Gonadal hormone activation of male courtship ultrasonic vocalizations and male copulatory behavior in castrated male deer mice (Peromyscus maniculatus bairdi)

Hearing, touching, and multisensory integration during mate choice

Mate choice is a potent generator of diversity and a fundamental pillar for sexual selection and evolution. Mate choice is a multistage affair, where complex sensory information and elaborate actions are used to identify, scrutinize, and evaluate potential mating partners. While widely accepted that communication during mate assessment relies on multimodal cues, most studies investigating the mechanisms controlling this fundamental behavior have restricted their focus to the dominant sensory modality used by the species under examination, such as vision in humans and smell in rodents. However, despite their undeniable importance for the initial recognition, attraction, and approach towards a potential mate, other modalities gain relevance as the interaction progresses, amongst which are touch and audition. In this review, we will: (1) focus on recent findings of how touch and audition can contribute to the evaluation and choice of mating partners, and (2) outline our current knowledge regarding the neuronal circuits processing touch and audition (amongst others) in the context of mate choice and ask (3) how these neural circuits are connected to areas that have been studied in the light of multisensory integration.

Courtship vocalizations: A potential biomarker of adult exposure to endocrine disrupting compounds?

In rodents, male courtship is stimulated by pheromones emitted by the sexually receptive female. In response, the male produces ultrasonic vocalizations, which appear to play a role in female attraction and facilitate copulation. The present review summarizes the main findings on courtship vocalizations and their tight regulation by sex steroid hormones. It describes studies that address the effects of exposure to endocrine disrupting compounds (EDC) on ultrasound production, as changes in hormone levels or their signaling pathways may interfere with the emission of ultrasonic vocalizations. It also discusses the potential use of this behavior as a noninvasive biomarker of adult exposure to EDC.

Adult male mice exposure to nonylphenol alters courtship vocalizations and mating

The neural circuitry processing male sexual behavior is tightly regulated by testosterone and its neural metabolite estradiol. The present study evaluated the effects of adult exposure to low doses of nonylphenol (NP), a widespread environmental contaminant, on the neuroendocrine regulation of testosterone and expression of sexual behavior. Oral exposure of C57BL/6J males to NP (0.5, 5 or 50 μg/kg/day) for 4 weeks did not affect circulating levels of testosterone or the kisspeptin system, a key regulator of the gonadotropic axis. In contrast, mice exposed to NP at 5 μg/kg/day emitted an increased number and duration of ultrasonic vocalizations, took longer to reach ejaculation and showed increased number of mounts, intromissions and thrusts. This was associated with normal olfactory preference and locomotor activity, and increased anxiety level. Analysis of the neural circuitry that underlies sexual behavior showed changes in the number of cells expressing androgen and estrogen receptors in males exposed to NP at 5 μg/kg/day. The neural circuitry underlying sexual behavior is thus highly sensitive to adult exposure to NP. Furthermore, almost all the observed effects were induced at 5 μg/kg/day of NP, indicating that this endocrine disrupter triggers a non-monotonic response in the adult male mouse brain.

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.

Ultrasonic vocalizations in golden hamsters (Mesocricetus auratus) reveal modest sex differences and nonlinear signals of sexual motivation

Vocal signaling is one of many behaviors that animals perform during social interactions. Vocalizations produced by both sexes before mating can communicate sex, identity and condition of the caller. Adult golden hamsters produce ultrasonic vocalizations (USV) after intersexual contact. To determine whether these vocalizations are sexually dimorphic, we analyzed the vocal repertoire for sex differences in: 1) calling rates, 2) composition (structural complexity, call types and nonlinear phenomena) and 3) acoustic structure. In addition, we examined it for individual variation in the calls. The vocal repertoire was mainly composed of 1-note simple calls and at least half of them presented some degree of deterministic chaos. The prevalence of this nonlinear phenomenon was confirmed by low values of harmonic-to-noise ratio for most calls. We found modest sexual differences between repertoires. Males were more likely than females to produce tonal and less chaotic calls, as well as call types with frequency jumps. Multivariate analysis of the acoustic features of 1-note simple calls revealed significant sex differences in the second axis represented mostly by entropy and bandwidth parameters. Male calls showed lower entropy and inter-quartile bandwidth than female calls. Because the variation of acoustic structure within individuals was higher than among individuals, USV could not be reliably assigned to the correct individual. Interestingly, however, this high variability, augmented by the prevalence of chaos and frequency jumps, could be the result of increased vocal effort. Hamsters motivated to produce high calling rates also produced longer calls of broader bandwidth. Thus, the sex differences found could be the result of different sex preferences but also of a sex difference in calling motivation or condition. We suggest that variable and complex USV may have been selected to increase responsiveness of a potential mate by communicating sexual arousal and preventing habituation to the caller.

The role of 5alpha-reductase activity in sexual behaviors of the green anole lizard

Both testosterone (T) and its metabolite, 5alpha-dihydrotestosterone (DHT), can facilitate male sexual behavior in the lizard Anolis carolinensis. The present study addresses the role of DHT synthesis in regulating male sexual behavior by inhibiting 5alpha-reductase, the enzyme that converts T into DHT. In two separate experiments (one replacement and one maintenance paradigm), breeding adult males were castrated and implanted with capsules of T, DHT, or a control capsule (blank, BL). The animals were then injected with the 5alpha-reductase inhibitor, FCE, or with steroid suspending vehicle (SSV) as a control. Both experiments produced similar results. Overall, T was most effective in eliciting courtship and copulatory behaviors above control levels. In both experiments, treatment with FCE attenuated the T-induced effects on courtship behavior, whereas the inhibition of 5alpha-reductase activity resulted in modest and inconsistent effects on the latency to intromission and the proportion of copulating males. DHT treatment did not significantly increase courtship or copulatory behaviors above control levels. These results suggest that (a) 5alpha-reductase activity is necessary but that DHT alone is not sufficient for stimulating courtship in male A. carolinensis; and (b) courtship behavior is more sensitive than copulatory behavior to the activity of the androgen metabolizing enzyme.

Castration affects the emission of an ultrasonic vocalization in a nocturnal primate, the grey mouse lemur (Microcebus murinus)

Male primates have evolved particular displays to advertise male fitness. In nocturnal prosimians it was assumed that such displays (vocal advertisement, marking behaviour) are hormonally dependent and influenced by olfactory cues of a receptive female. To test this assumption mouse lemur males of different gonadal status (males castrated as adults, normal males), or of varying olfactory input (males vomeronasalectomized or bulbectomized as adults) were paired with intact "stimulus" preoestrous females and vocalization, vocal rates, and marking behaviours were monitored. The vocal rate of only one particular vocalization, the "trill" advertisement call, was sexually dimorphic and changed depending on gonadal status. Vocalization rate declined significantly in sexually experienced, castrated males as did their marking behaviour. There was however no significant correlation between either calling rate or marking behaviour and plasma testosterone levels in gonadally intact males. Neither vomeronasalectomy nor bulbectomy had a significant effect on trill calling rate or marking behaviour. Results imply that testicular hormones have a greater impact on advertisement calling and marking motivation than olfactory information of preoestrous females. In contrast to rodents, ultrasonic calling rate seems to be not only dependent on gonadal status, but also on social experience.

Hormonal control of precopulatory sebaceous scent marking and ultrasonic mating vocalizations in male rats

The effects of testosterone (T) and its major metabolites, estradiol (E2) and dihydrotestosterone (DHT), on the restoration of sebaceous scent marking and 50-kHz ultrasonic vocalizations in male rats were measured in two studies (Experiments 1 and 2) employing different hormone levels. Silastic capsule administration of high and low doses of T (10 and 20 mm) or E2 (5%; 5 and 10 mm) completely restored marking to precastration levels. Both doses of DHT (30 and 40 mm) or no hormone replacement were without effect when tested in presence of estrous odor cues. In our testing paradigm, males appeared to mark glass objects with a sebaceous secretion rather than urine. Only the high dose of T (20 mm) restored vocalizations to intact levels, while animals receiving the high or low dose of E2 or DHT showed no restoration of behavior. In Experiment 3, habituation to estrous odor cues over weekly tests was not observed for marking or vocalizations. These results are discussed with respect to similarities and differences in the hormonal control of signaling behaviors by androgen metabolites in male rodents.

Microimplants of estradiol in the sexually dimorphic area of the hypothalamus activate ultrasonic vocal behavior in male Mongolian gerbils

The hormonal control of ultrasonic vocal behavior in the male Mongolian gerbil was examined by comparing the behavioral effects of androgen with those of estrogen administered to the preoptic-anterior hypothalamic area (POA-AH) in castrates. By measuring radioactivity released from solid "floating" POA-AH microimplants (mean diameter, 141 microns) of testosterone (3H-T, mean weight, 880 ng) in Experiment 1, we found that the steroid had a concentration gradient which fell rapidly from the edge of the microimplant, suggesting restricted diffusion. Using floating microimplants in Experiment 2, we studied the effects of testosterone propionate (TP, 650 ng), estradiol-17 beta benzoate (EB, 439 ng), or cholesterol (C, 478 ng) on rates of a frequency modulated ultrasonic vocalization emitted during sexual interactions. The effects on the upsweep call were compared with those on sexual mounting. The upsweep rate remained significantly below precastration levels in C implanted males. EB reinstated upsweep calling within 5 days, 3 days earlier than TP microimplants. Mounting in EB implanted males was maintained at precastration levels, whereas TP implantation restored mounting to precastration levels only after 5 days. EB was effective in inducing ultrasonic vocalizations when placed in, or near, the sexually dimorphic area (SDA) in the medial preoptic area (POM). Our results indicate that brain mechanisms underlying both ultrasonic vocalizations and mounting are directly sensitive to estradiol (E2) in the male gerbil. We conclude that E2 affects mechanisms in the SDA associated with ultrasonic calling and suggest that T is likely to act via aromatization products in the brain.

The emission and elicitation of mouse ultrasonic vocalizations: the effects of age, sex and gonadal status

Subject' mice of varying gonadal status (castrate males; intact or neonatally gonadectomized females and males) were paired for 3-min with intact 'stimulus' females and ultrasonic vocalizations were monitored. Vocalization patterns from home cages differed from the test pairings. The results suggested that the age, experience and gonadal status of the subject influenced the vocalizations from the pair. As the source of ultrasonic calls from these vocally intact pairs could not be individually identified, the 'subjects' were paired with a range of ultrasonically silent (inferior laryngeal nerve-transected) stimulus animals. Vocalizations were detected from all combinations of animals. Gonadally intact females were most effective in eliciting ultrasonic vocalizations from the subjects and gonadally intact males were least effective. The responses of castrate males were lower than from intact males. Anesthetized adults of either sex elicited only poor vocalization responses from other adults. Ultrasonic calls have often previously been studied using vocally intact 'subject' and 'stimulus' animals: the present results confirm the difficulty of establishing who is who in such situations.

Effects of laryngeal denervation on ultrasound production and male sexual behavior in rodents

Adult male house mice (Mus musculus) and deer mice (Peromyscus maniculatus bairdi) were used to investigate the effects of partial denervation of the larynx on ultrasound production. In addition the masculine sexual behavior of vocalizing and muted males was assessed on tests with receptive females. For both species, unilateral transections of the inferior laryngeal nerve (ILN) significantly reduced the proportion of males that emitted ultrasonic vocalizations. However, the inability to vocalize did not prevent the display of male sexual behavior. Bilateral transections of the superior laryngeal nerve (SLN) failed to abolish ultrasound production in males of both species. These nerve transections, however, reduced the amount of ultrasound recorded during postsurgical tests, and in the case of deer mice, cuts of the SLN increased the latency to the first vocalization. The present results confirm the importance of the ILN in the neural control of ultrasonic vocalizations and indicate that, at least in laboratory tests, the ability to emit ultrasounds is not a necessary condition for the display of mating behavior in male rodents.