He's healthy, but will he survive the plague? Possible constraints on mate choice for disease resistance

From perplexing to predictive: are we ready to forecast insect disease susceptibility in a warming world?

Insects are critical to our ecosystems, but we do not fully understand their future in our warming world. Rising temperatures are affecting insect physiology in myriad ways, including changes to their immune systems and the ability to fight infection. Whether predicted changes in temperature will contribute to insect mortality or success, and the role of disease in their future survival, remains unclear. Although heat can enhance immunity by activating the integrated defense system (e.g. via the production of protective molecules such as heat-shock proteins) and accelerating enzyme activity, heat can also compromise the immune system through energetic-resource trade-offs and damage. The responses to heat are highly variable among species. The reasons for this variability are poorly known, and we are lagging in our understanding of how and why the immune system responds to changes in temperature. In this Commentary, we highlight the variation in insect immune responses to heat and the likely underlying mechanisms. We suggest that we are currently limited in our ability to predict the effects of rising temperatures on insect immunity and disease susceptibility, largely owing to incomplete information, coupled with a lack of tools for data integration. Moreover, existing data are concentrated on a relatively small number of insect Orders. We provide suggestions for a path towards making more accurate predictions, which will require studies with realistic temperature exposures and housing design, and a greater understanding of both the thermal biology of the immune system and connections between immunity and the physiological responses to heat.

Pathogens, odors, and disgust in rodents

All animals are under the constant threat of attack by parasites. The mere presence of parasite threat can alter behavior before infection takes place. These effects involve pathogen disgust, an evolutionarily conserved affective/emotional system that functions to detect cues associated with parasites and infection and facilitate avoidance behaviors. Animals gauge the infection status of conspecific and the salience of the threat they represent on the basis of various sensory cues. Odors in particular are a major source of social information about conspecifics and the infection threat they present. Here we briefly consider the origins, expression, and regulation of the fundamental features of odor mediated pathogen disgust in rodents. We briefly review aspects of: (1) the expression of affective states and emotions and in particular, disgust, in rodents; (2) olfactory mediated recognition and avoidance of potentially infected conspecifics and the impact of pathogen disgust and its' fundamental features on behavior; (3) pathogen disgust associated trade-offs; (4) the neurobiological mechanisms, and in particular the roles of the nonapeptide, oxytocin, and steroidal hormones, in the expression of pathogen disgust and the regulation of avoidance behaviors and concomitant trade-offs. Understanding the roles of pathogen disgust in rodents can provide insights into the regulation and expression of responses to pathogens and infection in humans.

Sex and burrowing behavior and their implications with lytic activity in the sand-dwelling spider Allocosa senex

The immune response can be costly. Studies in several arthropod species have indicated a trade-off between immunity and other life-history traits, including reproduction. In sexually dimorphic species in which females and males largely differ in their life history strategies and related energetic demands, we can expect to find sex differences in immune functions. Sex differences in immunity are well documented in vertebrates; however, we largely lack data from invertebrate systems. Lytic activity, the immune system's ability to lysate bacteria and viruses, has been widely used as a proxy for the strength of the immune response in several invertebrates. With this in mind, we used the burrowing wolf spider Allocosa senex to test differences in lytic activity between females and males. We also studied whether digging behavior affects the immune responses in this species. While females of A. senex construct simple refuges where they stay during the day, males construct deep burrows, which they donate to females after copulation. In accordance with our hypothesis, females showed higher lytic activity compared with males, and those males who dug showed higher levels of lytic activity than those that did not dig. Furthermore, male body condition and lytic activity did not correlate with burrow length, a trait under female choice in this species. Our results show sexual dimorphism in lytic activity responses, which are likely related to differences in life-history strategies and energetic requirements of each sex in A. senex spiders.

Sexual selection favours good or bad genes for pathogen resistance depending on males' pathogen exposure

Resistance to pathogens is often invoked as an indirect benefit of female choice, but experimental evidence for links between father's sexual success and offspring resistance is scarce and equivocal. Two proposed mechanisms might generate such links. Under the first, heritable resistance to diverse pathogens depends on general immunocompetence; owing to shared condition dependence, male sexual traits indicate immunocompetence independently of the male's pathogen exposure. By contrast, other hypotheses (e.g. Hamilton-Zuk) assume that sexual traits only reveal heritable resistance if the males have been exposed to the pathogen. The distinction between the two mechanisms has been neglected by experimental studies. We show that Drosophila melanogaster males that are successful in mating contests (one female with two males) sire sons that are substantially more resistant to the intestinal pathogen Pseudomonas entomophila-but only if the males have themselves been exposed to the pathogen before the mating contest. By contrast, sons of males sexually successful in the absence of pathogen exposure are less resistant than sons of unsuccessful males. We detected no differences in daughters' resistance. Thus, while sexual selection may have considerable consequences for offspring resistance, these consequences may be sex-specific. Furthermore, contrary to the 'general immunocompetence' hypothesis, these consequences can be positive or negative depending on the epidemiological context under which sexual selection operates.

Trade-offs between age-related breeding improvement and survival senescence in highly polygynous elephant seals: Dominant males always do better

Life history trade-off theory predicts that current reproduction can negatively affect survival and future reproduction. Few studies have assessed breeding costs for males of polygynous species compared to females, despite substantial variation in breeding success among individual males (e.g. subordinate cf. dominant breeders). Specifically, differentiating between the cost of attending breeding seasons, and the additional cost of successfully securing and mating females is lacking. We investigated whether trade-offs are present in the highly polygynous male southern elephant seal (Mirounga leonina) using 34-years of individual-level data. We compare age-specific survival, recruitment and future breeding success probabilities of pre-breeders (males yet to recruit) and breeders (subordinate and dominant social ranks) using multievent models. Pre-breeders and breeders of overlapping ages had similar survival probabilities, suggesting that there was no attendance cost for early recruits. In addition, the probability of recruiting as a dominant breeder never exceeded recruitment probability as a subordinate breeder of the same age. Therefore, older pre-breeders that delayed attendance costs generally did not improve their breeding success (probability of being dominant) at recruitment more than younger recruits. Rather, recruitment age may be a function of individual quality, with lower quality individuals requiring more time to socially mature. When comparing subordinate and dominant breeders, we found clear evidence for survival senescence, with subordinate breeders having a higher baseline mortality. In contrast, age-specific future breeding success (probability of being dominant at t + 1) increased with age, with dominant breeders maintaining higher subsequent breeding success than subordinate breeders. The opposite trends in survival and future breeding success for both subordinate and dominant breeders may indicate a lifetime, population-level trade-off. However, we found no evidence to suggest that being a dominant breeder consecutively (and having a higher accumulated breeding cost) accelerated the rate of senescence when compared to individuals that were previously subordinate. Thus, males experienced actuarial senescence regardless of social rank, with dominant (and possibly high quality) breeders showing a reduced trade-off between survival and future breeding success. We make several novel contributions to understanding polygynous male life histories and southern elephant seal demography.

The role of social cognition in parasite and pathogen avoidance

The acquisition and use of social information are integral to social behaviour and parasite/pathogen avoidance. This involves social cognition which encompasses mechanisms for acquiring, processing, retaining and acting on social information. Social cognition entails the acquisition of social information about others (i.e. social recognition) and from others (i.e. social learning). Social cognition involves assessing other individuals and their infection status and the pathogen and parasite threat they pose and deciding about when and how to interact with them. Social cognition provides a framework for examining pathogen and parasite avoidance behaviours and their associated neurobiological mechanisms. Here, we briefly consider the relationships between social cognition and olfactory-mediated pathogen and parasite avoidance behaviours. We briefly discuss aspects of (i) social recognition of actual and potentially infected individuals and the impact of parasite/pathogen threat on mate and social partner choice; (ii) the roles of 'out-groups' (strangers, unfamiliar individuals) and 'in-groups' (familiar individuals) in the expression of parasite/pathogen avoidance behaviours; (iii) individual and social learning, i.e. the utilization of the pathogen recognition and avoidance responses of others; and (iv) the neurobiological mechanisms, in particular the roles of the nonapeptide, oxytocin and steroid hormones (oestrogens) associated with social cognition and parasite/pathogen avoidance.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Does juvenile hormone prompt oxidative stress in male damselflies?

In invertebrates, it has recently been reported that secondary sexual characteristics (SSCs) reflect the antioxidant defense of their bearers, but it is not known what physiological link maintains the honesty of those signals. Here, we used the damselfly Hetaerina americana to test whether juvenile hormone plays such a role. First, we analyzed whether oxidative damage is a real threat in natural damselfly populations by examining the accumulation of oxidized guanines as a function of age in males. Then, we injected paraquat (a pro-oxidant agent) and added the juvenile hormone analog methoprene (JHa) to the experimental group and the JHa vehicle (acetone) to the control group, to determine whether JHa increases the levels of pro-oxidants and antioxidants. We found that DNA oxidation increased with age, and that levels of hydrogen peroxide and superoxide dismutase, but not catalase or glutathione, were elevated in the JHa group compared with the control group. We propose that juvenile hormone is a mediator of the relationship between SSCs and antioxidant capacity and, based on the literature, we know that JHa suppresses the immune response. We therefore suggest that juvenile hormone is a molecular mediator of the general health of males, which is reflected in their SSCs.

An exploration of the links between parasites, trophic ecology, morphology, and immunogenetics in the Lake Tanganyika cichlid radiation

Differences in habitat and diet between species are often associated with morphological differences. Habitat and trophic adaptation have therefore been proposed as important drivers of speciation and adaptive radiation. Importantly, habitat and diet shifts likely impose changes in exposure to different parasites and infection risk. As strong selective agents influencing survival and mate choice, parasites might play an important role in host diversification. We explore this possibility for the adaptive radiation of Lake Tanganyika (LT) cichlids. We first compare metazoan macroparasites infection levels between cichlid tribes. We then describe the cichlids' genetic diversity at the major histocompatibility complex (MHC), which plays a key role in vertebrate immunity. Finally, we evaluate to what extent trophic ecology and morphology explain variation in infection levels and MHC, accounting for phylogenetic relationships. We show that different cichlid tribes in LT feature partially non-overlapping parasite communities and partially non-overlapping MHC diversity. While morphology explained 15% of the variation in mean parasite abundance, trophic ecology accounted for 16% and 22% of the MHC variation at the nucleotide and at the amino acid level, respectively. Parasitism and immunogenetic adaptation may thus add additional dimensions to the LT cichlid radiation.

Microbes and masculinity: Does exposure to pathogenic cues alter women's preferences for male facial masculinity and beardedness?

Women's preferences for men's androgen dependent secondary sexual traits are proposed to be phenotypically plastic in response to exposure to pathogens and pathogen disgust. While previous studies report that masculinity in facial shape is more attractive to women who have recently been exposed to pathogenic cues and who are high in self-reported pathogen disgust, facial hair may reduce male attractiveness under conditions of high pathogens as beards are a possible breeding ground for disease carrying ectoparasites. In the present study, we test whether women's preferences for beardedness and facial masculinity vary due to exposure to different pathogenic cues. Participants (N = 688, mean age + 1SD = 31.94 years, SD = 6.69, range = 18-67) rated the attractiveness of facial composite stimuli of men when they were clean-shaven or fully bearded. These stimuli were also manipulated in order to vary sexual dimorphism by ±50%. Ratings were conducted before and after exposure to one of four experimental treatments in which participants were primed to either high pathogens (e.g. infected cuts), ectoparasites (e.g. body lice), a mixture of pathogens and ectoparasites, or a control condition (e.g. innocuous liquids). Participants then completed the three-domain disgust scale measuring attitudes to moral, sexual and pathogen disgust. We predicted that women would prefer facial masculinity following exposure to pathogenic cues, but would show reduced preferences for facial hair following exposure to ectoparasites. Women preferred full beards over clean-shaven faces and masculinised over feminised faces. However, none of the experimental treatments influenced the direction of preferences for facial masculinity or beardedness. We also found no association between women's self-reported pathogen disgust and their preferences for facial masculinity. However, there was a weak positive association between moral disgust scores and preferences for facial masculinity, which might reflect conservatism and preferences for gender typicality in faces. Women's preferences for beards were positively associated with their pathogen disgust, which runs contrary to our predictions and may reflect preferences for high quality individuals who can withstand any costs of beardedness, although further replications are necessary before firm conclusions can be made. We conclude that there is little support for pathogenic exposure being a mechanism that underpins women's directional preferences for masculine traits.

Neuroscience of aesthetics

Aesthetic evaluations are appraisals that influence choices in important domains of human activity, including mate selection, consumer behavior, art appreciation, and possibly even moral judgment. The nascent field of neuroaesthetics is advancing our understanding of the role of aesthetic evaluations by examining their biological bases. Here, we conduct a selective review of the literature on neuroaesthetics to demonstrate that aesthetic experiences likely emerge from the interaction between emotion-valuation, sensory-motor, and meaning-knowledge neural systems. This tripartite model can in turn be evoked to explain phenomena central to aesthetics, such as context effects on preferences. Indeed, context-dependent appraisals that focus on objects rather than on outcomes could be an important factor distinguishing aesthetic experiences from other kinds of evaluations.

Health, Anticipated Partner Infidelity, and Jealousy in Men and Women

Health has been identified as an important variable involved in mate choice. Unhealthy organisms are generally less able to provide reproductively important resources to partners and offspring and are more likely to pass on communicable disease. Research on human mate preferences has shown that both men and women prefer healthy mates. Yet to date, little research has examined how health relates to one's own mating experiences. In the present study, 164 participants (87 women) who were currently in heterosexual romantic relationships completed measures of frequency and severity of health problems, anticipated partner infidelity, and intensity of jealousy felt in their current relationship. Mediation analyses showed that health problems predicted greater anticipated partner infidelity and jealousy scores and that anticipated partner infidelity mediated the links between health and jealousy for both frequency and severity of health problems, controlling for both sex and relationship duration. These findings suggest that unhealthy people perceive themselves to be at a mating disadvantage, experiencing associated differences in perceptions and emotions surrounding their romantic partners' fidelity.

Female partner preferences enhance offspring ability to survive an infection

Background

It is often suggested that mate choice enhances offspring immune resistance to infectious diseases. To test this hypothesis, we conducted a study with wild-derived house mice (Mus musculus musculus) in which females were experimentally mated either with their preferred or non-preferred male, and their offspring were infected with a mouse pathogen, Salmonella enterica (serovar Typhimurium).

Results

We found that offspring sired by preferred males were significantly more likely to survive the experimental infection compared to those sired by non-preferred males. We found no significant differences in the pathogen clearance or infection dynamics between the infected mice, suggesting that offspring from preferred males were better able to cope with infection and had improved tolerance rather than immune resistance.

Conclusion

Our results provide the first direct experimental evidence within a single study that partner preferences enhance offspring resistance to infectious diseases.

It is all in the face: carotenoid skin coloration loses attractiveness outside the face

Recently, the importance of skin colour for facial attractiveness has been recognized. In particular, dietary carotenoid-induced skin colour has been proposed as a signal of health and therefore attractiveness. While perceptual results are highly consistent, it is currently not clear whether carotenoid skin colour is preferred because it poses a cue to current health condition in humans or whether it is simply seen as a more aesthetically pleasing colour, independently of skin-specific signalling properties. Here, we tested this question by comparing attractiveness ratings of faces to corresponding ratings of meaningless scrambled face images matching the colours and contrasts found in the face. We produced sets of face and non-face stimuli with either healthy (high-carotenoid coloration) or unhealthy (low-carotenoid coloration) colour and asked participants for attractiveness ratings. Results showed that, while for faces increased carotenoid coloration significantly improved attractiveness, there was no equivalent effect on perception of scrambled images. These findings are consistent with a specific signalling system of current condition through skin coloration in humans and indicate that preferences are not caused by sensory biases in observers.

Male Facial Masculinity as a Cue to Health Outcomes

Evolutionary theories of human attraction draw heavily upon nonhuman literature, and currently the Immunocompetence Handicap Hypothesis dominates research into female attraction to male facial masculinity. Although some studies have shown links between masculinity and some measures of health, other data have failed to support the Immunocompetence Hypothesis as applied to human face preferences. Here we summarize that literature and present new data regarding links between masculinity and multiple measures of health condition in human males. Undergraduate males were photographed and their faces were assessed for sexual dimorphism using multiple methods and rated for apparent healthiness and attractiveness. Participants also reported recent health experiences both prior to being photographed and then again 10 weeks later. Although both attractiveness and rated health were associated with better actual health in the past and future (mainly indexed by lower antibiotic use), results were mixed for masculinity. With respect to respiratory illnesses, facial masculinity (assessed using morphometric techniques) was associated with better past health but with worse future health. Possible reasons for the complex and inconsistent findings are discussed and some potentially fruitful avenues of future research are outlined.

Adiposity, compared with masculinity, serves as a more valid cue to immunocompetence in human mate choice

According to the 'good genes' hypothesis, females choose males based on traits that indicate the male's genetic quality in terms of disease resistance. The 'immunocompetence handicap hypothesis' proposed that secondary sexual traits serve as indicators of male genetic quality, because they indicate that males can contend with the immunosuppressive effects of testosterone. Masculinity is commonly assumed to serve as such a secondary sexual trait. Yet, women do not consistently prefer masculine looking men, nor is masculinity consistently related to health across studies. Here, we show that adiposity, but not masculinity, significantly mediates the relationship between a direct measure of immune response (hepatitis B antibody response) and attractiveness for both body and facial measurements. In addition, we show that circulating testosterone is more closely associated with adiposity than masculinity. These findings indicate that adiposity, compared with masculinity, serves as a more important cue to immunocompetence in female mate choice.

Do men's faces really signal heritable immunocompetence?

In the literature on human mate choice, masculine facial morphology is often proposed to be an intersexual signal of heritable immunocompetence, and hence an important component of men's attractiveness. This hypothesis has received considerable research attention, and is increasingly treated as plausible and well supported. In this article, we propose that the strength of the evidence for the immunocompetence hypothesis is somewhat overstated, and that a number of difficulties have been under-acknowledged. Such difficulties include (1) the tentative nature of the evidence regarding masculinity and disease in humans, (2) the complex and uncertain picture emerging from the animal literature on sexual ornaments and immunity, (3) the absence of consistent, cross-cultural support for the predictions of the immunocompetence hypothesis regarding preferences for masculinized stimuli, and (4) evidence that facial masculinity contributes very little, if anything, to overall attractiveness in real men. Furthermore, alternative explanations for patterns of preferences, in particular the proposal that masculinity is primarily an intrasexual signal, have been neglected. We suggest that immunocompetence perspectives on masculinity, whilst appealing in many ways, should still be regarded as speculative, and that other perspectives-and other traits-should be the subject of greater attention for researchers studying human mate preferences.

Mate preferences and infectious disease: theoretical considerations and evidence in humans

Mate preferences may operate in part to mitigate the threats posed by infectious disease. In this paper, we outline various ways in which preferring healthy mates can offer direct benefits in terms of pathogen avoidance and indirect benefits in terms of heritable immunity to offspring, as well as the costs that may constrain mate preferences for health. We then pay special attention to empirical work on mate preferences in humans given the depth and breadth of research on human mating. We review this literature and comment on the degree to which human mate preferences may reflect preferences for health.

The evolution of sex-specific immune defences

Why do males and females often differ in their ability to cope with infection? Beyond physiological mechanisms, it has recently been proposed that life-history theory could explain immune differences from an adaptive point of view in relation to sex-specific reproductive strategies. However, a point often overlooked is that the benefits of immunity, and possibly the costs, depend not only on the host genotype but also on the presence and the phenotype of pathogens. To address this issue we developed an adaptive dynamic model that includes host–pathogen population dynamics and host sexual reproduction. Our model predicts that, although different reproductive strategies, following Bateman's principle, are not enough to select for different levels of immunity, males and females respond differently to further changes in the characteristics of either sex. For example, if males are more exposed to infection than females (e.g. for behavioural reasons), it is possible to see them evolve lower immunocompetence than females. This and other counterintuitive results highlight the importance of ecological feedbacks in the evolution of immune defences. While this study focuses on sex-specific natural selection, it could easily be extended to include sexual selection and thus help to understand the interplay between the two processes.