Robert C Brooks

UNSW Sydney | UNSW · Evolution & Ecology Research Centre

Can beautification empower women to act assertively? Some women report that beautification is an agentic and assertive act, whereas others find beautification to be oppressive and disempowering. To disentangle these effects, in the context of romantic attraction we conducted the first experimental tests of beautification—on psychological and behavi...

Sexual conflict may be manifested during social interactions, shaping the costs of reproduction in sexually reproducing species. This conflict, and the physical necessity of intromission, can intensify the already costly nature of reproduction for female mammals. To identify and partition the costs that males inflict on females during mating and re...

Previous research finds that both men and women perceive sexualized women as lacking in certain human qualities such as mental capacity and moral status. The mechanism underlying this effect, however, is unclear. The present two studies test how appearance-based judgements affect the degree to which a broad sample of women are objectified. In Study...

In this study, we apply economic principles to the heterosexual human mating market using data on the socio-demographics, biology, attractiveness, sexual behaviour, and reproductive history of 3,261 Australian online dating participants. More specifically, by using survey participants attractiveness ratings as a proxy for market value, we are able...

Objectives According to the ectoparasite avoidance hypothesis, natural selection has shaped human hairlessness to reduce the potential for the body to host disease carrying ectoparasites. However, men retain sexually dimorphic and conspicuous patches of facial and body hair. The ectoparasite avoidance hypothesis also proposes that sexual selection...

Example images used for targets presented alone and with opposite sex others.

Ad Hoc MLMs of rated attractiveness of male and female models by men and women, factoring in ‘target attire’.

Ad Hoc MLMs of rated economic status of male and female models by men and women, factoring in ‘target attire’.

Data for groups study 1 separated by target sex and measure (attractiveness, earnings).

Data for attire study 2 separated by measure (attractiveness, earnings).

This study uses the BIG 5 personality traits to quantitatively explore correlates of sexual frequency and reproductive success of a large sample (NMale = 2998; NFemale = 1480) of heterosexuals advertised to on an Australian dating website. Consistent with previous research we find that for both sexes, extraversion has a positive linear relationship...

Desirable characteristics of “opposite sex others,” such as physical attractiveness and economic status, can influence how individuals are judged, and this is different for men and women. However, under various social contexts where cues of higher or lower economic status is suggested, sex differences in judgments related to mate choice have not be...

Objective Mate choice copying (MCC) is a type of non-independent mate choice where the ‘probability of acceptance’ of a potential mate increases if they are observed to be chosen by others first. The phenomenon was first demonstrated in several non-human taxa, with studies on humans conducted shortly after. The effect has been consistently document...

Using a unique dataset of 7479 respondents to the online Australian Sex Survey (July–September 2016), we explored factors relevant for individuals who self-identify as one of the many possible nonbinary gender options (i.e., not man or woman). Our results identified significant sex differences in such factors; in particular, a positive association...

Although attempts to rectify intimate partner violence (IPV) predominantly target gender inequality as its socio-structural source, evolutionary insights cast doubt on the notion that gender equality unambiguously lessens IPV. Here we test whether the effect of gender equality on male-to-female IPV will depend upon men's relative position in the se...

Significance Female sexualization is increasing, and scholars are divided on whether this trend reflects a form of gendered oppression or an expression of female competitiveness. Here, we proxy local status competition with income inequality, showing that female sexualization and physical appearance enhancement are most prevalent in environments th...

Whether it is men or women who suppress female sexuality has important implications for understanding gendered relations, ultimately providing insight into one widespread cause of female disadvantage. The question of which sex suppresses female sexuality more avidly, however, neglects that our interests are never unambiguously masculine or feminine...

Maternal investment is costly to the mother but essential to offspring survival in altrical species. Infanticide by novel males results in loss of maternal investment, and maternal strategies have evolved to mitigate these losses. One such maternal strategy, the Bruce effect, involves spontaneous abortion by females of some mammal species when expo...

The ovulatory shift hypothesis proposes that women's preferences for masculine physical and behavioral traits are greater at the peri-ovulatory period than at other points of the menstrual cycle. However, many previous studies used self-reported menstrual cycle data to estimate fecundability rather than confirming the peri-ovulatory phase hormonall...

Convict transportation to Australia imposed heavily male-biased sex ratios in some areas, altering the convict-era mating market and generating long-running cultural effects that persist to the present day. We test whether convict-era sex ratios have altered marital and overall life satisfaction today, through their persistent effects on gender nor...

We argue that the CLASH model makes a number of questionable assumptions about the harshness and unpredictability of low-latitude environments, calling into question the life history strategy approach used, and that it is inconsistent with more nuanced global patterns of violence. We suggest an alternative account for less violence at high latitude...

Prevailing environmental factors influence preferences for attractive traits across many species. In humans, debate surrounds the role of environmental pathogens and economic development in determining facial attractiveness. We tested whether women and men's preferences for facial dimorphism, symmetry, skin tone, and adiposity differ among Melanesi...

Males and females in many species differ in how they age and how long they live. These differences have motivated much research, concerning both their evolution and the underlying mechanisms that cause them. We review how differences in male and female life histories have evolved to shape patterns of aging and some of the mechanisms and pathways in...

Facial hair is a prominent secondary sexual trait, particularly given the importance of the face in interpersonal communication. Bizarrely by animal standards, men expend considerable effort every day trimming, waxing or shaving this androgen-dependent trait. Why some men shave this cue of masculinity off, and why women's preferences for facial hai...

In many species, male secondary sexual traits have evolved via female choice as they confer indirect (i.e. genetic) benefits or direct benefits such as enhanced fertility or survival. In humans, the role of men's characteristically masculine androgen-dependent facial traits in determining men's attractiveness has presented an enduring paradox in st...

Fluctuating sexual selection caused by environmental heterogeneity can maintain variation in sexual signals. Sexual selection can also shape correlations among behavioral traits (behavioral syndromes) when certain behavioral combinations enjoy greater fitness than other combinations (i.e., under correlational sexual selection). Here, we tested the...

Sexual reproduction in animals requires close interactions with the opposite sex. These interactions may generate costs of reproduction, because mates can induce detrimental physiological or physical effects on one another, due to their interest in maximising their own fitness. To understand how a male’s presence influences aspects of female physio...

Human mate choice research often concerns sex differences in the importance of traits such as physical attractiveness and social status. A growing number of studies indicate that cues to social context, including other people who appear in stimulus photographs, can alter that individual's attractiveness. Fewer studies, however, consider judgements...

Excel spreadsheet containing data used in experiments. The spreadsheet contains the data used to perform the analysis. (XLSX)

MLMs for rated attractiveness of male and female models. The final MLMs of rated attractiveness of male and female models by men and women. (DOCX)

MLMs for rated earnings for target male and female participants. The final MLMs of rated earnings of male and female models by men and women. (DOCX)

Participant Information Sheet. Information about the study. This was presented to participants before they agreed to start the study. (DOCX)

Behavioural correlations between mating behaviour and antipredator behaviour are expected when sexual behaviour increases predation risk. However, the correlation would be different between males and females, particularly when the sexes experience different levels of predation risk. Here, we tested this idea using a water strider species, Gerris gr...

The post-natal environment in which young develop can substantially impact development, adult phenotype, and fitness. In wild mice, competition among litter-mates affects development rate and adult behaviour. We manipulated post-natal litter size in a cross-fostering design to investigate the effects of enlarged and reduced litter sizes on sexual s...

A variety of hypotheses have been suggested to explain the evolution of same-sex sexual behaviour (SSB) in male insects. Males may be poor at discriminating other males from females, inexperienced, or may need to learn how to distinguish females. Alternatively SSB could be a by-product of a plastic reproductive strategy to succeed in scramble compe...

Environmental factors, such as pathogen prevalence and resource scarcity, are thought to influence mate preferences for traits related to health and resource provisioning potential. Specific body dimensions, such as women's waist-to-hip-ratio (WHR), men's shoulder-to-hip ratio (SHR), and body mass index (BMI) have also been theorised to be associat...

Many studies have attempted to account for variation in male reproductive success by quantifying a single trait such as an ornament or a behavior, but male reproductive performance may be determined by a number of interacting traits. Although developmental nutrition is often a major determinant of adult body size and secondary sexual trait expressi...

Human bodies exemplify complex phenotypes, likely to be subject to complex evolutionary forces. Despite the importance of body shape to health, social interactions and self-esteem, our understanding of body evolution and integration remains simplistically focused on simple ratios like waist-hip ratio (WHR), and body mass index (BMI), or manipulatio...

Male mammals typically have shorter lifespans than females [1]. Sex differences in survival may result, in part, from sex-specific optima in investment in reproduction, with higher male mortality rates from sexual competition selecting for a "live-fast die-young" strategy in this sex [2]. In the wild, lifespan is also influenced by environmental co...

Male-male aggression can have a large influence on access to mates, particularly in highly territorial animals such as mice. It has been suggested that males with impaired antioxidant defence and a consequential increased susceptibility to oxidative stress may have a reduced ability to invest in aggressive behaviours, which could limit their mating...

Both juvenile (E j ) and adult (E a ) environment can alter developmental trajectories, independently or interactively (as environment by environment interaction; E j × E a ), to shape behaviour in later life. Within a population, however, the developmental response of behaviours to environments can vary among genotypes (G × E × E, multidimensional...

Significance Oxidative stress is expected to restrict investment in life history traits, including sexual signals used to attract mates. However, evidence supporting this prediction remains equivocal. We utilized superoxide dismutase knockout mice to provide, to our knowledge, the first direct evidence that oxidative stress impairs investment in im...

Human mate choice is complicated, with various individual differences and contextual factors influencing preferences for numerous traits. However, focused studies on human mate choice often do not capture the multivariate complexity of human mate choice. Here, we consider multiple factors simultaneously to demonstrate the advantages of a multivaria...

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 face...

Abstract Behavioral traits often change over an individual's lifetime. Experience, physiological senescence, and age-dependent differences in optimal behavior can, in theory, all cause longitudinal behavioral changes. Yet most studies of behavioral plasticity and selection on behavior focus on short-term population-level responses to social factors...

An individual’s fitness depends not only on their phenotype but also on the phenotypes of their competitors and contemporaries. Sexual attractiveness may be strongly influenced by an individual’s familiarity to potential mates or the rarity of the individual’s phenotype. Such effects can cause negative frequency-dependent selection, maintaining str...

Males from different populations of the same species often differ in their sexually selected traits. Variation in sexually selected traits can be attributed to sexual selection if phenotypic divergence matches the direction of sexual selection gradients among populations. However, phenotypic divergence of sexually selected traits may also be influe...

Interpopulation variance–covariance matrix among-population means (D) for the seven male traits for the six natural populations of Poecilia reticulata. Table S2 Interpopulation variance–covariance matrix of directional sexual selection gradients (B) acting on the seven male traits for the six natural populations of Poecilia reticulata.

Linear selection gradients for each male trait in the feral guppy populations in North Queensland: Alligator Creek (‘Ack’), Big Crystal Creek (‘Crc’), Mena Creek (‘Mnc’), Millaa Millaa Falls (‘Mlm’), Mulgrave River (‘Ulg’), Wadda Creek (‘Wdd’).

Conflicts between family members are expected to influence the duration and intensity of parental care. In mammals, the majority of this care occurs as resource transfer from mothers to offspring during gestation and lactation. Mating systems can have a strong influence on the severity of familial conflict - where female promiscuity is prevalent, c...

The trade-off between reproductive investment and lifespan is the single most important concept in life-history theory. A variety of sources of evidence support the existence of this trade-off, but the physiological costs of reproduction that underlie this relationship remain poorly understood. The Free Radical Theory of Ageing suggests that oxidat...

Correlated suites of behaviours, or behavioural syndromes, appear to be widespread, and yet few studies have explored how they arise and are maintained. One possibility holds that correlational selection can generate and maintain behavioural syndrome if certain behavioural combinations enjoy greater fitness than other combinations. Here we test thi...

In scramble competition mating systems, individual behaviours related to male–male competition are expected to be plastic across sociosexual environments. However, the evolutionary forces that shape individual differences in behavioural plasticity remain poorly understood. We measured behaviour associated with scramble competition in male water str...

In mammals, allocation to reproduction can either be primed or suppressed in relation to cues from other individuals. Some conspecifics (e.g. potential mates) may enhance an individual's ability to reproduce but others may have a detrimental effect on reproductive success. One widely studied response to conspecific cues, the 'Bruce effect', occurs...

Oxidative stress, an over-production of reactive oxygen species (ROS) in relation to defence mechanisms, is thought to be a major cause of male infertility. To help protect against the deleterious effects of ROS, animals have a variety of enzymatic antioxidants that reduce these molecules to less reactive forms. The physiological role of these anti...

Mate choice often depends on the properties of both sexes, such as the preference and responsiveness of the female and the sexual display traits of the male. Quantitative genetic studies, however, traditionally explore the outcome of an interaction between males and females based solely on the genotype of one sex, treating the other sex as a source...

Facial hair strongly influences people's judgments of men's socio-sexual attributes. However, the nature of these judgments is often contradictory. The levels of intermediate facial hair growth presented to raters and the stage of female raters' menstrual cycles might have influenced past findings. We quantified men's and women's judgments of attra...

Avoiding disease and acquiring resources have been recurrent challenges throughout human evolution. These abilities are particularly relevant to mate preferences, as pathogens and resources can both be transferred between mates and to mutual offspring. Based on 689 participants’ attractiveness ratings of manipulated online dating profiles, we teste...

Reproduction imposes significant costs and is characterized by an increased energy demand. As a consequence, individuals adjust their cellular structure and function to this physiological constraint. Because mitochondria are central to energy production, changes in their functional properties are likely to occur during reproduction. Such changes co...

Life history theory suggests that investment in reproduction can trade off against growth, longevity and both reproduction and performance later in life. One possible reason for this trade-off is that reproduction directly causes somatic damage. Oxidative stress, an overproduction of reactive oxygen species in relation to cellular defences, can cor...

Few mammalian organs vary as dramatically among species as the placenta. This variation is remarkable considering that the placenta's primary function-transfer of nutrients and waste between mother and offspring-does not differ among species. Evolutionary changes in placental morphology remain poorly understood, with suggestions that parent-offspri...

Phenotypic integration and plasticity are central to our understanding of how complex phenotypic traits evolve. Evolutionary change in complex quantitative traits can be predicted using the multivariate breeders' equation, but such predictions are only accurate if the matrices involved are stable over evolutionary time. Recent study, however, sugge...

Sexual conflict theory suggests that male and female interests often diverge, causing selection to favour sex‐specific reproductive strategies that maximize the fitness of an individual at the expense of its mate. Sexually antagonistic selection can lead to conflicts over the timing and frequency of mating events, mate choice and the delivery of pa...

In many parts of Asia, the Middle East and North Africa, women and children are so undervalued, neglected, abused, and so often killed, that sex ratios are now strongly male biased. In recent decades, sex-biased abortion has exacerbated the problem. In this article I highlight several important insights from evolutionary biology into both the origi...

1. The global transfer of species by human vectors is continuing despite the use of managerial controls such as antifouling biocides and pesticide applications. The process of introduction now exposes species to novel conditions which may select for tolerance to a contaminant. Invader establishment success is influenced by both the supply of invasi...

Senescence is shaped by age-dependent trade-offs between fitness components. Because males and females invest different resources in reproduction, the trade-offs behind age-dependent reproductive effort should be resolved differently in the sexes. In this study, we assess the effects of diet (high carbohydrate and low protein vs. equal carbohydrate...

The global spread of invasive species may be facilitated by adaptation to the practices that humans use to manage those species. For example, marine invertebrates that adapt to metal-based antifouling biocides on ship hulls may be more likely to be introduced to and establish in metal-polluted environments. We tested this idea by studying clonal va...

The links between fitness, health, sexual signals and mate choice are complex and subject to ongoing study. In 1999, von Schantz et al. made the valuable suggestion that oxidative stress may be an important missing piece of this complex puzzle. Their suggestion has been enthusiastically tested, with over 300 studies citing their paper, but most eff...

Numts are an integral component of many eukaryote genomes offering a snapshot of the evolutionary process that led from the incorporation of an α-proteobacterium into a larger eukaryotic cell some 1.8 billion years ago. Although numt sequence can be harnessed as molecular marker, these sequences often remain unidentified and are mistaken for genuin...

Result of sliding window analysis. Number of segregating sites per 200 bases (step size: 50 bases). (TIFF)

Amino acid alignment of 8 orthopteran, one coleopteran and three dipteran species. The scoring scheme ranges from 0 for the least conserved alignment position, up to 10 for the most conserved alignment position. (PDF)

Test of general applicability. Presence of Mitochondrial and Nuclear DNA in Serial Dilution Series' T-DNA I & T-DNA II in genomic DNA extract from blood of the Australian Dingo. T-DNA I was used as template in A–B and T-DNA II in C-D. The highest dilution step in which the presence of nuclear DNA was revealed was step 3 (10−2, B) and step 4 (10−3)...

Initial amplification and sequencing using a range of conserved and newly designed primers resulted in the co-amplification of numt sequences. The presence of numt sequences was discovered after alignments revealed multiple mismatches between overlapping fragments (Figure S1A). Fragments revealing consistently a high number of ‘heteroplasmic’ sites...

Amino acid alignment of 8 orthopteran, one coleopteran and three dipteran species and secondary structure prediction. Secondary structure for each sequence is represented by a color. Secondary structure for each sequence is represented by a colour. If a sequence in the alignment has no colours assigned, this means that either there is no DSSP infor...

Primer sequences for the amplification and subsequent sequencing. (XLSX)

Pair-wise comparison of ND3 sequences between 8 ensiferan, one coleopteran and three dipteran species. Values identify the number of identical nucleotides between single pairs [%]. (XLS)

Pair-wise comparison of ND3 sequences between 8 ensiferan, one coleopteran and three dipteran species. Values identify the number of identical amino acids between single pairs [%]. (XLS)

The juvenile environment provides numerous cues of the intensity of competition and the availability of mates in the near environment. As research demonstrates that the developing individuals can use these cues to alter their developmental trajectories, and therefore, adult phenotypes, we examined whether social cues available during development ca...

Condition-dependent sexually selected traits are thought to indicate an individual’s quality or breeding value for fitness. Variation in developmental environments, however, introduces much complexity to resource allocation, and therefore, to phenotypic expression. The extent to which environment-specific developmental tactics interact with resourc...

Phenotypically plastic mating behavior may allow males to modify their reproductive behavior to suit the prevailing social conditions, but we do not know if males only react to immediate social stimuli or change their inherent mate preferences according to their social history. Here we examine the effect of social experiences on the subsequent repr...

Evolutionary theories of aging state that the force of natural selection declines with age, resulting in trait senescence. However, sexual selection theory predicts that costly traits that signal mate value should increase in expression as survival prospects decline. Mortality rates and fertility tend to show strong signatures of senescence, wherea...

Selection has led to the evolution of a variety of different mating strategies, each adapted to different competitive challenges. But what happens if the competitive challenges depend on the social environment? Here we discuss and review examples of socially cued anticipatory plasticity: irreversible developmental tactics in which resource allocati...