Production and appreciation of humor as sexually selected traits

Both men and women prefer someone with a “good sense of humor” as a relationship partner. However, two recent studies have shown that men are not attracted to funny women, suggesting the sexes use the phrase good sense of humor differently. To investigate this question, we measured the importance participants placed on a partner's production of humor vs. receptivity to their own humor. Men emphasized the importance of their partners' receptivity to their own humor, whereas women valued humor production and receptivity equally. In a second task, participants chose whether they preferred a person who only produced humor or a person who only appreciated their own humor for several types of relationships. Women preferred those who produced humor for all types of relationships, whereas men preferred those who were receptive to their own humor, particularly for sexual relationships. Our results suggest that sexual selection may have operated on men's and women's preferences during humorous interaction in dramatically different ways.     

Menstrual cycle phase alters women's sexual preferences for composers of more complex music

Over 140 years ago Charles Darwin first argued that birdsong and human music, having no clear survival benefit, were obvious candidates for sexual selection. Whereas the first contention is now universally accepted, his theory that music is a product of sexual selection through mate choice has largely been neglected. Here, I provide the first, to my knowledge, empirical support for the sexual selection hypothesis of music evolution by showing that women have sexual preferences during peak conception times for men that are able to create more complex music. Two-alternative forced-choice experiments revealed that woman only preferred composers of more complex music as short-term sexual partners when conception risk was highest. No preferences were displayed when women chose which composer they would prefer as a long-term partner in a committed relationship, and control experiments failed to reveal an effect of conception risk on women''s preferences for visual artists. These results suggest that women may acquire genetic benefits for offspring by selecting musicians able to create more complex music as sexual partners, and provide compelling support for Darwin''s assertion ‘that musical notes and rhythm were first acquired by the male or female progenitors of mankind for the sake of charming the opposite sex’.     

Birds reveal their personality when singing

Background

Individual differences in social behaviour may have consequences for mate choice and sexual signalling, because partners should develop preferences for personalities that maximize reproductive output. Here we propose that behavioural traits involved in sexual advertisement may serve as good indicators of personality, which is fundamental for sexual selection to operate on temperament. Bird song has a prominent and well-established role in sexual selection, and it displays considerable variation among individuals with a potentially strong personality component. Therefore, we predicted that features of song would correlate with estimates of personality.

Methodology/Principal Findings

In a field study of free-living male collared flycatchers, Ficedula albicollis, we characterised personality based on the exploration of an altered breeding environment, and based on the risk taken when a potential predator was approaching during a simulated territorial interaction. We found that explorative and risk-taker individuals consistently sang at lower song posts than shy individuals in the presence of a human observer. Moreover, males from lower posts established pair-bonds relatively faster than males from higher posts.

Conclusions/Significance

Our results may demonstrate that risk taking during singing correlates with risk taking during aggression and with exploration, thus personality may be manifested in different contexts involving sexual advertisement. These findings are in accordance with the hypothesis that the male''s balance between investment in reproduction and risk taking is reflected in sexual displays, and it may be important information for choosy females that seek partners with personality traits enhancing breeding success.     

The adaptive value of humor and laughter

Most explanations for humor neglect important types of humor, such as tickling and word play; or raise difficult evolutionary theoretical problems, such as group selection, dubious fitness benefits, and excessive complexity of design; or ignore the data on humor and laughter. The present theory was based on the following observations. Tickling entails a mock attack at vulnerable body spots, and may provide youngsters with practice in defending themselves. The child's laughter is pleasant and encourages the tickler to persist. Similarly, juvenile primates including children encourage roughhousing by laughter and other emotional expressions. We also laugh at humorous content that provides striking counter-examples (incongruities), as in word play, or that informs us about fitness-relevant topics such as sexual, aggressive, and social poise scenarios. The present theory is that the pleasure of humor motivates us to seek out poignant, fitness- enhancing input of this sort. Laughter evolved to allow us to continue to recieve amusement. Laughter is a pleasant social signal that prompts the humorist to persist in providing this edifying stimulation. In response to true wit, laughter conveys appreciation and gratitude—an intention to reciprocate for having received a stimulating idea. Thus, humor benefits both humorist and laughter. This theory and others are evaluated in the light of evolutionary principles and relevant data.     

Sharing a joke: The effects of a similar sense of humor on affiliation and altruism

Cooperation requires that individuals are able to identify, and preferentially associate with, others who have compatible preferences and the shared background knowledge needed to solve interpersonal coordination problems. This body of shared knowledge constitute a substantial proportion of what is called ‘culture’. It has been argued that, for this reason, individuals prefer to associate with others who share their culture, and also that shared appreciation of humor provides a particularly effective means of identifying others with the relevant preferences and knowledge. The present experiment uses a ‘dummy rating procedure’ to compare the effects of sharing an appreciation of non-humorous (first lines of novels) and humorous (jokes) cultural stimuli on interpersonal affiliation, altruism and assessment. The results show that the degree of shared appreciation for both sets of stimuli had a positive effect on Affiliation; only humorous stimuli had an effect on Altruism; and neither effected the Assessment of others' personal traits. Thus, the results support the general theory that shared culture promotes affiliation, and provide evidence of the special role of humor in interpersonal relations.     

Females prefer to associate with males with longer intromittent organs in mosquitofish

Sexual selection is a major force behind the rapid evolution of male genital morphology among species. Most within-species studies have focused on sexual selection on male genital traits owing to events during or after copulation that increase a male''s share of paternity. Very little attention has been given to whether genitalia are visual signals that cause males to vary in their attractiveness to females and are therefore under pre-copulatory sexual selection. Here we show that, on average, female eastern mosquitofish Gambusia holbrooki spent more time in association with males who received only a slight reduction in the length of the intromittent organ (‘gonopodium’) than males that received a greater reduction. This preference was, however, only expressed when females chose between two large males; for small males, there was no effect of genital size on female association time.     

Immune Investment Is Explained by Sexual Selection and Pace-of-Life,but Not Longevity in Parrots (Psittaciformes)

Investment in current reproduction should come at the expense of traits promoting future reproduction, such as immunity and longevity. To date, comparative studies of pace-of-life traits have provided some support for this, with slower paced species having greater immune function. Another means of investment in current reproduction is through secondary sexual characters (SSC). Investment in SSC''s is considered costly, both in terms of immunity and longevity, with greater costs being borne by species with more elaborate traits. Yet within species, females prefer more ornate males and those males are typically immunologically superior. Because of this, predictions about the relationship between immunity and SSC''s across species are not clear. If traits are costly, brighter species should have reduced immune function, but the opposite is true if SSC''s arise from selection for more immunocompetent individuals. My approach was to investigate immune investment in relation to SSC''s, pace-of-life and longevity while considering potentially confounding ecological factors. To do so I assessed leukocyte counts from in a novel group, the Psittaciformes. Investment in SSC''s best explained investment in immunity: species with brighter plumage had higher leukocyte counts and those with a greater degree of sexual dichromatism had fewer. Ecological variables and pace-of-life models tended to be poor predictors of immune investment. However, shorter incubation periods were associated with lower leukocyte counts supporting the notion that species with a fast pace-of-life invest less in immunity. These results suggest that investment in reproduction in terms of fast pace-of-life and sexual dichromatism results in reduced immunity; however, investment in plumage colour per se does not impose a cost on immunity across species.     

Characterizing selection in black‐throated blue warblers using a sexual network approach

Our understanding of trait evolution is built upon studies that examine the correlation between traits and fitness, most of which implicitly assume all individuals experience similar selective environments. However, accounting for differences in selective pressures, such as variation in the social environment, can advance our understanding of how selection shapes individual traits and subsequent fitness. In this study, we test whether variation in the social environment affects selection on individual phenotype. We apply a new sexual network framework to quantify each male's social environment as the mean body size of his primary competitors. We test for direct and social selection on male body size using a 10‐year data set on black‐throated blue warblers (Setophaga caerulescens), a territorial species for which body size is hypothesized to mediate competition for mates. We found that direct selection on body size was weak and nonsignificant, as was social selection via the body size of the males' competitors. Analysing both types of selection simultaneously allows us to firmly reject a role for body size in competitive interactions between males and subsequent male fitness in this population. We evaluate the application of the sexual network approach to empirical data and suggest that other phenotypic traits such as song characteristics and plumage may be more relevant than body size for male–male competition in this small passerine bird.     

Evolutionary quantitative genetics of behavioral responses to handling in a wild passerine

Behavioral differences between individuals that are consistent over time characterize animal personality. The existence of such consistency contrasts to the expectation based on classical behavioral theory that facultative behavior maximizes individual fitness. Here, we study two personality traits (aggression and breath rate during handling) in a wild population of blue tits during 2007–2012. Handling aggression and breath rate were moderately heritable (h² = 0.35 and 0.20, respectively) and not genetically correlated (r_A = 0.06) in adult blue tits, which permits them to evolve independently. Reciprocal cross‐fostering (2007–2010) showed that offspring reared by more aggressive males have a higher probability to recruit. In addition, offspring reared by pairs mated assortatively for handling aggression had a higher recruitment probability, which is the first evidence that both parents' personalities influence their reproductive success in the wild in a manner independent of their genetic effects. Handling aggression was not subjected to survival selection in either sex, but slow‐breathing females had a higher annual probability of survival as revealed by capture–mark–recapture analysis. We find no evidence for temporal fluctuations in selection, and thus conclude that directional selection (via different fitness components) acts on these two heritable personality traits. Our findings show that blue tit personality has predictable fitness consequences, but that facultative adjustment of an individual's personality to match the fitness maximum is likely constrained by the genetic architecture of personality. In the face of directional selection, the presence of heritable variation in personality suggests the existence of a trade‐off that we have not identified yet.     

Morphological and genetic predictors of parental care in the North American barn swallow Hirundo rustica erythrogaster

Sexually dimorphic traits often signal the fitness benefits an individual can provide to potential mates. In species with altricial young, these signals may also predict the level of parental care an individual is expected to provide to shared offspring. In this study, we tested three hypotheses that traditionally relate sexually dimorphic traits to parental care in two populations of North American barn swallows Hirundo rustica erythrogaster. The good parent hypothesis predicts a positive relationship between an individual's ornamentation and his or her care whereas the differential allocation (more care given by individuals when paired to high quality mates) and reproductive compensation (more care given by individuals when paired to low quality mates) hypotheses predict that an individual's level of parental investment is relative to the quality of their mate. Male and female North American barn swallows have colorful ventral feathers and elongated tail streamers, but there is evidence that ventral color, not tail streamer length, predicts measures of seasonal reproductive success. Accounting for the positive correlation between within‐pair feeding rates and other potentially confounding variables in all of our models, we found no support for the good parent hypothesis because in both males and females, traits shown to be under sexual selection did not predict feeding rates in either sex. However, our data reveal that male coloration, and not streamer length, predicted a female's provisioning rate to shared offspring (females fed more when paired with darker individuals) in two separate populations, supporting the differential allocation, but not the reproductive compensation hypothesis. Because genetic traits have also been shown to affect parental investment, we evaluated this variable as well and found that a male's paternity did not have significant effects on either male or female feeding rates. Overall, our results suggest that females do not pair with darker males in order to gain direct benefits in terms of his expected levels of parental care to shared offspring, but do themselves invest greater levels of care when paired to darker males. Further, our results are consistent with previous studies which suggest that ventral feather color, not streamer length, is a target of sexual selection in North American populations of barn swallow because females invested more in their offspring when paired to darker mates.     

Sexual differences in larval life history traits of acanthocephalan cystacanths

Sexual differences in life history traits, such as size dimorphism, presumably arise via sexual selection and are most readily observed in adults. For complex life-cycle parasites, however, sexual selection may also have consequences for larval traits, e.g., growth in intermediate hosts. Two acanthocephalan species (Acanthocephalus lucii and Echinorhynchus borealis) were studied to determine, whether larval life histories differ between males and females. The size of female A. lucii cystacanths had a much stronger relationship with intermediate host size than males, suggesting females invest more in growth and are consequently more limited by resources. No relationship between host size and cystacanth size was observed for E. borealis. For both species, female cystacanths survived longer in a culture medium composed entirely of salts, which could suggest that females have greater energy reserves than males. A comparative analysis across acanthocephalan species indicated that sexual size dimorphism at the adult stage correlates with cystacanth dimorphism. However, the relationship was not isometric; cystacanths do not reach the same level of sexual dimorphism as adults, possibly due to resource constraints. Our results suggest that larval life histories diverge between males and females in some acanthocephalans, and this is seemingly a consequence of sexual selection acting on adults.     

Beauty and the beast: mechanisms of sexual selection in humans

Literature in evolutionary psychology suggests that mate choice has been the primary mechanism of sexual selection in humans, but this conclusion conforms neither to theoretical predictions nor available evidence. Contests override other mechanisms of sexual selection; that is, when individuals can exclude their competitors by force or threat of force, mate choice, sperm competition, and other mechanisms are impossible. Mates are easier to monopolize in two dimensional mating environments, such as land, than in three-dimensional environments, such as air, water, and trees. Thus, two-dimensional mating environments may tend to favor the evolution of contests. The two-dimensionality of the human mating environment, along with phylogeny, the spatial and temporal clustering of mates and competitors, and anatomical considerations, predict that contest competition should have been the primary mechanism of sexual selection in men. A functional analysis supports this prediction. Men's traits are better designed for contest competition than for other sexual selection mechanisms; size, muscularity, strength, aggression, and the manufacture and use of weapons probably helped ancestral males win contests directly, and deep voices and facial hair signal dominance more effectively than they increase attractiveness. However, male monopolization of females was imperfect, and female mate choice, sperm competition, and sexual coercion also likely shaped men's traits. In contrast, male mate choice was probably central in women's mating competition because ancestral females could not constrain the choices of larger and more aggressive males through force, and attractive women could obtain greater male investment. Neotenous female features and body fat deposition on the breasts and hips appear to have been shaped by male mate choice.     

Negative frequency-dependent preferences and variation in male facial hair

Negative frequency-dependent sexual selection maintains striking polymorphisms in secondary sexual traits in several animal species. Here, we test whether frequency of beardedness modulates perceived attractiveness of men''s facial hair, a secondary sexual trait subject to considerable cultural variation. We first showed participants a suite of faces, within which we manipulated the frequency of beard thicknesses and then measured preferences for four standard levels of beardedness. Women and men judged heavy stubble and full beards more attractive when presented in treatments where beards were rare than when they were common, with intermediate preferences when intermediate frequencies of beardedness were presented. Likewise, clean-shaven faces were least attractive when clean-shaven faces were most common and more attractive when rare. This pattern in preferences is consistent with negative frequency-dependent selection.     

Sexual selection explains sex-specific growth plasticity and positive allometry for sexual size dimorphism in a reef fish

In 1950, Rensch noted that in clades where males are the larger sex, sexual size dimorphism (SSD) tends to be more pronounced in larger species. This fundamental allometric relationship is now known as ‘Rensch''s rule’. While most researchers attribute Rensch''s rule to sexual selection for male size, experimental evidence is lacking. Here, we suggest that ultimate hypotheses for Rensch''s rule should also apply to groups of individuals and that individual trait plasticity can be used to test those hypotheses experimentally. Specifically, we show that in the sex-changing fish Parapercis cylindrica, larger males have larger harems with larger females, and that SSD increases with harem size. Thus, sexual selection for male body size is the ultimate cause of sexual size allometry. In addition, we experimentally illustrate a positive relationship between polygyny potential and individual growth rate during sex change from female to male. Thus, sexual selection is the ultimate cause of variation in growth rate, and variation in growth rate is the proximate cause of sexual size allometry. Taken together, our results provide compelling evidence in support of the sexual selection hypothesis for Rensch''s rule and highlight the potential importance of individual growth modification in the shaping of morphological patterns in Nature.     

Sexual selection and animal personality

Consistent individual behavioural tendencies, termed “personalities”, have been identified in a wide range of animals. Functional explanations for personality have been proposed, but as yet, very little consideration has been given to a possible role for sexual selection in maintaining differences in personality and its stability within individuals. We provide an overview of the available literature on the role of personality traits in intrasexual competition and mate choice in both human and non‐human animals and integrate this into a framework for considering how sexual selection can generate and maintain personality. For this, we consider the evolution and maintenance of both main aspects of animal personality: inter‐individual variation and intra‐individual consistency.     

Good Liars Are Neither ‘Dark’ Nor Self-Deceptive

Deception is a central component of the personality ''Dark Triad'' (Machiavellianism, Psychopathy and Narcissism). However, whether individuals exhibiting high scores on Dark Triad measures have a heightened deceptive ability has received little experimental attention. The present study tested whether the ability to lie effectively, and to detect lies told by others, was related to Dark Triad, Lie Acceptability, or Self-Deceptive measures of personality using an interactive group-based deception task. At a group level, lie detection accuracy was correlated with the ability to deceive others—replicating previous work. No evidence was found to suggest that Dark Triad traits confer any advantage either to deceive others, or to detect deception in others. Participants who considered lying to be more acceptable were more skilled at lying, while self-deceptive individuals were generally less credible and less confident when lying. Results are interpreted within a framework in which repeated practice results in enhanced deceptive ability.     

Does individual personality predict male mating preference for female body size in the Trinidadian guppy?

Within populations, individual animals vary considerably in their behaviour, including mate choice and personality. There is mounting interest in the potential covariation between these two behaviours within individuals, such that personality would influence mate choice. We experimentally tested this proposition under controlled laboratory conditions using male Trinidadian guppies (Poecilia reticulata) as a model study system. We assayed repeatedly the mating preference of individual males for females based on their body size. Additionally, we assayed repeatedly two ecologically relevant personality traits in males, namely exploration of a novel environment and boldness under a simulated predation threat. Finally, we analysed whether male mating preference and personality traits were repeatable, and tested whether the personality of individual males was correlated (covaried) with their mating preference scores. Although all but one of the measures of exploration and boldness behaviour were repeatable over time, male mating preference scores were not repeatable. Measures of male exploration and boldness were not inter-correlated among individuals, suggesting the absence of a behavioural syndrome between exploration and boldness. Unexpectedly, males did not exhibit on average a significant mating preference for larger females over smaller ones; they chose randomly between the paired stimulus females. Overall, we found no compelling evidence for a relationship between individual personality traits and mating preference in male guppies, suggesting that personality does not predict mate choice, at least in our study population and under our experimental conditions. We discuss potential factors, other than male personality and body length, that might maintain inter-individual variation in male mating preferences in the guppy in the wild.     

Bateman's principle is reversed in a cooperatively breeding bird

Bateman''s principle is not only used to explain sex differences in mating behaviour, but also to determine which sex has the greater opportunity for sexual selection. It predicts that the relationship between the number of mates and the number of offspring produced should be stronger for males than for females. Yet, it is unclear whether Bateman''s principle holds in cooperatively breeding systems where the strength of selection on traits used in intrasexual competition is high in both sexes. We tested Bateman''s principle in the cooperatively breeding superb starling (Lamprotornis superbus), finding that only females showed a significant, positive Bateman gradient. We also found that the opportunity for selection was on average higher in females, but that its strength and direction oscillated through time. These data are consistent with the hypothesis that sexual selection underlies the female trait elaboration observed in superb starlings and other cooperative breeders. Even though the Bateman gradient was steeper for females than for males, the year-to-year oscillation in the strength and direction of the opportunity for selection likely explains why cooperative breeders do not exhibit sexual role reversal. Thus, Bateman''s principle may not hold in cooperative breeders where both sexes appear to be under mutually strong sexual selection.     

Personality predicts ectoparasite abundance in an asocial sciurid

Parasitism is a consequence of complex interactions between host, parasite, and their shared environment, and host behavior can influence parasite risk. Animal personality (i.e., consistent behavioral differences that are repeatable across time and context) can influence parasitism with more explorative individuals typically hosting greater parasite loads. Host “sociality” is known to impact parasite risk with more social individuals typically at higher risk of acquiring or transmitting parasites, but other behaviors could also be important. We quantified personality in least chipmunks (Tamias minimus), including repeatability of behavioral traits, and determined whether these personality traits affected ectoparasite prevalence and abundance. We measured personality using standardized hole‐board tests and quantified ectoparasitism of 39 least chipmunks over 2 years at a site in southeastern Manitoba, Canada. We found that activity and exploration were repeatable within the context of the hole‐board test for least chipmunks, which suggests that these traits reflect personality. More exploratory individuals hosted a greater abundance of ectoparasites compared to less exploratory individuals. Our results are consistent with past studies implicating personality as a factor in host–parasite dynamics and suggest that exploration may be an important behavioral correlate of parasite acquisition.     

It is not always about body size: evidence of Rensch's rule in a male weapon

In many species, sexual dimorphism increases with body size when males are the larger sex but decreases when females are the larger sex, a macro-evolutionary pattern known as Rensch''s rule (RR). Although empirical studies usually focus exclusively on body size, Rensch''s original proposal included sexual differences in other traits, such as ornaments and weapons. Here, we used a clade of harvestmen to investigate whether two traits follow RR: body size and length of the fourth pair of legs (legs IV), which are used as weapons in male–male fights. We found that males were slightly smaller than females and body size did not follow RR, whereas legs IV were much longer in males and followed RR. We propose that sexual selection might be stronger on legs IV length than on body size in males, and we discuss the potential role of condition dependence in the emergence of RR.