetic model for conducting many experiments to understand how steroids control social behavior. Since there has been a general lack of parallel methodological developments in other species, many of the findings cannot be generalized. This is especially the case for teleost fish, in which a whole-genome duplication produced novel paralogs for key steroid hormone signaling genes. In this review, we summarize technical advancements over the history of the field of neuroendocrinology that have led to important insights in our understanding of the control of social behavior by steroids. We demonstrate that early mouse genetic models to understand these mechanisms suffered from several issues that were remedied by more precise transgenic technological advancements. We then highlight the importance of CRISPR/Cas9 gene editing tools that will in time bridge the gap between mice and non-traditional model species for understanding principles of steroid hormone action in the modulation of social behavior. We specifically highlight the role of teleost fish in bridging this gap because they are 1) highly genetically tractable and 2) provide a novel advantage in achieving precise genetic control. The field of neuroendocrinology is entering a new "gene editing revolution" that will lead to novel discoveries about the roles of steroid hormones in the regulation and evolutionary trajectories of social behavior.

Mapping the vocal circuitry of Alston's singing mouse with pseudorabies virus

Vocalizations are often elaborate, rhythmically structured behaviors. Vocal motor patterns require close coordination of neural circuits governing the muscles of the larynx, jaw, and respiratory system. In the elaborate vocalization of Alston's singing mouse (Scotinomys teguina) each note of its rapid, frequency-modulated trill is accompanied by equally rapid modulation of breath and gape. To elucidate the neural circuitry underlying this behavior, we introduced the polysynaptic retrograde neuronal tracer pseudorabies virus (PRV) into the cricothyroid and digastricus muscles, which control frequency modulation and jaw opening, respectively. Each virus singly labels ipsilateral motoneurons (nucleus ambiguus for cricothyroid, and motor trigeminal nucleus for digastricus). We find that the two isogenic viruses heavily and bilaterally colabel neurons in the gigantocellular reticular formation, a putative central pattern generator. The viruses also show strong colabeling in compartments of the midbrain including the ventrolateral periaqueductal gray and the parabrachial nucleus, two structures strongly implicated in vocalizations. In the forebrain, regions important to social cognition and energy balance both exhibit extensive colabeling. This includes the paraventricular and arcuate nuclei of the hypothalamus, the lateral hypothalamus, preoptic area, extended amygdala, central amygdala, and the bed nucleus of the stria terminalis. Finally, we find doubly labeled neurons in M1 motor cortex previously described as laryngeal, as well as in the prelimbic cortex, which indicate these cortical regions play a role in vocal production. The progress of both viruses is broadly consistent with vertebrate-general patterns of vocal circuitry, as well as with circuit models derived from primate literature.

Testosterone pulses paired with a location induce a place preference to the nest of a monogamous mouse under field conditions

Changing social environments such as the birth of young or aggressive encounters present a need to adjust behavior. Previous research examined how long-term changes in steroid hormones mediate these adjustments. We tested the novel concept that the rewarding effects of transient testosterone pulses (T-pulses) in males after social encounters alters their spatial distribution on a territory. In free-living monogamous California mice (Peromyscus californicus), males administered three T-injections at the nest spent more time at the nest than males treated with placebo injections. This mimics T-induced place preferences in the laboratory. Female mates of T-treated males spent less time at the nest but the pair produced more vocalizations and call types than controls. Traditionally, transient T-changes were thought to have transient behavioral effects. Our work demonstrates that in the wild, when T-pulses occur in a salient context such as a territory, the behavioral effects last days after T-levels return to baseline.

Behavioural and physiological plasticity in social hierarchies

Individuals occupying dominant and subordinate positions in social hierarchies exhibit divergent behaviours, physiology and neural functioning. Dominant animals express higher levels of dominance behaviours such as aggression, territorial defence and mate-guarding. Dominants also signal their status via auditory, visual or chemical cues. Moreover, dominant animals typically increase reproductive behaviours and show enhanced spatial and social cognition as well as elevated arousal. These biobehavioural changes increase energetic demands that are met via shifting both energy intake and metabolism and are supported by coordinated changes in physiological systems including the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes as well as altered gene expression and sensitivity of neural circuits that regulate these behaviours. Conversely, subordinate animals inhibit dominance and often reproductive behaviours and exhibit physiological changes adapted to socially stressful contexts. Phenotypic changes in both dominant and subordinate individuals may be beneficial in the short-term but lead to long-term challenges to health. Further, rapid changes in social ranks occur as dominant animals socially ascend or descend and are associated with dynamic modulations in the brain and periphery. In this paper, we provide a broad overview of how behavioural and phenotypic changes associated with social dominance and subordination are expressed in neural and physiological plasticity. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Estrogen signaling modulates behavioral selection toward pups and amygdalohippocampal area in the rhomboid nucleus of the bed nucleus of the stria terminalis circuit

Gonadal steroid hormone influences behavioral choice of adult animals toward pups, parental or aggressive. We previously reported that long-term administration of 17β-estradiol (E2) to male mice during sexual maturation induces aggressive behavior toward conspecific pups, which is called "infanticide," and significantly enhanced excitatory synaptic transmission in the rhomboid nucleus of bed nucleus of the stria terminalis (BSTrh), which is an important brain region for infanticide. However, it is unclear how estrogen receptor-dependent signaling after sexual maturity regulates neural circuits including the BSTrh. Here we revealed that E2 administration to gonadectomized mice in adulthood elicited infanticidal behavior and enhanced excitatory synaptic transmission in the BSTrh by increasing the probability of glutamate release from the presynaptic terminalis. Next, we performed whole-brain mapping of E2-sensitive brain regions projecting to the BSTrh and found that amygdalohippocampal area (AHi) neurons that project to the BSTrh densely express estrogen receptor 1 (Esr1). Moreover, E2 treatment enhanced synaptic connectivity in the AHi-BSTrh pathway. Together, these results suggest that reinforcement of excitatory inputs from AHi neurons into the BSTrh by estrogen receptor-dependent signaling may contribute to the expression of infanticide.

The influence of social context on pinyon mouse (Peromyscus truei) vocalizations

Mice in the diverse genus Peromyscus are emerging as important models in the study of acoustic communication. However, reports on vocal repertoires exist for only 8 of the 56 currently recognized species. Descriptions of acoustic content and context are necessary to facilitate comparative studies. In this study, we present the first recordings of wild-captured pinyon mice (Peromyscus truei) in the laboratory in different social contexts. Similar to other Peromyscus species, pinyon mice produced four general types of vocalizations, including sustained vocalizations (SVs), barks, simple sweeps, and complex sweeps. SVs were produced primarily by females in social isolation, highlighting the potential significance of SVs in female advertisement behavior. While agonistic barks were rare, simple and complex sweeps were reliably produced in response to olfactory cues from same- and opposite-sex mice, and in paired contexts. Opposite-sex pairs produced significantly more complex sweeps than same-sex pairs, consistent with the well-supported function of sweeps in coordinating close-distance interactions. In addition, females produced sweeps with lower peak frequencies following separation from their social partner, indicative of a sex-specific mechanism to facilitate advertisement to potential mates and competitors. Together, our findings highlight the importance of social context in rodent vocal production, the significance of female vocal behavior, and the continued need to sample understudied species to better understand similarities and differences in rodent communication systems.

Non-individualistic ultrasonic and audible isolation calls throughout ontogeny in a rodent, Eolagurus luteus

Acoustic individuality is present in diverse taxa of mammals and birds, becoming especially prominent in those age groups for which discriminating conspecifics by voice is critically important. This study compares, for the first time, the ontogenetic changes of acoustic individuality of ultrasonic and audible calls (USVs and AUDs) across 12 age-classes (from neonates to adults) in captive yellow steppe lemmings Eolagurus luteus. We found that, in this rodent species, the isolation-induced USVs and AUDs are not individually distinct at any age. We discuss that this result is unusual, because discriminating individuals by individualistic vocal traits may be important for such a social species as yellow steppe lemming. We also discuss the potential role of acoustic individuality in studies including rodent models.

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.

Conservation and dimorphism in androgen receptor distribution in Alston's singing mouse (Scotinomys teguina)

Because of their roles in courtship and intrasexual competition, sexual displays are often sexually dimorphic, but we know little about the mechanisms that produce such dimorphism. Among mammals, one example is the vocalization of Alston's singing mouse (Scotinomys teguina), which consists of a series of rapidly repeated, frequency-modulated notes. The rate and duration of songs is sexually dimorphic and androgen responsive. To understand the neuronal mechanisms underlying this sexual dimorphism, we map the sites of androgen sensitivity throughout the brain, focusing analysis along a pathway that spans from limbic structures to vocal motor regions. We find widespread expression of AR immunoreactivity (AR-ir) throughout limbic structures important for social behavior and vocalization, including the lateral septum, extended amygdala, preoptic area and hypothalamus. We also find extensive AR staining along previously documented vocal motor pathways, including the periaqueductal gray, parabrachial nucleus, and nucleus ambiguus, the last of which innervates intrinsic laryngeal muscles. Lastly, AR-ir is also evident in sensory areas such as the medial geniculate, inferior, and superior colliculi. A quantitative analysis revealed that males exhibited more AR-ir than females, a pattern that was most pronounced in the hypothalamus. Despite the elaboration of vocalization in singing mice, comparison with prior literature suggests that the broad pattern of AR-ir may be conserved across a wide range of rodents. Together these data identify brain nuclei well positioned to shape the sexually dimorphic vocalization of S. teguina and suggest that such androgen modulation of vocalization is evolutionary conserved among rodents.

Androgen Receptor Modulates Multimodal Displays in the Bornean Rock Frog (Staurois parvus)

Multimodal communication is common in the animal kingdom. It occurs when animals display by stimulating two or more receiver sensory systems, and often arises when selection favors multiple ways to send messages to conspecifics. Mechanisms of multimodal display behavior are poorly understood, particularly with respect to how animals coordinate the production of different signals. One important question is whether all components in a multimodal display share an underlying physiological basis, or whether different components are regulated independently. We investigated the influence of androgen receptors (ARs) on the production of both visual and vocal signal components in the multimodal display repertoire of the Bornean rock frog (Staurois parvus). To assess the role of AR in signal production, we treated reproductively active adult males with the antiandrogen flutamide (FLUT) and measured the performance of each component signal in the multimodal display. Our results show that blocking AR inhibited the production of multiple visual signals, including a conspicuous visual signal known as the "foot flag," which is produced by rotating the hind limb above the body. However, FLUT treatment caused no measurable change in vocal signaling behavior, or in the frequency or fine temporal properties of males' calls. Our study, therefore, suggests that activation of AR is not a physiological prerequisite to the coordination of multiple signals, in that it either does not regulate all signaling behaviors in a male's display repertoire or it does so only in a context-dependent manner.

Seasonal regulation of behaviour: what role do hormone receptors play?

Many animals differentially express behaviours across the annual cycle as life stages are coordinated with seasonal environmental conditions. Understanding of the mechanistic basis of such seasonal changes in behaviour has traditionally focused on the role of changes in circulating hormone levels. However, it is increasingly apparent that other endocrine regulation mechanisms such as changes in local hormone synthesis and receptor abundance also play a role. Here I review what is known about seasonal changes in steroid hormone receptor abundance in relation to seasonal behaviour in vertebrates. I find that there is widespread, though not ubiquitous, seasonal variation in the expression of steroid hormone receptors in the brain, with such variation being best documented in association with courtship, mating and aggression. The most common pattern of seasonal variation is for there to be upregulation of sex steroid receptors with the expression of courtship and mating behaviours, when circulating hormone levels are also high. Less well-documented are cases in which seasonal increases in receptor expression could compensate for low circulating hormone levels or seasonal downregulation that could serve a protective function. I conclude by identifying important directions for future research.

Pygmy mouse songs reveal anatomical innovations underlying acoustic signal elaboration in rodents

Elaborate animal communication displays are often accompanied by morphological and physiological innovations. In rodents, acoustic signals used in reproductive contexts are produced by two distinct mechanisms, but the underlying anatomy that facilitates such divergence is poorly understood. 'Audible' vocalizations with spectral properties between 500 Hz and 16 kHz are thought to be produced by flow-induced vocal fold vibrations, whereas 'ultrasonic' vocalizations with fundamental frequencies above 19 kHz are produced by an aerodynamic whistle mechanism. Baiomyine mice (genus Baiomys and Scotinomys) produce complex frequency-modulated songs that span these traditional distinctions and represent important models to understand the evolution of signal elaboration. We combined acoustic analyses of spontaneously vocalizing northern pygmy mice (Baiomystaylori) in air and light gas atmosphere with morphometric analyses of their vocal apparatus to infer the mechanism of vocal production. Increased fundamental frequencies in heliox indicated that pygmy mouse songs are produced by an aerodynamic whistle mechanism supported by the presence of a ventral pouch and alar cartilage. Comparative analyses of the larynx and ventral pouch size among four additional ultrasonic whistle-producing rodents indicated that the unusually low 'ultrasonic' frequencies (relative to body size) of pygmy mice songs are associated with an enlarged ventral pouch. Additionally, mice produced shorter syllables while maintaining intersyllable interval duration, thereby increasing syllable repetition rates. We conclude that while laryngeal anatomy sets the foundation for vocal frequency range, variation and adjustment of central vocal motor control programs fine tunes spectral and temporal characters to promote acoustic diversity within and between species.

Songs versus colours versus horns: what explains the diversity of sexually selected traits?

Papers on sexual selection often highlight the incredible diversity of sexually selected traits across animals. Yet, few studies have tried to explain why this diversity evolved. Animals use many different types of traits to attract mates and outcompete rivals, including colours, songs, and horns, but it remains unclear why, for example, some taxa have songs, others have colours, and others horns. Here, we first conduct a systematic survey of the basic diversity and distribution of different types of sexually selected signals and weapons across the animal Tree of Life. Based on this survey, we describe seven major patterns in trait diversity and distributions. We then discuss 10 unanswered questions raised by these patterns, and how they might be addressed. One major pattern is that most types of sexually selected signals and weapons are apparently absent from most animal phyla (88%), in contrast to the conventional wisdom that a diversity of sexually selected traits is present across animals. Furthermore, most trait diversity is clustered in Arthropoda and Chordata, but only within certain clades. Within these clades, many different types of traits have evolved, and many types appear to have evolved repeatedly. By contrast, other major arthropod and chordate clades appear to lack all or most trait types, and similar patterns are repeated at smaller phylogenetic scales (e.g. within insects). Although most research on sexual selection focuses on female choice, we find similar numbers of traits (among sampled species) are involved in male contests (44%) and female choice (55%). Overall, these patterns are largely unexplained and unexplored, as are many other fundamental questions about the evolution of these traits. We suggest that understanding the diversity of sexually selected traits may require a shift towards macroevolutionary studies at relatively deep timescales (e.g. tens to hundreds of millions of years ago).

An insular view of the social decision-making network

Social animals must detect, evaluate and respond to the emotional states of other individuals in their group. A constellation of gestures, vocalizations, and chemosignals enable animals to convey affect and arousal to others in nuanced, multisensory ways. Observers integrate social information with environmental and internal factors to select behavioral responses to others via a process call social decision-making. The Social Decision Making Network (SDMN) is a system of brain structures and neurochemicals that are conserved across species (mammals, reptiles, amphibians, birds) that are the proximal mediators of most social behaviors. However, how sensory information reaches the SDMN to shape behavioral responses during a social encounter is not well known. Here we review the empirical data that demonstrate the necessity of sensory systems in detecting social stimuli, as well as the anatomical connectivity of sensory systems with each node of the SDMN. We conclude that the insular cortex is positioned to link integrated social sensory cues to this network to produce flexible and appropriate behavioral responses to socioemotional cues.

Social context mediates testosterone's effect on snort acoustics in male hyrax songs

Testosterone affects physical and motivational states, both of which may strongly influence vocalization structure and acoustics. The loud complex calls (i.e., songs) of male rock hyraxes (Procavia capensis) are used as honest signals for advertising physical and social states. The snort, a low frequency, noisy element of the song, encodes information on the singer's age and social rank via harshness, as measured by jitter (i.e., acoustic frequency stability) and duration; suggesting that the snort concomitantly advertises both vocal stability and aggression. Our past findings revealed that testosterone levels are related to both vocal elements and social status of male hyraxes, suggesting that hormonal mechanisms mediate the motivation for aggressive and courtship behaviors. Here we examined whether long-term androgen levels are related to snort acoustics and song structure by comparing levels of testosterone in hair with acoustic and structural parameters. We found that songs performed by individuals with higher testosterone levels include more singing bouts and longer, smoother snorts, but only in those songs induced by external triggers. It is possible that hyraxes with higher levels of testosterone possess the ability to perform higher-quality singing, but only invest in situations of high social arousal and potential benefit. Surprisingly, in spontaneous songs, hyraxes with high testosterone were found to snort more harshly than low-testosterone males. The context dependent effects of high testosterone on snort acoustics suggest that the aggressive emotional arousal associated with testosterone is naturally reflected in the jittery hyrax snort, but that it can be masked by high-quality performance.

Characterization of vocalizations emitted in isolation by California mouse (Peromyscus californicus) pups throughout the postnatal period

Rodent species, such as monogamous and biparental California mice, produce vocalizations as a means of communication. A temporal examination of vocalizations produced by California mice pups in isolation was performed. Pup recordings were performed for 3 min at ∼10.00 and 14.00 hrs on early postnatal days (PND) 2-4, 7, 21, and 28. Once initial recordings were finished, pups were returned to the home cage with parents and any siblings for 5 minutes to determine if active biparental responses resulted in an enhanced vocalization response when pups were isolated and retested. We also sought to determine whether potential reduction in vocalizations by older pups might be due to procedure-habituation procedure associated with less anxiety and/or possibly decreased need for parental care. Vocalizations were measured in weanling (30 days of age) "naïve" pups not previously isolated. Results show older pups took significantly longer to vocalize, indicated by increased latency before producing their initial syllable compared to earlier ages. With increasing age, pups demonstrated decreased syllable duration, reduced number and duration of phrases, and decreased number of syllables per phrase. No differences in pup vocalizations were observed before and after being placed back with parents, suggestive biparental potentiation may not exist in California mice pups. Comparison of the naïve to habituated weanling pups indicated the former group had more total calls but no other differences in vocalization parameters were detected between these 2 groups. Collectively, the findings suggest that as California mice pups mature and approach weaning they generally vocalize less in isolation. (PsycINFO Database Record

Role of the androgen receptor in the central nervous system

The involvement of gonadal androgens in functions of the central nervous system was suggested for the first time about half a century ago. Since then, the number of functions attributed to androgens has steadily increased, ranging from regulation of the hypothalamic-pituitary-gonadal axis and reproductive behaviors to modulation of cognition, anxiety and other non-reproductive functions. This review focuses on the implication of the neural androgen receptor in these androgen-sensitive functions and behaviors.

Trade-offs in the production of animal vocal sequences: insights from the structure of wild chimpanzee pant hoots

Background

Vocal sequences - utterances consisting of calls produced in close succession - are common phenomena in animal communication. While many studies have explored the adaptive benefits of producing such sequences, very little is known about how the costs and constraints involved in their production affect their form. Here, we investigated this issue in the chimpanzee (Pan troglodytes schweinfurthii) pant hoot, a long and structurally complex vocal sequence comprising four acoustically distinct phases – introduction, build-up, climax and let-down.

Results

We found that in each of these phases, and for the sequence as a whole, there was a negative relationship between the number of calls produced and their average duration. There was also a negative relationship between the total duration of some adjacent phases. Significant relationships between the fundamental frequency of calls and their number or duration were found for some phases of the sequence, but the direction of these relationships differed between particular phases.

Conclusions

These results indicate that there are trade-offs in terms of signal duration at two levels in pant-hoot production: between call number and duration, and between the relative durations of successive phases. These trade-offs are likely to reflect biomechanical constraints on vocal sequence production. Phase-specific trade-offs also appear to occur between fundamental frequency and call number or duration, potentially reflecting that different phases of the sequence are associated with distinct types of information, linked in different ways to call pitch. Overall, this study highlights the important role of costs and constraints in shaping the temporal and acoustic structure of animal vocal sequences.

Electronic supplementary material

The online version of this article (10.1186/s12983-017-0235-8) contains supplementary material, which is available to authorized users.

Testosterone and reproductive effort in male primates

Considerable evidence suggests that the steroid hormone testosterone mediates major life-history trade-offs in vertebrates, promoting mating effort at the expense of parenting effort or survival. Observations from a range of wild primates support the "Challenge Hypothesis," which posits that variation in male testosterone is more closely associated with aggressive mating competition than with reproductive physiology. In both seasonally and non-seasonally breeding species, males increase testosterone production primarily when competing for fecund females. In species where males compete to maintain long-term access to females, testosterone increases when males are threatened with losing access to females, rather than during mating periods. And when male status is linked to mating success, and dependent on aggression, high-ranking males normally maintain higher testosterone levels than subordinates, particularly when dominance hierarchies are unstable. Trade-offs between parenting effort and mating effort appear to be weak in most primates, because direct investment in the form of infant transport and provisioning is rare. Instead, infant protection is the primary form of paternal investment in the order. Testosterone does not inhibit this form of investment, which relies on male aggression. Testosterone has a wide range of effects in primates that plausibly function to support male competitive behavior. These include psychological effects related to dominance striving, analgesic effects, and effects on the development and maintenance of the armaments and adornments that males employ in mating competition.

Effects of quinestrol on the vocal behavior of mice during courtship interactions

Vocalizations are a crucial part of courtship and mating in a wide variety of species. Mating behavior, including courtship calls, is modulated by sex steroid hormones. Male mice produce courtship ultrasonic vocalizations to attract females during heterosexual encounters. However, rare is the knowledge on whether vocal behavior of mice changes under sterilant treatment which will affect gonadal hormone levels. In the present study, we treat male mice with quinestrol, which interferes with the release of the gonadotropin-releasing hormone (GnRH) and has a significant anti-fertility effect in rodents. We compared the differences in the syllable structures (including peak intensity, peak frequency, duration, and bandwidth), total number of calls, and harmonic syllable proportions between quinestrol treated and control male mice. Male mice treated with quinestrol produced more courtship calls and more harmonic syllables than control mice, whereas the parameters of call syllables showed no significant change between the two groups. The results indicate that normal male vocal behavior during sexual interactions could be retained or even reinforced after quinestrol treatment. In addition, female mice approached male mice treated with quinestrol more than control mice, suggesting that the treated male mice were more attractive to the female mice than the controls. Thus, competitive reproductive interference is enhanced. Further, findings provided behavior mechanism in vocal context of the fertility control in mice.

Sequential information in a great ape utterance

Birdsong is a prime example of acoustically sophisticated vocal behaviour, but its complexity has evolved mainly through sexual selection to attract mates and repel sexual rivals. In contrast, non-human primate calls often mediate complex social interactions, but are generally regarded as acoustically simple. Here, we examine arguably the most complex call in great ape vocal communication, the chimpanzee (Pan troglodytes schweinfurthii) 'pant hoot'. This signal consists of four acoustically distinct phases: introduction, build-up, climax and let-down. We applied state-of-the-art Support Vector Machines (SVM) methodology to pant hoots produced by wild male chimpanzees of Budongo Forest, Uganda. We found that caller identity was apparent in all four phases, but most strongly in the low-amplitude introduction and high-amplitude climax phases. Age was mainly correlated with the low-amplitude introduction and build-up phases, dominance rank (i.e. social status) with the high-amplitude climax phase, and context (reflecting activity of the caller) with the low-amplitude let-down phase. We conclude that the complex acoustic structure of chimpanzee pant hoots is linked to a range of socially relevant information in the different phases of the call, reflecting the complex nature of chimpanzee social lives.

Hormones and the Evolution of Complex Traits: Insights from Artificial Selection on Behavior

Although behavior may often be a fairly direct target of natural or sexual selection, it cannot evolve without changes in subordinate traits that cause or permit its expression. In principle, changes in endocrine function could be a common mechanism underlying behavioral evolution because they are well positioned to mediate integrated responses to behavioral selection. More specifically, hormones can influence both motivational (e.g., brain) and performance (e.g., muscles) components of behavior simultaneously and in a coordinated fashion. If the endocrine system is often "used" as a general mechanism to effect responses to selection, then correlated responses in other aspects of behavior, life history, and organismal performance (e.g., locomotor abilities) should commonly occur because any cell with appropriate receptors could be affected. Ways in which behavior coadapts with other aspects of the phenotype can be studied directly through artificial selection and experimental evolution. Several studies have targeted rodent behavior for selective breeding and reported changes in other aspects of behavior, life history, and lower-level effectors of these organismal traits, including endocrine function. One example involves selection for high levels of voluntary wheel running, one aspect of physical activity, in four replicate High Runner (HR) lines of mice. Circulating levels of several hormones (including insulin, testosterone, thyroxine, triiodothyronine) have been characterized, three of which-corticosterone, leptin, and adiponectin-differ between HR and control lines, depending on sex, age, and generation. Potential changes in circulating levels of other behaviorally and metabolically relevant hormones, as well as in other components of the endocrine system (e.g., receptors), have yet to be examined. Overall, results to date identify promising avenues for further studies on the endocrine basis of activity levels.

The relationship between testosterone and long-distance calling in wild male chimpanzees

Long-distance calling is a common behaviour in animals that has various important social functions. At a physiological level, calling is often mediated by gonadal hormones such as testosterone (T), particularly when its function is linked to intra-sexual competition for mates or territory. T also plays an important role in the development of vocal characteristics associated with dominance in humans. However, the few available studies of T and vocal behaviour in non-human primates suggest that in primates T has less influence on call production than in other animals. We tested this hypothesis by studying the relationship between T concentrations and pant hooting in wild male chimpanzees (Pan troglodytes schweinfurthii) of the Kanyawara community in the Kibale National Park, Uganda. We found three kinds of correlation. Hourly T averages were positively associated with hourly rates of pant-hooting. Monthly T levels were likewise correlated with monthly rates of pant hooting after controlling for other influences such as fission-fusion rates. Finally, males with high T levels had higher peak frequency at the start of the call climax. These results suggest that T affects the production of pant-hoots in chimpanzees. This implies that the pant-hoot call plays a role in male-male competition. We propose that even in cognitively sophisticated species, endocrine mechanisms can contribute to regulating vocal production.

Beyond aggression: Androgen-receptor blockade modulates social interaction in wild meerkats

In male vertebrates, androgens are inextricably linked to reproduction, social dominance, and aggression, often at the cost of paternal investment or prosociality. Testosterone is invoked to explain rank-related reproductive differences, but its role within a status class, particularly among subordinates, is underappreciated. Recent evidence, especially for monogamous and cooperatively breeding species, suggests broader androgenic mediation of adult social interaction. We explored the actions of androgens in subordinate, male members of a cooperatively breeding species, the meerkat (Suricata suricatta). Although male meerkats show no rank-related testosterone differences, subordinate helpers rarely reproduce. We blocked androgen receptors, in the field, by treating subordinate males with the antiandrogen, flutamide. We monitored androgen concentrations (via baseline serum and time-sequential fecal sampling) and recorded behavior within their groups (via focal observation). Relative to controls, flutamide-treated animals initiated less and received more high-intensity aggression (biting, threatening, feeding competition), engaged in more prosocial behavior (social sniffing, grooming, huddling), and less frequently initiated play or assumed a 'dominant' role during play, revealing significant androgenic effects across a broad range of social behavior. By contrast, guarding or vigilance and measures of olfactory and vocal communication in subordinate males appeared unaffected by flutamide treatment. Thus, androgens in male meerkat helpers are aligned with the traditional trade-off between promoting reproductive and aggressive behavior at a cost to affiliation. Our findings, based on rare endocrine manipulation in wild mammals, show a more pervasive role for androgens in adult social behavior than is often recognized, with possible relevance for understanding tradeoffs in cooperative systems.

Vocalizations convey sex, seasonal phenotype, and aggression in a seasonal mammal

Seasonal variation in social behavior is often accompanied by seasonal variation in communication. In mammals, how seasonal environmental cues influence aggressive vocalizations remains underexplored. Photoperiod is the primary cue coordinating seasonal responses in most temperate zone animals, including Siberian hamsters (Phodopus sungorus), a species that undergoes reproductive inhibition and increased aggression in winter. During same-sex aggressive encounters, hamsters emit both broadband calls (BBCs) and ultrasonic vocalizations (USVs) that indicate aggression and the vocalizer's sex, respectively; however, it is not known whether these rodents adjust specific elements of their vocal repertoire to reflect their photoperiod-induced seasonal phenotypes. To address this, we recorded vocalizations emitted during dyadic interactions between male or female pairs of hamsters housed in long or short photoperiods and measured serum testosterone levels. USV emission rate remained stable across photoperiods, but proportional use of USV subtypes varied in novel ways: 'jump' USVs were sensitive to seasonal phenotype, but not the vocalizer's sex, whereas 'plain' USVs were sensitive only to the sex of the vocalizer. BBC emission rate varied with seasonal phenotype; short-day non-reproductive hamsters produced more BBCs and demonstrated increased aggression compared with reproductive hamsters. Testosterone, however, was not related to vocalization rates. Collectively, these findings demonstrate that changes in the vocal repertoire of Siberian hamsters reflect sex, aggression, and seasonal phenotype, suggesting that both BBCs and USVs are important signals used during same-sex social encounters.

Male fidelity expressed through rapid testosterone suppression of ultrasonic vocalizations to novel females in the monogamous California mouse

The steroid hormone testosterone (T) is a well-known mediator of male sexual behavior in vertebrates. However, less is known about T's rapid effects on sexual behavior, particularly those involving ultrasonic vocalizations (USVs), a mode of communication that can influence mate acquisition in rodents. Using the monogamous California mouse, Peromyscus californicus, we tested whether T rapidly alters male USV production by giving T or saline injections to non-paired (sexually naïve) males and paired (paternally experienced and pair-bonded) males immediately prior to a brief exposure to an unrelated, novel female. Among non-paired males, no differences in the total number of USVs were observed; however, T increased the proportion of simple sweeps produced. Among paired males, T decreased the number of USVs produced, and this change was driven by a reduction in simple sweeps. These results suggest a differential rapid effect of T pulses between non-paired and paired males upon exposure to a novel female. Additionally, we observed a positive correlation in the production of USVs made between males and novel females, and this relationship was altered by T. Given the importance of USVs in sexual communication, our study supports an essential concept of monogamy in that mate fidelity is reinforced by decreased responsiveness to prospective mates outside of the pair bond. The central mechanism in pair bonded males that decreases their responsiveness to novel females appears to be one that T can trigger. This is among the first studies to demonstrate that T can inhibit sexually related behaviors and do so rapidly.

Vocal ontogeny in neotropical singing mice (Scotinomys)

Isolation calls produced by dependent young are a fundamental form of communication. For species in which vocal signals remain important to adult communication, the function and social context of vocal behavior changes dramatically with the onset of sexual maturity. The ontogenetic relationship between these distinct forms of acoustic communication is surprisingly under-studied. We conducted a detailed analysis of vocal development in sister species of Neotropical singing mice, Scotinomys teguina and S. xerampelinus. Adult singing mice are remarkable for their advertisement songs, rapidly articulated trills used in long-distance communication; the vocal behavior of pups was previously undescribed. We recorded 30 S. teguina and 15 S. xerampelinus pups daily, from birth to weaning; 23 S. teguina and 11 S. xerampelinus were recorded until sexual maturity. Like other rodent species with poikilothermic young, singing mice were highly vocal during the first weeks of life and stopped vocalizing before weaning. Production of first advertisement songs coincided with the onset of sexual maturity after a silent period of ≧2 weeks. Species differences in vocal behavior emerged early in ontogeny and notes that comprise adult song were produced from birth. However, the organization and relative abundance of distinct note types was very different between pups and adults. Notably, the structure, note repetition rate, and intra-individual repeatability of pup vocalizations did not become more adult-like with age; the highly stereotyped structure of adult song appeared de novo in the first songs of young adults. We conclude that, while the basic elements of adult song are available from birth, distinct selection pressures during maternal dependency, dispersal, and territorial establishment favor major shifts in the structure and prevalence of acoustic signals. This study provides insight into how an evolutionarily conserved form of acoustic signaling provides the raw material for adult vocalizations that are highly species specific.

Androgen modulation of Foxp1 and Foxp2 in the developing rat brain: impact on sex specific vocalization

Sex differences in vocal communication are prevalent in both the animals and humans. The mechanism(s) mediating gender differences in human language are unknown, although, sex hormones, principally androgens, play a central role in the development of vocalizations in a wide variety of animal species. The discovery of FOXP2 has added an additional avenue for exploring the origins of language and animal communication. The FOXP2 gene is a member of the forkhead box P (FOXP) family of transcription factors. Prior to the prenatal androgen surge in male fetuses, we observed no sex difference for Foxp2 protein levels in cultured cells. In contrast, 24 hours after the onset of the androgen surge, we found a sex difference for Foxp2 protein levels in cultured cortical cells with males having higher levels than females. Furthermore, we observed the potent nonaromatizable androgen dihydrotestosterone altered not only Foxp2 mRNA and protein levels but also Foxp1. Androgen effects on both Foxp2 and Foxp1 were found to occur in the striatum, cerebellar vermis, and cortex. Immunofluorescence microscopy and coimmunoprecipitation demonstrate Foxp2 and the androgen receptor protein interact. Databases for transcription factor binding sites predict a consensus binding motif for androgen receptor on the Foxp2 promoter regions. We also observed a sex difference in rat pup vocalization with males vocalizing more than females and treatment of females with dihydrotestosterone eliminated the sex difference. We propose that androgens might be an upstream regulator of both Foxp2 and Foxp1 expression and signaling. This has important implications for language and communication as well as neuropsychiatric developmental disorders involving impairments in communication.

Seasonal expression of androgen receptor in scented gland of muskrat (Ondatra zibethicus)

Muskrat is a seasonal breeder, males of which secret musk from paired perineal scented glands found beneath the skin at the ventral base of the tail for attracting female during the breeding season. The aim of this study was to investigate the seasonal changes of expression of androgen receptor (AR) in the scented gland of muskrat during the breeding and nonbreeding seasons. Histologically, glandular cells, interstitial cells and excretory tubules were identified in scented glands in both seasons, whereas epithelial cells were sparse in the nonbreeding season. AR was observed in glandular cells of scented glands during the breeding and nonbreeding seasons with stronger immunostaining during the breeding season compared to the nonbreeding season. Consistent with the immunohistochemical results, AR protein level was higher in the scented glands of the breeding season, and then decreased to a relatively low level in the nonbreeding season. The mean mRNA level of Ar was significantly higher in the breeding season than in the nonbreeding season. In addition, plasma gonadotropins and testosterone concentrations were remarkably higher in the breeding season than those in the nonbreeding season. These results suggested that muskrat scented gland was the direct target organ of androgen, and stronger expression of AR in scented glands during the breeding season suggested that androgens may directly influence scented glandular function of the muskrats and also courtship behavior as we inferred.

The role of ultrasonic vocalizations in mouse communication

Human speech and language underlie many aspects of social behavior and thus understanding their ultimate evolutionary function and proximate genetic and neural mechanisms is a fundamental goal in neuroscience. Mouse ultrasonic vocalizations have recently received enormous attention as possible models for human speech. This attention has raised the question of whether these vocalizations are learned and what roles they play in communication. In this review, we first discuss recent evidence that ultrasonic vocalizations are not learned. We then review current evidence addressing how adult vocalizations may communicate courtship, territorial and/or other information. While there is growing evidence that these signals play key roles in communication, many important questions remain unanswered.

Interspecific dominance via vocal interactions mediates altitudinal zonation in neotropical singing mice

Interspecific aggression between ecologically similar species may influence geographic limits by mediating competitive exclusion at the range edge. Advertisement signals that mediate competitive interactions within species may also provide social information that contributes to behavioral dominance and spatial segregation among species. We studied the mechanisms underlying altitudinal range limits in Neotropical singing mice (Scotinomys), a genus of muroid rodent in which males vocalize to repel rivals and attract mates. We first delineated replacement zones and described temperature regimes on three mountains in Costa Rica and Panama where Chiriquí singing mice (S. xerampelinus) abruptly replace Alston's singing mice (S. teguina). Next, we conducted interspecific behavioral trials and reciprocal removal experiments to examine if interspecific aggression mediated species replacement. Finally, we performed reciprocal playback experiments to investigate whether response to song matched competitive interactions. Behavioral trials and removal experiments suggest that S. xerampelinus is behaviorally dominant and excludes S. teguina from higher, cooler altitudes. Playback experiments indicate that subordinate S. teguina is silenced and repelled by heterospecific song, whereas S. xerampelinus responded to heterospecifics with approach and song rates comparable to responses to conspecifics. Thus, interspecific communication reflects underlying dominance and suggests that acoustic signaling contributes to altitudinal zonation of ecologically similar congeners. Our findings implicate the use of social information in structuring spatial distributions of animal communities across landscapes and provide insight into how large-scale patterns are generated by individual interactions.