Female Choice or Male Sex Drive? The Advantages of Male Body Size during Mating in Drosophila Melanogaster

The mating success of larger male Drosophila melanogaster in the laboratory and the wild has been traditionally been explained by female choice, even though the reasons are generally hard to reconcile. Female choice can explain this success by virtue of females taking less time to mate with preferred males, but so can the more aggressive or persistent courtships efforts of large males. Since mating is a negotiation between the two sexes, the behaviors of both are likely to interact and influence mating outcomes. Using a series of assays, we explored these negotiations by testing for the relative influence of male behaviors and its effect on influencing female courtship arousal threshold, which is the time taken for females to accept copulation. Our results show that large males indeed have higher copulation success compared to smaller males. Competition between two males or an increasing number of males had no influence on female sexual arousal threshold;—females therefore may have a relatively fixed ‘arousal threshold’ that must be reached before they are ready to mate, and larger males appear to be able to manipulate this threshold sooner. On the other hand, the females’ physiological and behavioral state drastically influences mating; once females have crossed the courtship arousal threshold they take less time to mate and mate indiscriminately with large and small males. Mating quicker with larger males may be misconstrued to be due to female choice; our results suggest that the mating advantage of larger males may be more a result of heightened male activity and relatively less of female choice. Body size per se may not be a trait under selection by female choice, but size likely amplifies male activity and signal outputs in courtship, allowing them to influence female arousal threshold faster.     

Low Pitched Voices Are Perceived as Masculine and Attractive but Do They Predict Semen Quality in Men?

Women find masculinity in men's faces, bodies, and voices attractive, and women's preferences for men's masculine features are thought to be biological adaptations for finding a high quality mate. Fertility is an important aspect of mate quality. Here we test the phenotype-linked fertility hypothesis, which proposes that male secondary sexual characters are positively related to semen quality, allowing females to obtain direct benefits from mate choice. Specifically, we examined women's preferences for men's voice pitch, and its relationship with men's semen quality. Consistent with previous voice research, women judged lower pitched voices as more masculine and more attractive. However men with lower pitched voices did not have better semen quality. On the contrary, men whose voices were rated as more attractive tended to have lower concentrations of sperm in their ejaculate. These data are more consistent with a trade off between sperm production and male investment in competing for and attracting females, than with the phenotype-linked fertility hypothesis.     

Voice pitch influences voting behavior

It may be adaptive for voters to recognize good leadership qualities among politicians. Men with lower-pitched voices are found to be more dominant and attractive than are men with higher-pitched voices. Candidate attractiveness and vocal quality relate to voting behavior, but no study has tested the influence of voice pitch on voting-related perceptions. We tested whether voice pitch influenced perceptions of politicians and how these perceptions related to voting behavior. In Study 1, we manipulated voice pitch of recordings of US presidents and asked participants to attribute personality traits to the voices and to choose the voice they preferred to vote for. We found that lower-pitched voices were associated with favorable personality traits more often than were higher-pitched voices and that people preferred to vote for politicians with lower-pitched rather than higher-pitched voices. Furthermore, lower voice pitch was more strongly associated with physical prowess than with integrity in a wartime voting scenario. Thus, sensitivity to vocal cues to dominance was heightened during wartime. In Study 2, we found that participants preferred to vote for the candidate with the lower-pitched voice when given the choice between two unfamiliar men's voices speaking a neutral sentence. Taken together, our results suggest that candidates' voice pitch has an important influence on voting behavior and that men with lower-pitched voices may have an advantage in political elections.     

Voice pitch alters mate-choice-relevant perception in hunter–gatherers

In humans, voice pitch is thought to be a cue of underlying quality and an important criterion for mate choice, but data from non-Western cultures have not been provided. Here we test attributions to and preferences for voices with raised and lowered pitch in hunter–gatherers. Using a forced-choice playback experiment, we found that both men and women viewed lower pitched voices in the opposite sex as being better at acquiring resources (e.g. hunting and gathering). While men preferred higher pitched women''s voices as marriage partners, women showed no overall preference for voice pitch in men. However, women who were currently breastfeeding had stronger preferences for higher pitched male voices whereas women not currently breastfeeding preferred lower pitched voices. As testosterone is considered a costly signal associated with dominance, heritable immunity to infection and low paternal investment, women''s preferences potentially reflect a trade-off between securing good genes and paternal investment. Men''s preferences for higher pitched female voices are probably due to an evolved preference for markers of fecundity, reflected in voice pitch.     

Preferences for very low and very high voice pitch in humans

Manipulations of voice pitch have been shown to alter attractiveness ratings, but whether preferences extend to very low or very high voice pitch is unknown. Here, we manipulated voice pitch in averaged men's and women's voices by 2 Hz intervals to create a range of male and female voices speaking monopthong vowel sounds and spanning a range of frequencies from normal to very low and very high pitch. With these voices, we used the method of constant stimuli to measure preferences for voice. Nineteen university students (ages: 20-25) participated in three experiments. On average, men preferred high-pitched women's voices to low-pitched women's voices across all frequencies tested. On average, women preferred men's voices lowered in pitch, but did not prefer very low men's voices. The results of this study may reflect selection pressures for men's and women's voices, and shed light on a perceptual link between voice pitch and vocal attractiveness.     

Benefits for Voice Learning Caused by Concurrent Faces Develop over Time

Recognition of personally familiar voices benefits from the concurrent presentation of the corresponding speakers’ faces. This effect of audiovisual integration is most pronounced for voices combined with dynamic articulating faces. However, it is unclear if learning unfamiliar voices also benefits from audiovisual face-voice integration or, alternatively, is hampered by attentional capture of faces, i.e., “face-overshadowing”. In six study-test cycles we compared the recognition of newly-learned voices following unimodal voice learning vs. bimodal face-voice learning with either static (Exp. 1) or dynamic articulating faces (Exp. 2). Voice recognition accuracies significantly increased for bimodal learning across study-test cycles while remaining stable for unimodal learning, as reflected in numerical costs of bimodal relative to unimodal voice learning in the first two study-test cycles and benefits in the last two cycles. This was independent of whether faces were static images (Exp. 1) or dynamic videos (Exp. 2). In both experiments, slower reaction times to voices previously studied with faces compared to voices only may result from visual search for faces during memory retrieval. A general decrease of reaction times across study-test cycles suggests facilitated recognition with more speaker repetitions. Overall, our data suggest two simultaneous and opposing mechanisms during bimodal face-voice learning: while attentional capture of faces may initially impede voice learning, audiovisual integration may facilitate it thereafter.     

Social dialect and men's voice pitch influence women's mate preferences

Low male voice pitch may communicate potential benefits for offspring in the form of heritable health and/or dominance, whereas access to resources may be indicated by correlates of socioeconomic status, such as sociolinguistic features. Here, we examine if voice pitch and social dialect influence women's perceptions of men's socioeconomic status and attractiveness. In Study 1, women perceived lower pitched male voices as higher in socioeconomic status than higher pitched male voices. In Study 2, women independently perceived lower pitched voices and higher status sociolinguistic dialects as higher in socioeconomic status and attractiveness. We also found a significant interaction wherein women preferred lower pitched men's voices more often when dialects were lower in sociolinguistic status than when they were higher in sociolinguistic status. Women also perceived lower pitched voices as higher in socioeconomic status more often when dialects were higher in sociolinguistic status than when lower in sociolinguistic status. Finally, women's own self-rated socioeconomic status was positively related to their preferences for voices with higher status sociolinguistic dialects, but not to their preferences for voice pitch. Hence, women's preferences for traits associated with potentially biologically heritable benefits, such as low voice pitch, are moderated by the presence of traits associated with resource accrual, such as social dialect markers. However, women's preferences for language markers of resource accrual may be functionally independent from preferences for potential biological indicators of heritable benefits, such as voice pitch.     

The Trade-Off between Female Fertility and Longevity during the Epidemiological Transition in the Netherlands

Several hypotheses have been put forward to explain the relationship between women’s fertility and their post-reproductive longevity. In this study, we focus on the disposable soma theory, which posits that a negative relationship between women’s fertility and longevity can be understood as an evolutionary trade-off between reproduction and survival. We examine the relationship between fertility and longevity during the epidemiological transition in the Netherlands. This period of rapid decline in mortality from infectious diseases offers a good opportunity to study the relationship between fertility and longevity, using registry data from 6,359 women born in The Netherlands between 1850 and 1910. We hypothesize that an initially negative relationship between women’s fertility and their longevity gradually turns less negative during the epidemiological transition, because of decreasing costs of higher parities. An initially inversed U-shaped association between fertility and longevity changes to zero during the epidemiological transition. This does suggest a diminishing environmental pressure on fertility. However, we find no evidence of an initial linear trade-off between fertility and post-reproductive survival.     

Preference for Leaders with Masculine Voices Holds in the Case of Feminine Leadership Roles

Human voice pitch research has focused on perceptions of attractiveness, strength, and social dominance. Here we examine the influence of pitch on selection of leaders, and whether this influence varies by leadership role. Male and female leaders with lower-pitched (i.e., masculine) voices are generally preferred by both men and women. We asked whether this preference shifts to favor higher-pitch (i.e., feminine) voices within the specific context of leadership positions that are typically held by women (i.e., feminine leadership roles). In hypothetical elections for two such positions, men and women listened to pairs of male and female voices that differed only in pitch, and were asked which of each pair they would vote for. As in previous studies, men and women preferred female candidates with masculine voices. Likewise, men preferred men with masculine voices. Women, however, did not discriminate between male voices. Overall, contrary to research showing that perceptions of voice pitch can be influenced by social context, these results suggest that the influence of voice pitch on perceptions of leadership capacity is largely consistent across different domains of leadership.     

Vocal Identity Recognition in Autism Spectrum Disorder

Voices can convey information about a speaker. When forming an abstract representation of a speaker, it is important to extract relevant features from acoustic signals that are invariant to the modulation of these signals. This study investigated the way in which individuals with autism spectrum disorder (ASD) recognize and memorize vocal identity. The ASD group and control group performed similarly in a task when asked to choose the name of the newly-learned speaker based on his or her voice, and the ASD group outperformed the control group in a subsequent familiarity test when asked to discriminate the previously trained voices and untrained voices. These findings suggest that individuals with ASD recognized and memorized voices as well as the neurotypical individuals did, but they categorized voices in a different way: individuals with ASD categorized voices quantitatively based on the exact acoustic features, while neurotypical individuals categorized voices qualitatively based on the acoustic patterns correlated to the speakers'' physical and mental properties.     

Vocal characteristics of presidential candidates can predict the outcome of actual elections

During political elections, voters rely on various cues that signal good social leadership, such as indicators of physical strength and masculinity. In adult men, masculine traits are related to testosterone levels, and one of those traits is low-pitched voice. Hence, lower pitch in a presidential candidate may be related to the election's outcome. This prediction is supported by experimental evidence showing that people prefer to vote for a candidate with a low-pitched voice. The aim of this study was to investigate the relationship between presidential candidates' vocal characteristics and actual election outcomes in 51 presidential elections held across the world. After analysis of the voices of opposing candidates, results showed that winners had lower-pitched voices with less pitch variability. Moreover, regression analysis revealed an interaction effect of voice pitch and voice pitch variability on the election outcome. Candidates with lower-pitched voices had greater likelihood of winning the election if they had higher pitch variability. This study extends previous findings, shows the importance of assessing vocal characteristics other than voice pitch, and offers external validity for the experimental evidence that candidates' vocal characteristics are related to the election outcome.     

Rhesus macaques compensate for reproductive delay following ecological adversity early in life

Adversity early in life can shape the reproductive potential of individuals through negative effects on health and life span. However, long‐lived populations with multiple reproductive events may present alternative life history strategies to optimize reproductive schedules and compensate for shorter life spans. Here, we quantify the effects of major hurricanes and density dependence as sources of early‐life ecological adversity on Cayo Santiago rhesus macaque female reproduction and decompose their effects onto the mean age‐specific fertility, reproductive pace, and lifetime reproductive success (LRS). Females experiencing major hurricanes exhibit a delayed reproductive debut but maintain the pace of reproduction past debut and show a higher mean fertility during prime reproductive ages, relative to unaffected females. Increasing density at birth is associated to a decrease in mean fertility and reproductive pace, but such association is absent at intermediate densities. When combined, our study reveals that hurricanes early in life predict a delay‐overshoot pattern in mean age‐specific fertility that supports the maintenance of LRS. In contrast to predictive adaptive response models of accelerated reproduction, this long‐lived population presents a novel reproductive strategy where females who experience major natural disasters early in life ultimately overcome their initial reproductive penalty with no major negative fitness outcomes. Density presents a more complex relation with reproduction that suggests females experiencing a population regulated at intermediate densities early in life will escape density dependence and show optimized reproductive schedules. Our results support hypotheses about life history trade‐offs in which adversity‐affected females ensure their future reproductive potential by allocating more energy to growth or maintenance processes at younger adult ages.     

Using Energetic Models to Investigate the Survival and Reproduction of Beaked Whales (family Ziphiidae)

Mass stranding of several species of beaked whales (family Ziphiidae) associated with exposure to anthropogenic sounds has raised concern for the conservation of these species. However, little is known about the species’ life histories, prey or habitat requirements. Without this knowledge, it becomes difficult to assess the effects of anthropogenic sound, since there is no way to determine whether the disturbance is impacting the species’ physical or environmental requirements. Here we take a bioenergetics approach to address this gap in our knowledge, as the elusive, deep-diving nature of beaked whales has made it hard to study these effects directly. We develop a model for Ziphiidae linking feeding energetics to the species’ requirements for survival and reproduction, since these life history traits would be the most likely to be impacted by non-lethal disturbances. Our models suggest that beaked whale reproduction requires energy dense prey, and that poor resource availability would lead to an extension of the inter-calving interval. Further, given current information, it seems that some beaked whale species require relatively high quality habitat in order to meet their requirements for survival and reproduction. As a result, even a small non-lethal disturbance that results in displacement of whales from preferred habitats could potentially impact a population if a significant proportion of that population was affected. We explored the impact of varying ecological parameters and model assumptions on survival and reproduction, and find that calf and fetus survival appear more readily affected than the survival of adult females.     

The natural history of a monogamous coral-reef fish, Valenciennea strigata (Gobiidae): 2. behavior, mate fidelity and reproductive success

This study examined the behavior and reproduction of a monogamous coral-reef fish, Valenciennea strigata, to determine mate fidelity and the proximate causes of monogamy. Most fish were found in monogamous pairs that remained together over several rounds of reproduction. Pairs stayed within close proximity to each other and their burrows. Females fed at a higher rate than their mates, while males spent more time maintaining burrows. Females spawned every 13 days; males guarded eggs in the burrow for 2–3 days. Although females limited the RS of males, males did not mate polygynously under natural conditions. Reproductive success (RS) was affected primarily by survival, and secondarily by size. Both sexes enforced monogamy by guarding their mates. Three factors facilitated mate guarding: (1) all males were able to hold a nest site, (2) both sexes showed strong site fidelity, and (3) residents had an advantage in contests over mates. Thus, mates were economically defensible. Additionally, females formed a crescent of dark pigments on their abdomen that resembled a gravid condition; these marks may enhance continuation of the pair bond. Both sexes preferred large mates, and pairs were positively assorted by size. Males benefited from guarding large females because fecundity increased with size. Females may benefit from the burrowing of males, and larger males should be better burrowers.     

The Unique Impact of Menstruation on the Female Voice: Implications for the Evolution of Menstrual Cycle Cues

Research shows that hormonal changes in women across the menstrual cycle affect vocal production. Most work has documented shifts at high fertility times (i.e., ovulation) or during premenstruation. However, hormonal changes at menstruation also affect female physiology and behavior and could affect vocal production. The present studies investigated perceptual differences in voices recorded during menstruation compared with recordings taken at other times of the menstrual cycle. Results show that male raters could reliably identify voices recorded during menstruation with or without the presence of a voice recorded closest to ovulation. In addition, voices recorded at menstruation were identified as being the most unattractive. These findings indicate that voice recordings taken at times of lowest fertility may uniquely impact women’s vocal production and that perceptions of voice based on cycle phase are not specific to the time of heightened fertility. Implications for the evolution of human menstrual cycle cues are discussed.     

Drifting behaviour as an alternative reproductive strategy for social insect workers

Restricted reproduction is traditionally posited as the defining feature of eusocial insect workers. The discovery of worker reproduction in foreign colonies challenges this view and suggests that workers’ potential to pursue selfish interests may be higher than previously believed. However, whether such reproductive behaviour truly relies on a reproductive decision is still unknown. Workers’ reproductive decisions thus need to be investigated to assess the extent of workers’ reproductive options. Here, we show in the bumblebee Bombus terrestris that drifting is a distinct strategy by which fertile workers circumvent competition in their nest and reproduce in foreign colonies. By monitoring workers’ movements between colonies, we show that drifting is a remarkably dynamic behaviour, widely expressed by both fertile and infertile workers. We demonstrate that a high fertility is, however, central in determining the propensity of workers to enter foreign colonies as well as their subsequent reproduction in host colonies. Moreover, our study shows that the drifting of fertile workers reflects complex decision-making processes associated with in-nest reproductive competition. This novel finding therefore adds to our modern conception of cooperation by showing the previously overlooked importance of alternative strategies which enable workers to assert their reproductive interests.     

The Acoustic Structure and Information Content of Female Koala Vocal Signals

Determining the information content of animal vocalisations can give valuable insights into the potential functions of vocal signals. The source-filter theory of vocal production allows researchers to examine the information content of mammal vocalisations by linking variation in acoustic features with variation in relevant physical characteristics of the caller. Here I used a source-filter theory approach to classify female koala vocalisations into different call-types, and determine which acoustic features have the potential to convey important information about the caller to other conspecifics. A two-step cluster analysis classified female calls into bellows, snarls and tonal rejection calls. Additional results revealed that female koala vocalisations differed in their potential to provide information about a given caller’s phenotype that may be of importance to receivers. Female snarls did not contain reliable acoustic cues to the caller’s identity and age. In contrast, female bellows and tonal rejection calls were individually distinctive, and the tonal rejection calls of older female koalas had consistently lower mean, minimum and maximum fundamental frequency. In addition, female bellows were significantly shorter in duration and had higher fundamental frequency, formant frequencies, and formant frequency spacing than male bellows. These results indicate that female koala vocalisations have the potential to signal the caller’s identity, age and sex. I go on to discuss the anatomical basis for these findings, and consider the possible functional relevance of signalling this type of information in the koala’s natural habitat.     

Vocal Fry May Undermine the Success of Young Women in the Labor Market

Vocal fry is speech that is low pitched and creaky sounding, and is increasingly common among young American females. Some argue that vocal fry enhances speaker labor market perceptions while others argue that vocal fry is perceived negatively and can damage job prospects. In a large national sample of American adults we find that vocal fry is interpreted negatively. Relative to a normal speaking voice, young adult female voices exhibiting vocal fry are perceived as less competent, less educated, less trustworthy, less attractive, and less hirable. The negative perceptions of vocal fry are stronger for female voices relative to male voices. These results suggest that young American females should avoid using vocal fry speech in order to maximize labor market opportunities.     

Adaptive Dynamics of Extortion and Compliance

Direct reciprocity is a mechanism for the evolution of cooperation. For the iterated prisoner’s dilemma, a new class of strategies has recently been described, the so-called zero-determinant strategies. Using such a strategy, a player can unilaterally enforce a linear relationship between his own payoff and the co-player’s payoff. In particular the player may act in such a way that it becomes optimal for the co-player to cooperate unconditionally. In this way, a player can manipulate and extort his co-player, thereby ensuring that the own payoff never falls below the co-player’s payoff. However, using a compliant strategy instead, a player can also ensure that his own payoff never exceeds the co-player’s payoff. Here, we use adaptive dynamics to study when evolution leads to extortion and when it leads to compliance. We find a remarkable cyclic dynamics: in sufficiently large populations, extortioners play a transient role, helping the population to move from selfish strategies to compliance. Compliant strategies, however, can be subverted by altruists, which in turn give rise to selfish strategies. Whether cooperative strategies are favored in the long run critically depends on the size of the population; we show that cooperation is most abundant in large populations, in which case average payoffs approach the social optimum. Our results are not restricted to the case of the prisoners dilemma, but can be extended to other social dilemmas, such as the snowdrift game. Iterated social dilemmas in large populations do not lead to the evolution of strategies that aim to dominate their co-player. Instead, generosity succeeds.     

Does coping style predict optimization? An experimental test in a wild passerine bird

A number of studies have suggested that avian brood size is individually optimized. Yet, optimal reproductive decisions likely vary owing to among-individual differences in environmental sensitivity. Specifically, ‘proactive’ individuals who do not track environmental changes may be less able to produce optimal brood sizes than ‘reactive’ individuals who have more precise local environmental knowledge. To test this, we quantified exploratory behaviour (a proxy for proactivity) in a great tit (Parus major) population, manipulated brood sizes (reduced, control, enlarged) and evaluated whether individuals of dissimilar coping style differed in their level of optimization. If reactive females behaved optimally, any deviation from their original brood size should lower fitness, whereas this should not be the case for proactive females. Reactive females indeed performed best at their natural brood size, whereas proactive females performed best when raising an enlarged brood. These findings imply that proactive females produced sub-optimal brood sizes. We speculate that proactive females might (i) take decisions based on biased perception of their environment, (ii) face energetic constraints in offspring production and/or (iii) be more willing to invest into current reproduction when given the option. Our findings provide experimental evidence for coping style-related differences in optimal reproductive decisions and life-history strategies.