Evolution of identity signals: frequency-dependent benefits of distinctive phenotypes used for individual recognition

Evidence for a selective link between cooperation and individual recognition

The ability to recognize others is a frequent assumption of models of the evolution of cooperation. At the same time, cooperative behavior has been proposed as a selective agent favoring the evolution of individual recognition abilities. Although theory predicts that recognition and cooperation may co-evolve, data linking recognition abilities and cooperative behavior with evidence of selection are elusive. Here, we provide evidence of a selective link between individual recognition and cooperation in the paper wasp Polistes fuscatus through a combination of clinal, common garden, and population genomics analyses. We identified latitudinal clines in both rates of cooperative nesting and color pattern diversity, consistent with a selective link between recognition and cooperation. In behavioral experiments, we replicated previous results demonstrating individual recognition in cooperative and phenotypically diverse P. fuscatus from New York. In contrast, wasps from a less cooperative and phenotypically uniform Louisiana population showed no evidence of individual recognition. In a common garden experiment, groups of wasps from northern populations formed more stable and individually biased associations, indicating that recognition facilitates group stability. The strength of recent positive selection on cognition-associated loci likely to mediate individual recognition is substantially greater in northern compared with southern P. fuscatus populations. Collectively, these data suggest that individual recognition and cooperative nesting behavior have co-evolved in P. fuscatus because recognition helps stabilize social groups. This work provides evidence of a specific cognitive phenotype under selection because of social interactions, supporting the idea that social behavior can be a key driver of cognitive evolution.

'Do I know you?' Categorizing individuals on the basis of familiarity in kea (Nestor notabilis)

Categorizing individuals on the basis of familiarity is an adaptive way of dealing with the complexity of the social environment. It requires the use of conceptual familiarity and is considered higher order learning. Although, it is common among many species, ecological need might require and facilitate individual differentiation among heterospecifics. This may be true for laboratory populations just as much as for domesticated species and those that live in urban contexts. However, with the exception of a few studies, populations of laboratory animals have generally been given less attention. The study at hand, therefore, addressed the question whether a laboratory population of kea parrots (Nestor notabilis) were able to apply the concept of familiarity to differentiate between human faces in a two-choice discrimination task on the touchscreen. The results illustrated that the laboratory population of kea were indeed able to differentiate between familiar and unfamiliar human faces in a two-choice discrimination task. The results provide novel empirical evidence on abstract categorization capacities in parrots while at the same time providing further evidence of representational insight in kea.

Social experience drives the development of holistic face processing in paper wasps

Most recognition is based on identifying features, but specialization for face recognition in some taxa relies on a different mechanism, termed 'holistic processing' where facial features are bound together into a gestalt which is more than the sum of its parts. Although previous work suggests that extensive experience may be required for the development of holistic processing, we lack experiments that test how age and experience interact to influence holistic processing. Here, we test how age and experience influence the development of holistic face processing in Polistes fuscatus paper wasps. Previous work has shown that P. fuscatus use facial patterns to individually identify conspecifics and wasps use holistic processing to discriminate between conspecific faces. We tested face processing in three groups of P. fuscatus: young (1-week-old), older, experienced (2-weeks-old, normal experience), and older, inexperienced (2-weeks-old, 1 week normal social experience and 1 week social isolation). Older, experienced wasps used holistic processing to discriminate between conspecific faces. In contrast, older inexperienced wasps used featural rather than holistic mechanisms to discriminate between faces. Young wasps show some evidence of holistic face processing, but this ability was less refined than older, experienced wasps. Notably, wasps only required 2 weeks of normal experience to develop holistic processing, while previous work suggests that humans may require years of experience. Overall, P. fuscatus wasps rapidly develop holistic processing for conspecific faces. Experience rather than age facilitates the transition between featural and holistic face processing mechanisms.

Color is necessary for face discrimination in the Northern paper wasp, Polistes fuscatus

Visual individual recognition requires animals to distinguish among conspecifics based on appearance. Though visual individual recognition has been reported in a range of taxa including primates, birds, and insects, the features that animals require to discriminate between individuals are not well understood. Northern paper wasp females, Polistes fuscatus, possess individually distinctive color patterns on their faces, which mediate individual recognition. However, it is currently unclear what role color plays in the facial recognition system of this species. Thus, we sought to test two possible roles of color in wasp facial recognition. On one hand, color may be important simply because it creates a pattern. If this is the case, then wasps should perform similarly when discriminating color or grayscale images of the same faces. Alternatively, color itself may be important for recognition of an image as a "face", which would predict poorer performance on grayscale discrimination relative to color images. We found wasps performed significantly better when discriminating between color faces compared to grayscale versions of the same faces. In fact, wasps trained on grayscale faces did not perform better than chance, indicating that color is necessary for the recognition of an image as a face by the wasp visual system.

Individual recognition and long-term memory of inanimate interactive agents and humans in dogs

Investigation of individual recognition (IR) is difficult due to the lack of proper control of cues and previous experiences of subjects. Utilization of artificial agents (Unidentified Moving Objects: UMOs) may offer a better approach than using conspecifics or humans as partners. In Experiment 1, we investigated whether dogs are able to develop IR of UMOs (that is stable for at least 24 h) or that they only retain a more generalised memory about them. The UMO helped dogs to obtain an unreachable ball and played with them. One day, one week or one month later, we tested whether dogs display specific behaviour toward the familiar UMO over unfamiliar ones (four-way choice test). Dogs were also re-tested in the same helping context and playing interaction. Subjects did not approach the familiar UMO sooner than the others; however, they gazed at the familiar UMO earlier during re-testing of the problem solving task, irrespectively of the delay. In Experiment 2, we repeated the same procedure with human partners, applying a two-way choice test after a week delay, to study whether lack of IR was specific to the UMO. Dogs did not approach the familiar human sooner than the unfamiliar, but they gazed at the familiar partner earlier during re-testing. Thus, dogs do not seem to recognise an individual UMO or human after a short experience, but they remember the interaction with the novel partner in general, even after a long delay. We suggest that dogs need more experience with a specific social partner for the development of long-term memory.

Dynamic neurogenomic responses to social interactions and dominance outcomes in female paper wasps

Social interactions have large effects on individual physiology and fitness. In the immediate sense, social stimuli are often highly salient and engaging. Over longer time scales, competitive interactions often lead to distinct social ranks and differences in physiology and behavior. Understanding how initial responses lead to longer-term effects of social interactions requires examining the changes in responses over time. Here we examined the effects of social interactions on transcriptomic signatures at two times, at the end of a 45-minute interaction and 4 hours later, in female Polistes fuscatus paper wasp foundresses. Female P. fuscatus have variable facial patterns that are used for visual individual recognition, so we separately examined the transcriptional dynamics in the optic lobe and the non-visual brain. Results demonstrate much stronger transcriptional responses to social interactions in the non-visual brain compared to the optic lobe. Differentially regulated genes in response to social interactions are enriched for memory-related transcripts. Comparisons between winners and losers of the encounters revealed similar overall transcriptional profiles at the end of an interaction, which significantly diverged over the course of 4 hours, with losers showing changes in expression levels of genes associated with aggression and reproduction in paper wasps. On nests, subordinate foundresses are less aggressive, do more foraging and lay fewer eggs compared to dominant foundresses and we find losers shift expression of many genes in the non-visual brain, including vitellogenin, related to aggression, worker behavior, and reproduction within hours of losing an encounter. These results highlight the early neurogenomic changes that likely contribute to behavioral and physiological effects of social status changes in a social insect.

How visual system configuration can play a role in individual recognition: a visual modeling study

Many species rely on individual recognition (i.e., the use of individual signals to identify and remember a conspecific) to tune their social interactions. However, little is known about how the configuration of the sensory system may affect the perception of individual recognition signals over space. Utilizing a visual modeling approach, we quantified (1) the threshold distance between the receiver and the signaler at which individual recognition can no longer accurately occur, and (2) the regions of the head most likely to contain the individual recognition signals. We used chickens (Gallus gallus) as our study species, as they use visual individual recognition and additionally have a well-studied visual system. We took pictures of different individuals and followed a visual modeling approach considering color vision, visual acuity, and pattern processing of the receiver. We found that distance degrades the quality of information in potential individual recognition signals. We estimated that the neighbor distance at which a receiver may have difficulty recognizing a conspecific was between 0.25 and 0.30 m in chickens, which may be related to a decrease in available features of the potential signal. This signal perception threshold closely matches the recognition distance predicted by previous behavioral approaches. Additionally, we found that certain regions of the head (beak, cheek, comb, eye) may be good candidates for individual recognition signals. Overall, our findings support that recognition in chickens occurs at short distances due to constraints imposed by their visual system, which can affect the costs and benefits associated with social spacing in groups.

Individual recognition is associated with holistic face processing in Polistes paper wasps in a species-specific way

Most recognition is based on identifying features, but specialization for face recognition in primates relies on a different mechanism, termed 'holistic processing' where facial features are bound together into a gestalt which is more than the sum of its parts. Here, we test whether individual face recognition in paper wasps also involved holistic processing using a modification of the classic part-whole test in two related paper wasp species: Polistes fuscatus, which use facial patterns to individually identify conspecifics, and Polistes dominula, which lacks individual recognition. We show that P. fuscatus use holistic processing to discriminate between P. fuscatus face images but not P. dominula face images. By contrast, P. dominula do not rely on holistic processing to discriminate between conspecific or heterospecific face images. Therefore, P. fuscatus wasps have evolved holistic face processing, but this ability is highly specific and shaped by species-specific and stimulus-specific selective pressures. Convergence towards holistic face processing in distant taxa (primates, wasps) as well as divergence among closely related taxa with different recognition behaviour (P. dominula, P. fuscatus) suggests that holistic processing may be a universal adaptive strategy to facilitate expertise in face recognition.

Ecological and social drivers of neighbor recognition and the dear enemy effect in a poison frog

Navigating social relationships frequently rests on the ability to recognize familiar individuals using phenotypic characteristics. Across diverse taxa, animals vary in their capacities for social recognition, but the ecological and social sources of selection for recognition are often unclear. In a comparative study of two closely related species of poison frogs, we identified a species difference in social recognition of territory neighbors and investigated potential sources of selection underlying this difference. In response to acoustic playbacks, male golden rocket frogs (Anomaloglossus beebei) recognized the calls of neighbors and displayed a “dear enemy effect” by responding less aggressively to neighbors’ calls than strangers’ calls. In contrast, male Kai rocket frogs (Anomaloglossus kaiei) were equally aggressive to the calls of neighbors and strangers. This species difference in behavior is associated with key differences in reproductive ecology and characteristics of territories. Golden rocket frogs defend reproductive resources in the form of bromeliads, which is expected to create a threat asymmetry between neighbors and strangers favoring decreased aggression to neighbors. In contrast, Kai rocket frogs do not defend reproductive resources. Further, compared with Kai rocket frog territories, golden rocket frog territories occur at higher densities and are defended for longer periods of time, creating a more complex social environment with more opportunities for repeated but unnecessary aggression between neighbors, which should favor the ability to recognize and exhibit less aggression toward neighbors. These results suggest that differences in reproductive ecology can drive changes in social structure that select for social recognition.

Coevolution of cognitive abilities and identity signals in individual recognition systems

Social interactions are mediated by recognition systems, meaning that the cognitive abilities or phenotypic diversity that facilitate recognition may be common targets of social selection. Recognition occurs when a receiver compares the phenotypes produced by a sender with a template. Coevolution between sender and receiver traits has been empirically reported in multiple species and sensory modalities, though the dynamics and relative exaggeration of traits from senders versus receivers have received little attention. Here, we present a coevolutionary dynamic model that examines the conditions under which senders and receivers should invest effort in facilitating individual recognition. The model predicts coevolution of sender and receiver traits, with the equilibrium investment dependent on the relative costs of signal production versus cognition. In order for recognition to evolve, initial sender and receiver trait values must be above a threshold, suggesting that recognition requires some degree of pre-existing diversity and cognitive abilities. The analysis of selection gradients demonstrates that the strength of selection on sender signals and receiver cognition is strongest when the trait values are furthest from the optima. The model provides new insights into the expected strength and dynamics of selection during the origin and elaboration of individual recognition, an important feature of social cognition in many taxa. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Vive la difference! Self/non-self recognition and the evolution of signatures of identity in arms races with parasites

In arms races with parasites, hosts can evolve defences exhibiting extensive variability within populations, which signals individual identity ('signatures'). However, few such systems have evolved, suggesting that the conditions for their evolution are uncommon. We review (a) polymorphic egg markings that allow hosts of brood-parasitic birds to recognize and reject parasitic eggs, and (b) polymorphic tissue antigens encoded in the major histocompatibility complex (MHC), which present self- and pathogen-derived peptides to T cells of the immune system. Despite the profound differences between these systems, they share analogous features: (i) self/non-self discrimination by a highly specific recognition system (bird eyes and T-cell antigen receptor, respectively), which antagonists may escape by evolving evasion or mimicry; (ii) a self substrate upon which diversifying selection can act (eggs, and MHC molecules); (iii) acquired knowledge of self (resulting in acceptance of own eggs, and immune tolerance); and (iv) fitness costs associated with attack on self or lack of parasite detection. We suggest that these features comprise a set of requirements for parasites to drive the evolution of identity signatures in hosts, which diminish the likelihood of recognition errors. This may help to explain the variety of trajectories arising from arms races in different antagonistic contexts. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Signal function drives phenotypic and genetic diversity: the effects of signalling individual identity, quality or behavioural strategy

Animal coloration is influenced by selection pressures associated with communication. During communication, signallers display traits that inform receivers and modify receiver behaviour in ways that benefit signallers. Here, we discuss how selection on signallers to convey different kinds of information influences animal phenotypes and genotypes. Specifically, we address the phenotypic and genetic consequences of communicating three different kinds of information: individual identity, behavioural strategy and quality. Previous work has shown signals that convey different kinds of information differ in terms of the (i) type of selection acting on signallers (e.g. directional, stabilizing, or negative frequency dependent), and (ii) developmental basis of signals (i.e. heritability, genetic architecture). These differences result in signals that convey different information having consistently different phenotypic properties, including the amount, modality and continuity of intraspecific variation. Understanding how communication influences animal phenotypes may allow researchers to quickly identify putative functions of colour variation prior to experimentation. Signals that convey different information will also have divergent evolutionary consequences. For example, signalling individual identity can increase genetic diversity, signalling quality may decrease diversity, and signalling strategy can constrain adaptation and contribute to speciation. Considering recent advances in genomic resources, our framework highlights new opportunities to resolve the evolutionary consequences of selection on communication across diverse taxa and signal types.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Identity Signaling and Patterns of Cooperative Behavior

Recognition systems play a central role in mediating cooperative behavior among individuals in a population. Despite the importance of discriminating among potential recipients of cooperation, the evolutionary forces that maintain diversity in traits used for kin recognition are poorly understood. Greenbeard-based models of kin recognition in which alleles for cooperative behavior also control recognition of those alleles in potential cooperators suggest that discrimination based on a greenbeard locus leads to positive frequency dependence, eroding diversity at the very genes responsible for recognition. As a result, the phenotypic diversity used for kin recognition has been widely assumed to be cues rather than signals of genetic identity. Diversity in identity cues is maintained by selection on other traits for reasons unrelated to recognition. A major problem with greenbeard-based models is that greenbeard recognition systems are uncommon among animals, which tend to learn kin phenotypes. We develop a simple model showing that learning a kin recognition template is sufficient to increase and maintain diversity in genetic traits used for kin recognition. Thus, our results suggest that phenotypes used for recognition may be true signals of genetic identity. As such, phenotypes are expected to evolve to facilitate recognition. Increased diversity in genetically-based recognition signals is also predicted to initiate a positive feedback loop between recognition efficiency and levels of cooperation. Finally, we discuss how the genetic architecture of recognition traits may influence kin discrimination abilities.

How does male-male competition generate negative frequency-dependent selection and disruptive selection during speciation?

Natural selection has been shown to drive population differentiation and speciation. The role of sexual selection in this process is controversial; however, most of the work has centered on mate choice while the role of male-male competition in speciation is relatively understudied. Here, we outline how male-male competition can be a source of diversifying selection on male competitive phenotypes, and how this can contribute to the evolution of reproductive isolation. We highlight how negative frequency-dependent selection (advantage of rare phenotype arising from stronger male-male competition between similar male phenotypes compared with dissimilar male phenotypes) and disruptive selection (advantage of extreme phenotypes) drives the evolution of diversity in competitive traits such as weapon size, nuptial coloration, or aggressiveness. We underscore that male-male competition interacts with other life-history functions and that variable male competitive phenotypes may represent alternative adaptive options. In addition to competition for mates, aggressive interference competition for ecological resources can exert selection on competitor signals. We call for a better integration of male-male competition with ecological interference competition since both can influence the process of speciation via comparable but distinct mechanisms. Altogether, we present a more comprehensive framework for studying the role of male-male competition in speciation, and emphasize the need for better integration of insights gained from other fields studying the evolutionary, behavioral, and physiological consequences of agonistic interactions.

Heritable variation in colour patterns mediating individual recognition

Understanding the developmental and evolutionary processes that generate and maintain variation in natural populations remains a major challenge for modern biology. Populations of Polistes fuscatus paper wasps have highly variable colour patterns that mediate individual recognition. Previous experimental and comparative studies have provided evidence that colour pattern diversity is the result of selection for individuals to advertise their identity. Distinctive identity-signalling phenotypes facilitate recognition, which reduces aggression between familiar individuals in P. fuscatus wasps. Selection for identity signals may increase phenotypic diversity via two distinct modes of selection that have different effects on genetic diversity. Directional selection for increased plasticity would greatly increase phenotypic diversity but decrease genetic diversity at associated loci. Alternatively, heritable identity signals under balancing selection would maintain genetic diversity at associated loci. Here, we assess whether there is heritable variation underlying colour pattern diversity used for facial recognition in a wild population of P. fuscatus wasps. We find that colour patterns are heritable and not Mendelian, suggesting that multiple loci are involved. Additionally, patterns of genetic correlations among traits indicated that many of the loci underlying colour pattern variation are unlinked and independently segregating. Our results support a model where the benefits of being recognizable maintain genetic variation at multiple unlinked loci that code for phenotypic diversity used for recognition.

Selection on Coding and Regulatory Variation Maintains Individuality in Major Urinary Protein Scent Marks in Wild Mice

Recognition of individuals by scent is widespread across animal taxa. Though animals can often discriminate chemical blends based on many compounds, recent work shows that specific protein pheromones are necessary and sufficient for individual recognition via scent marks in mice. The genetic nature of individuality in scent marks (e.g. coding versus regulatory variation) and the evolutionary processes that maintain diversity are poorly understood. The individual signatures in scent marks of house mice are the protein products of a group of highly similar paralogs in the major urinary protein (Mup) gene family. Using the offspring of wild-caught mice, we examine individuality in the major urinary protein (MUP) scent marks at the DNA, RNA and protein levels. We show that individuality arises through a combination of variation at amino acid coding sites and differential transcription of central Mup genes across individuals, and we identify eSNPs in promoters. There is no evidence of post-transcriptional processes influencing phenotypic diversity as transcripts accurately predict the relative abundance of proteins in urine samples. The match between transcripts and urine samples taken six months earlier also emphasizes that the proportional relationships across central MUP isoforms in urine is stable. Balancing selection maintains coding variants at moderate frequencies, though pheromone diversity appears limited by interactions with vomeronasal receptors. We find that differential transcription of the central Mup paralogs within and between individuals significantly increases the individuality of pheromone blends. Balancing selection on gene regulation allows for increased individuality via combinatorial diversity in a limited number of pheromones.

Individual recognition via scent is critical for many aspects of behavior including parental care, competition, cooperation and mate choice. While animal scents can differ in a huge number of dimensions, recent work has shown that only some specialized semiochemicals in scent marks are behaviorally relevant for individual recognition. How is individuality in specialized semiochemical blends produced and maintained in populations? At the extremes, individuality may depend on either a plethora of semiochemical isoforms or on combinatorial variation in a small number of shared isoforms across individuals. Analyzing the major urinary protein (MUP) pheromone blends of a wild population of house mice, we find evidence in favor of a combinatorial diversity model for the production and maintenance of individuality. Balancing selection maintains MUP proteins at moderate frequencies in the population, though interactions with the pheromone receptors appear to limit the extent of pheromone diversity in the system. By contrast, differential transcription of proteins greatly increases individuality in pheromone blends with balancing selection maintaining diversity in promoter regions associated with gene expression patterns. Selection maintaining combinatorial diversity in a limited set of behaviorally important semiochemicals may be a widespread mechanism generating and maintaining individuality in scent across taxa.

Assessing the potential information content of multicomponent visual signals: a machine learning approach

Careful investigation of the form of animal signals can offer novel insights into their function. Here, we deconstruct the face patterns of a tribe of primates, the guenons (Cercopithecini), and examine the information that is potentially available in the perceptual dimensions of their multicomponent displays. Using standardized colour-calibrated images of guenon faces, we measure variation in appearance both within and between species. Overall face pattern was quantified using the computer vision 'eigenface' technique, and eyebrow and nose-spot focal traits were described using computational image segmentation and shape analysis. Discriminant function analyses established whether these perceptual dimensions could be used to reliably classify species identity, individual identity, age and sex, and, if so, identify the dimensions that carry this information. Across the 12 species studied, we found that both overall face pattern and focal trait differences could be used to categorize species and individuals reliably, whereas correct classification of age category and sex was not possible. This pattern makes sense, as guenons often form mixed-species groups in which familiar conspecifics develop complex differentiated social relationships but where the presence of heterospecifics creates hybridization risk. Our approach should be broadly applicable to the investigation of visual signal function across the animal kingdom.

Morphological and population genomic evidence that human faces have evolved to signal individual identity

Facial recognition plays a key role in human interactions, and there has been great interest in understanding the evolution of human abilities for individual recognition and tracking social relationships. Individual recognition requires sufficient cognitive abilities and phenotypic diversity within a population for discrimination to be possible. Despite the importance of facial recognition in humans, the evolution of facial identity has received little attention. Here we demonstrate that faces evolved to signal individual identity under negative frequency-dependent selection. Faces show elevated phenotypic variation and lower between-trait correlations compared with other traits. Regions surrounding face-associated single nucleotide polymorphisms show elevated diversity consistent with frequency-dependent selection. Genetic variation maintained by identity signalling tends to be shared across populations and, for some loci, predates the origin of Homo sapiens. Studies of human social evolution tend to emphasize cognitive adaptations, but we show that social evolution has shaped patterns of human phenotypic and genetic diversity as well.

Coevolution of visual signals and eye morphology in Polistes paper wasps

To be effective, signals must propagate through the environment and be detected by receivers. As a result, signal form evolves in response to both the constraints imposed by the transmission environment and receiver perceptual abilities. Little work has examined the extent to which signals may act as selective forces on receiver sensory systems to improve the efficacy of communication. If receivers benefit from accurate signal assessment, selection could favour sensory organs that improve discrimination of established signals. Here, we provide evidence that visual resolution coevolves with visual signals in Polistes wasps. Multiple Polistes species have variable facial patterns that function as social signals, whereas other species lack visual signals. Analysis of 19 Polistes species shows that maximum eye facet size is positively associated with both eye size and presence of visual signals. Relatively larger facets within the eye's acute zone improve resolution of small images, such as wasp facial signals. Therefore, sensory systems may evolve to optimize signal assessment. Sensory adaptations to facilitate signal detection may represent an overlooked area of the evolution of animal communication.

Social knowledge and signals in primates

Primates are notable for having a rich and detailed understanding of their social environment and there has been great interest in the evolution and function of social knowledge in primates. Indeed, primates have been shown to have impressive understandings of not only other group members but also the complex relationships among them. To be useful, however, social knowledge requires memories from previous encounters and observations about individual traits that are stable. Here, we argue that social systems or traits that make social knowledge more costly or less accurate will favor signals that either supplement or replace social knowledge. Thus, the relationship between signals and social knowledge can be complementary or antagonistic depending on the type of signal. Our goal in this review is to elucidate the relationships between signals and social knowledge in primates. We categorize signals into three types, each with different relationships to social knowledge. (1) Identity signals directly facilitate social knowledge, (2) current-state signals supplement information gained through social knowledge, and (3) badges of status replace social knowledge. Primates rely extensively on identity information, but it remains to be determined to what extent this is based on receiver perception of individual variation or senders using identity signals. Primates frequently utilize current-state signals including signals of intent to augment their interactions with familiar individuals. Badges of status are rare in primates, and the cases where they are used point to a functional and evolutionary trade-off between badges of status and social knowledge. However, the nature of this relationship needs further exploration.

Female choosiness leads to the evolution of individually distinctive males

Individual recognition is a taxonomically widespread ability that underlies a diverse suite of behaviors including the identification of individual nest-mates, agonistic opponents, and mating partners. However, as yet relatively little is known about the circumstances under which the requisite signal diversity can evolve. Here, we develop a model describing a novel mechanism of individual identity evolution via sexual selection. Females choose among a subset of males, but can select the most attractive male only when he bears a unique identity signal. This mimics a species in which mate assessment and choice are temporally separate, such as when females observe males in direct conflict and must subsequently locate the winner. When females in our model are choosy at least 10% of the time, diversity at individuality signaling loci evolves as a by-product of selection on male attractiveness more rapidly than does diversity at equivalent loci evolving only under neutral processes. Even at lower discrimination rates, drifting signal diversity gives the female choice mechanism sufficient traction to drive up average male attractiveness. The mechanism we describe here can significantly increase signal diversity at even low rates of discrimination by females.

Revisiting social recognition systems in invertebrates

Since the 1970s, the ability of some invertebrate species to recognize individual conspecifics has attracted increased scientific interest. However, there is still confusion in the literature, possibly due to the lack of unambiguous criteria for classifying social recognition in its different forms. Here, we synthesize the results of studies on invertebrates and provide a framework with the purpose of identifying research needs and directions for future investigations. Following in part Sherman et al.'s (Behavioural ecology: an evolutionary approach. Blackwell Science, Oxford, pp 69-96, 1997) definition of 'recognition systems' and Tibbetts and Dale's (Trends Ecol Evol 22:529-537, 2007) classification of 'individual recognition,' we first discuss different case studies that exemplify the categories of 'familiar recognition' and 'class-level recognition.' Then, through the analysis of the invertebrate literature, we illustrate eight key properties that characterize 'true individual recognition' systems. We are confident that the proposed framework will provide opportunities for exciting discoveries of the cognitive abilities in invertebrates.

Specialized face learning is associated with individual recognition in paper wasps

We demonstrate that the evolution of facial recognition in wasps is associated with specialized face-learning abilities. Polistes fuscatus can differentiate among normal wasp face images more rapidly and accurately than nonface images or manipulated faces. A close relative lacking facial recognition, Polistes metricus, however, lacks specialized face learning. Similar specializations for face learning are found in primates and other mammals, although P. fuscatus represents an independent evolution of specialization. Convergence toward face specialization in distant taxa as well as divergence among closely related taxa with different recognition behavior suggests that specialized cognition is surprisingly labile and may be adaptively shaped by species-specific selective pressures such as face recognition.

Selection for individual recognition and the evolution of polymorphic identity signals in Polistes paper wasps

Individual recognition (IR) requires individuals to uniquely identify their social partners based on phenotypic variation. Because IR is so specific, distinctive phenotypes that stand out from the crowd facilitate efficient recognition. Over time, the benefits of unique appearances are predicted to produce a correlation between IR and phenotypic variation. Here, we test whether there is an association between elevated phenotypic polymorphism and IR in paper wasps. Previous work has shown that Polistes fuscatus use variable colour patterns for IR. We test whether two less variable wasp species, Polistes dominulus and Polistes metricus, are capable of IR. As predicted, neither species is capable of IR, suggesting that highly variable colour patterns are confined to Polistes species with IR. This association suggests that elevated phenotypic variation in taxa with IR may be the result of selection for identity signals rather than neutral processes. Given that IR is widespread among social taxa, selection for identity signalling may be an underappreciated mechanism for the origin and maintenance of polymorphism.