Aversive Responses of Female Mice to the Odors of Parasitized Males: Neuromodulatory Mechanisms and Implications for Mate Choice

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.

Suppression of host nocifensive behavior by parasitoid wasp venom

The parasitoid wasp Ampulex compressa envenomates the brain of its host the American cockroach (Periplaneta americana), thereby making it a behaviorally compliant food supply for its offspring. The target of venom injection is a locomotory command center in the brain called the central complex. In this study, we investigate why stung cockroaches do not respond to injuries incurred during the manipulation process by the wasp. In particular, we examine how envenomation compromises nociceptive signaling pathways in the host. Noxious stimuli applied to the cuticle of stung cockroaches fail to evoke escape responses, even though nociceptive interneurons projecting to the brain respond normally. Hence, while nociceptive signals are carried forward to the brain, they fail to trigger robust nocifensive behavior. Electrophysiological recordings from the central complex of stung animals demonstrate decreases in peak firing rate, total firing, and duration of noxious-evoked activity. The single parameter best correlated with altered noxious-evoked behavioral responses of stung cockroaches is reduced duration of the evoked response in the central complex. Our findings demonstrate how the reproductive strategy of a parasitoid wasp is served by venom-mediated elimination of aversive, nocifensive behavior in its host.

Ventromedial prefrontal cortex activity differentiates sick from healthy faces: Associations with inflammatory responses and disease avoidance motivation

BACKGROUND

Humans are able to discern the health status of others using olfactory and visual cues, and subsequently shift behavior to make infection less likely. However, little is known about how this process occurs. The present study examined the neural regions involved in differentiating healthy from sick individuals using visual cues.

METHODS

While undergoing a functional magnetic resonance imaging scan, participants (N = 42) viewed facial photos of 30 individuals (targets) who had been injected with an inflammatory challenge--low-dose endotoxin (i.e., sick) or placebo (i.e., healthy), and rated how much they liked each face. We examined regions implicated in processing either threat (amygdala, anterior insula) or cues that signal safety (ventromedial prefrontal cortex [VMPFC]), and how this activity related to their liking of targets and cytokine levels (interleukin-6, tumor necrosis factor-α) exhibited by the targets.

RESULTS

Photos of sick faces were rated as less likeable compared to healthy faces, and the least liked faces were those individuals with the greatest inflammatory response. While threat-related regions were not significantly active in response to viewing sick faces, the VMPFC was more active in response to viewing healthy (vs. sick) faces. Follow-up analyses revealed that participants tended to have lower VMPFC activity when viewing the least liked faces and the faces of those with the greatest inflammatory response.

CONCLUSIONS

This work builds on prior work implicating the VMPFC in signaling the presence of safe, non-threatening visual stimuli, and suggests the VMPFC may be sensitive to cues signaling relative safety in the context of pathogen threats.

Parasite avoidance behaviours in aquatic environments

Parasites, including macroparasites, protists, fungi, bacteria and viruses, can impose a heavy burden upon host animals. However, hosts are not without defences. One aspect of host defence, behavioural avoidance, has been studied in the terrestrial realm for over 50 years, but was first reported from the aquatic environment approximately 20 years ago. Evidence has mounted on the importance of parasite avoidance behaviours and it is increasingly apparent that there are core similarities in the function and benefit of this defence mechanism between terrestrial and aquatic systems. However, there are also stark differences driven by the unique biotic and abiotic characteristics of terrestrial and aquatic (marine and freshwater) environments. Here, we review avoidance behaviours in a comparative framework and highlight the characteristics of each environment that drive differences in the suite of mechanisms and cues that animals use to avoid parasites. We then explore trade-offs, potential negative effects of avoidance behaviour and the influence of human activities on avoidance behaviours. We conclude that avoidance behaviours are understudied in aquatic environments but can have significant implications for disease ecology and epidemiology, especially considering the accelerating emergence and re-emergence of parasites.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Sexual attractiveness of male chemicals and vocalizations in mice

Male-female interaction is important for finding a suitable mating partner and for ensuring reproductive success. Male sexual signals such as pheromones transmit information and social and sexual status to females, and exert powerful effects on the mate preference and reproductive biology of females. Likewise, male vocalizations are attractive to females and enhance reproductive function in many animals. Interestingly, females' preference for male pheromones and vocalizations is associated with their genetic background, to avoid inbreeding. Moreover, based on acoustic cues, olfactory signals have significant effects on mate choice in mice, suggesting mate choice involves multisensory integration. In this review, we synopsize the effects of both olfactory and auditory cues on female behavior and neuroendocrine functions. We also discuss how these male signals are integrated and processed in the brain to regulate behavior and reproductive function.

Sexual selection and the brain

Sex differences are intrinsically interesting, particularly in the brain. When sexually dimorphic structures mediate learning, and when such learning ability is necessary to compete for mates, then such differences are best understood within the framework of sexual selection. By categorizing recent studies of sex differences in the brain by their role in mate competition, theories of sexual selection can be used to predict and characterize the occurrence of dimorphisms among species with different mating systems.

Genes, odours and the recognition of parasitized individuals by rodents

Social recognition, whereby animals identify and recognize other individual conspecifics, is a crucial prerequisite for a wide range of social behaviours. There are relationships among social odours (chemical signals), parasite recognition and avoidance that are associated with hormonal, neural and genomic mechanisms in rodents. Rodents use social odours to: (i) distinguish between infected and uninfected individuals; (ii) recognize specific infected individuals; and (iii) avoid and display aversive responses to infected individuals. There are genomic correlates of this parasite recognition and avoidance in which genes expressing the neuropeptide oxytocin have roles. In this article, we provide a framework ("micronet") by which the genetic, hormonal and neural interactions associated with social behaviours and recognition and avoidance of parasitized individuals can be explored.

A study of mate choice in mice with experimental Taenia crassiceps cysticercosis: can males choose?

The detection and avoidance of infected males has been proposed as a component of female mate choice. However, male mate choice and sexual behaviour in the face of infected females have been rarely investigated. Here, we assessed the exact role displayed by each protagonist in mate choice during experimental Taenia crassiceps (Zeder, 1800) Rudolphi, 1810 cysticercosis. Mate choice was studied at two levels: (1) based on animal odour cues, using a two-choice test and (2) during dyadic interactions, using sexual behaviour and courtship analysis, but also ultrasonic vocalization recordings. Our results showed that female BALB/c mice (Mus musculus L., 1758) spent more time investigating odours in the two-choice test than males, but they were also able to repel infected males during dyadic interactions. Males did not display a clear mating choice with female odours, but during dyadic sexual interactions, they showed a significant aggressiveness towards infected females. Males were able to refrain from copulating with infected females. As shown by a number of studies on female mice, males were able to detect, select, and avoid infected females, and thus express a real mate choice. These observations provided an additional support to the sperm/ejaculate cost and male mate-choice hypothesis.

Parasites reduce attractiveness and reproductive success in male grain beetles

Sexual characters may reveal the quality of a potential mate, including the mate's level of infection with parasites. Females that prefer males with low levels of infection or no infection may benefit in several ways. Direct benefits may include avoidance of infection, acquistition of larger nuptial gifts or enhancement in fecundity due to differences in male fertility. Females may also benefit indirectly by producing offspring that are more resistant to infections. We measured female preference for odours produced by male grain beetles, Tenebrio molitor, that were either infected by a tapeworm, Hymenolepis diminuta, or uninfected. This parasite is not transmitted directly between conspecifics. Females were attracted to odours of all males, but they were less attracted to those from parasitized males. To the contrary, females were preferentially attracted to infected females. Males did not show any biased attraction to odours from infected and uninfected male beetles. Females that mated with highly infected males produced fewer offspring than females mated to uninfected males, indicating parasitic infection inflicts multiple costs to males. These results are consistent with models of parasite-mediated sexual selection. Copyright 2000 The Association for the Study of Animal Behaviour.

Parasitized female mice display reduced aversive responses to the odours of infected males

The present study showed that parasites influence both the responses of uninfected females to males and the responses of female hosts to infected males. In female laboratory mice one of the consequences of exposure to the olfactory cues associated with an infected male was a reduction of the reactivity to a thermal surface, i.e. pain inhibition or analgaesia. Uninfected oestrous and non-oestrous female mice displayed marked analgaesic responses after exposure to the odours of males infected with either the enteric single-host nematode parasite, Heligmosomoides polygyrus, or the protozoan parasite, Eimeria vermiformis. The uninfected oestrous females distinguished between infected and physically stressed males, displaying a greater analgaesic response to the odours of infected males. These analgaesic responses and their anxiety/ fearfulness-associated behavioural correlates could elicit either a reduced interest in, or avoidance of, parasitized males by females. Oestrous female mice infected with H. polygyrus displayed a reduced analgaesic response to the odours of the infected males and differentially responded to the odours of males infected with either the same (H. polygyrus) or a different parasite (E. vermiformis). An exposure time of 1 min elicited minimal responses to the odours of males infected with the same parasite, H. polygyrus, and an attenuated, though significant, non-opioid peptide-mediated analgaesic response to males infected with E. vermiformis. An exposure time of 30 min elicited similar markedly reduced endogenous opioid peptide-mediated analgaesic responses to the odours of both of the categories of infected males. The responses to the odours of a stressed male were, however, unaffected by the parasitic infection. The reduced analgaesic responses of the parasitized females to the odours of infected males may involve either enhanced odour familiarity and responses to group odour templates and/or neuromodulatory shifts resulting in reduced fearfulness and potentially greater interest in the infected males.

Discrimination by female mice between the odours of parasitized and non-parasitized males

The detection and avoidance of parasitized males has been proposed to be a component of female mate choice. We investigated whether or not female laboratory mice, Mus musculus domesticus, could discriminate between parasitized and non-parasitized males on the basis of odour. Female mice were given a choice between the urine and other odorous secretions of either a male mouse sub-clinically infected for five days with the naturally occurring, enteric, single host, protozoan parasite, Eimeria vermiformis, or an uninfected male. Females showed a marked preference for the odours of non-parasitized male mice over those of the parasitized males; as measured by number of investigations, time spent per investigation, and total investigation time of the odours in a choice situation. Female mice also displayed an overwhelming initial, or first choice, preference for the odours of the non-parasitized male mice over those of the parasitized males. These observations show that female mice can distinguish between the odours of parasitized and non-parasitized males, and discriminate against parasitized males on the basis of odour. We suggest that the detection and avoidance of infected males by female mice through odour cues may function to reduce parasite transmission and potentially serve as a component of female mate selection or choice.

Physiological bases for parasite-induced alterations of host behaviour

Parasitism is defined in various ways as an intimate relationship in which one partner, the parasite, lives on or in another, the host, generally at the expense of the latter. Parasitism commonly results in a unique array of host physiological responses and adaptations. Most studies of the physiological effects of parasitism have focused on the pathological consequence of infection and disease. While many physiological changes contribute to pathogenesis, it is now recognized that parasitic infections at sub-clinical levels also produce physiological effects that either ameliorate or may not contribute to the disease process. Moreover, these physiological changes are often manifested by altered host behaviour. Behavioural studies have enabled an ecological- and evolutionary-oriented evaluation of host responses. In this fashion, physiological effects may be assessed as to whether they affect fitness and confer benefit or harm to one or both of the symbionts involved. We briefly examine how these physiological responses, specifically neural, endocrine, neuromodulatory, and immunomodulatory components, may interact to modify host behaviors. We consider the adaptiveness of these responses and how the behavioural patterns elicited may simultaneously appear adaptive for the parasite as well as the host. In addition, we address how parasite-host physiological and behavioural interactions may be altered during the course of parasitism.