The neural mechanisms of mate choice: a hypothesis

Scientists have described many physical and behavioral traits in avian and mammalian species that evolved to attract mates. But the brain mechanisms by which conspecifics become attracted to these traits is unknown. This paper maintains that two aspects of mate choice evolved in tandem: 1) traits that evolved in the "display producer" to attract mates and, 2) corresponding neural mechanisms in the "display chooser" that enable them to become attracted to these display traits. Then it discusses our (in-progress) fMRI brain scanning project on human romantic attraction, what we believe is a developed form of "courtship attraction" common to avian and mammalian species as well as the primary neural mechanism underlying avian and mammalian mate choice. The paper hypothesizes that courtship attraction is associated with elevated levels of central dopamine and norepinephrine and decreased levels of central serotonin in reward pathways of the brain. It also proposes that courtship attraction is part of a triune brain system for mating, reproduction and parenting. 1)The sex drive evolved to motivate birds and mammals to court any conspecifics. 2) The attraction system evolved to enable individuals to discriminate among potential mating partners and focus courtship activities on particular individuals, thereby conserving mating time and energy. 3) The neural circuitry for attachment evolved to enable individuals to complete species-specific parental duties.     

The empirical question of thresholds and mechanisms of mate choice

Summary Theoretical discussions concerning how animals might best sample and select mates have suggested that individuals could base decisions either on a sample of mates (sampled-based decisions) or on a threshold of comparison (threshold-based decisions). Recent theoretical work demonstrates that threshold-based mating decisions generate higher expected fitness than sample-based mating decisions when search costs exist. Empirical results from most unmanipulated systems, however, either conclude that females make sample-based decisions or are inconclusive. A few experimental studies designed to detect mating thresholds purport to demonstrate threshold-based choice but an examination of these studies indicates such conclusions were premature. We believe that few examples of threshold-based choice exist because protocols designed to identify mating thresholds were often inconsistent with models of threshold choice. We suggest that future empirical work strive not to document mating thresholdsper se. Rather, future work might best reveal decision rules by manipulating the distribution of quality among potential mates; such manipulations predict uniquely how females using sample-based and threshold-based decision rules should behave.     

Alternative hypotheses linking the immune system and mate choice for good genes

Why do males often have extravagant morphological and behavioural traits, and why do females prefer to mate with such males? The answers have been the focus of considerable debate since Darwin''s The descent of man, and selection in relation to sex appeared in 1871. Recently the broadening of investigation to include fields outside evolutionary biology has shed new light on mate choice and sexual selection. Here, we focus on a specific set of hypotheses relating the biology of resisting disease-causing organisms with the production of condition-dependent sexual signals (advertisements). We present a framework that distinguishes three different hypotheses about trade-offs within the immune system that affect general condition. Hamilton and Zuk''s original hypothesis suggests that hosts fight off disease through resistance to particular pathogens, which consequently lowers resistance to other pathogens. Changes in pathogens over evolutionary time in turn favours changes in which genes confer the best resistance. Alternatively, the immunocompetence hypotheses suggests that the energetic costs of mounting a response to any pathogen compete for resources with other things, such as producing or maintaining advertisements. Finally, improving resistance to pathogens could increase the negative impacts of the immune system on the host, via immunopathologies such as allergies or autoimmune diseases. If both disease and immunopathology affect condition, then sexual advertisements could signal a balance between the two. Studies of hypothesized links between genes, condition, the immune system and advertisements likely will require careful consideration of which hypothesis is being considered, and may necessitate different measures of immune system responses and different experimental protocols.     

Consistency of mate choice in eye and hair colour: Testing possible mechanisms

Partner preferences are formed by several mechanisms, including an imprinting-like effect (parent-similarity) and homogamy (self-similarity). It is still unknown, however, whether these preferences remain stable throughout an individual’s lifetime. We have therefore tested the consistency of mate choice in eye and hair colour both in a shortand long-term context. In other words, we tested whether people systematically choose partners with a particular eye and hair colour. We asked 1,048 respondents to indicate the eye and hair colour of themselves, their opposite-sex and same-sex parent, and all the romantic partners they had in their lives. Our results show that people consistently choose partners of a particular eye and hair colour in both short- and long-term contexts, which suggests that people do have their ‘types’. Nevertheless, the consistency was significantly higher in a long-term context than in a short-term context. Furthermore, the eye colour of one's partner was predicted by the eye colour of one's opposite-sex as well as same-sex parent, but the strongest parental effect was found when both parents had same eye colour. There were no significant results for hair colour. Our results thus suggest that preferences for eye colour are determined by the imprinting-like effect rather than by homogamy, and that they remain stable over time. These findings also indirectly support an assumption of stability of this imprinting-like effect in humans, since people consistently choose partners with their opposite-sex parent's eye-colour.     

Sexual selection and mate choice

The past two decades have seen extensive growth of sexual selection research. Theoretical and empirical work has clarified many components of pre- and postcopulatory sexual selection, such as aggressive competition, mate choice, sperm utilization and sexual conflict. Genetic mechanisms of mate choice evolution have been less amenable to empirical testing, but molecular genetic analyses can now be used for incisive experimentation. Here, we highlight some of the currently debated areas in pre- and postcopulatory sexual selection. We identify where new techniques can help estimate the relative roles of the various selection mechanisms that might work together in the evolution of mating preferences and attractive traits, and in sperm-egg interactions.     

Stress hormones and mate choice

A few recent studies have suggested that glucocorticoid stress hormones can play a role in sexual selection. In terms of mate choice, these studies have shown that individuals can exhibit preferences for mates with either low baseline or peak glucocorticoid levels. This appears to occur because stress hormones can be key mediators of many condition-dependent, sexually selected traits that serve as honest signals of mate quality. We suggest that our understanding of how sexual selection can influence the glucocorticoid stress response will be expanded by studying the interactions among glucocorticoid stress hormones, sex steroids and body condition.     

Sex roles and mutual mate choice matter during mate sampling

The roles of females and males in mating competition and mate choice have lately proven more variable, between and within species, than previously thought. In nature, mating competition occurs during mate search and is expected to be regulated by the numbers of potential mates and same-sex competitors. Here, we present the first study to test how a temporal change in sex roles affects mating competition and mate choice during mate sampling. Our model system (the marine fish Gobiusculus flavescens) is uniquely suitable because of its change in sex roles, from conventional to reversed, over the breeding season. As predicted from sex role theory, courtship was typically initiated by males and terminated by females early in the breeding season. The opposite pattern was observed late in the season, at which time several females often simultaneously courted the same male. Mate-searching females visited more males early than late in the breeding season. Our study shows that mutual mate choice and mating competition can have profound effects on female and male behavior. Future work needs to consider the dynamic nature of mating competition and mate choice if we aim to fully understand sexual selection in the wild.     

Visual mate choice in poison frogs

We investigated female mate choice on the basis of visual cues in two populations of Dendrobates pumilio, the strawberry poison frog, from the Bocas del Toro Archipelago in Panama, Central America. Mate choice experiments were carried out by presenting subject females of each of two morphs of this species (orange and green) from two different island populations (Nancy Key and Pope Island) with object frogs (one of each morph) under glass at one end of a terrarium. Recorded calls were played simultaneously from behind both object frogs. The experiments were carried out under two light regimes: (i) white light, and (ii) relatively monochromatic filtered blue light. Subject females from each population displayed a significant preference for their own morph under white light, but not under blue light. These results indicate that female D. pumilio use visual cues in mate choice, and suggest that colour may be the visual cue they use.     

Sexual imprinting in human mate choice

Animal and human studies have shown that individuals choose mates partly on the basis of similarity, a tendency referred to as homogamy. Several authors have suggested that a specific innate recognition mechanism, phenotypic matching, allows the organism to detect similar others by their resemblance to itself. However, several objections have been raised to this theory on both empirical and theoretical grounds. Here, we report that homogamy in humans is attained partly by sexual imprinting on the opposite-sex parent during childhood. We hypothesized that children fashion a mental model of their opposite-sex parent's phenotype that is used as a template for acquiring mates. To disentangle the effects of phenotypic matching and sexual imprinting, adopted daughters and their rearing families were examined. Judges found significant resemblance on facial traits between daughter's husband and her adoptive father. Furthermore, this effect may be modified by the quality of the father-daughter relationship during childhood. Daughters who received more emotional support from their adoptive father were more likely to choose mates similar to the father than those whose father provided a less positive emotional atmosphere.     

Indirect mate choice, direct mate choice and species recognition in a bower-building cichlid fish lek

Sexual selection arising through female mate choice typically favours males with larger, brighter and louder signals. A critical challenge in sexual selection research is to determine the degree to which this pattern results from direct mate choice, where females select individual males based on variation in signalling traits, or indirect mate choice, where male competition governs access to reproductively active females. We investigated female mate choice in a lekking Lake Malawi cichlid fish, Hemitilapia oxyrhynchus, in which males build and aggressively defend sand 'bowers'. Similar to previous studies, we found that male reproductive success was positively associated with bower height and centrality on the lek. However, this pattern resulted from males holding these territories encountering more females, and thus their greater success was due to indirect mate choice. Following initial male courtship, an increase in the relative mating success of some males was observed, but this relative increase was unrelated to bower size or position. Crucially, experimentally manipulating bowers to resemble those of a co-occurring species had no appreciable effect on direct choice by females or male spawning success. Together, these results suggest indirect mate choice is the dominant force determining male-mating success in this species, and that bowers are not signals used in direct mate choice by females. We propose that, in this species, bowers have a primary function in intraspecific male competition, with the most competitive males maintaining larger and more central bowers that are favoured by sexual selection due to higher female encounter rates.     

Advancing mate choice studies in salmonids

Reviews in Fish Biology and Fisheries - Mate choice in most organisms is not random, but determined by a suite of interacting traits and environmental factors. While the selective pressures...     

Hybrid zone maintenance by non-adaptive mate choice

In a spatial simulation we show that formation of a stable, narrow hybrid zone requires no selection, only that individuals can recognize their own population type since this is a prerequisite for assortative mating. The European crow occurs in two species/subspecies that meet in a long hybrid zone, the black carrion crow Corvus corone and the grey and black hooded crow C. cornix. In a previous study (Brodin A, Haas F (2006) Anim Behav 72:139) we mimicked sexual imprinting in nestling crows with artificial neural networks, using a learning process that simulates proximate retina perception. The networks were trained on 700 images to recognize either carrion, hooded or hybrid phenotypes as their own. After training the networks were exposed to 300 new, unfamiliar pictures of crows. The networks that had been trained on pure subspecies then showed strong preference for their own type. Networks trained on hybrid crows showed weaker preference for their own type. Assuming that these preferences will determine mate choice preferences we have combined them with empirical data on dispersal of young crows to investigate what kind of geographical distribution pattern this would create. We then assume that the two subspecies meet and hybridize along a straight border. In only 60 generations a stable, narrow hybrid zone that resembles the real hybrid zone was formed. This zone remained stable over time. With minor adjustments of parameters the results would not only fit the width but also the shapes of the clines in the field. An erratum to this article can be found at