Individual Recognition | SpringerLink

Definition
Animal social behavior depends on recognition. There are many types of recognition, including self, kin, mate, gender, friend, offspring, predator, and prey. All recognition involves (i) cue production by the signaler, (ii) cue perception and template matching by the receiver, and (iii) a behavioral response by the receiver (Sherman et al. 1997).
Multiple definitions of individual recognition have been proposed that differ slightly in their specificity and complexity. The most common definition of individual recognition proposes that receivers discriminate a signaler from others based on the signaler’s unique characteristics and associate the unique characteristics with individual-specific information about the signaler (Tibbetts and Dale 2007) (Fig. 1). For example, king penguins (Aptenodytes patagonicus) exhibit individual recognition because each chick has individual-specific calls (signaler’s cue). Parents learn their chick’s unique calls and respond in a unique way; they treat their own chick differently than other chicks (Aubin and Jouventin 1998).
Fig. 1
The criteria for individual recognition. Signalers have unique cues and receivers behave in a unique way toward the signaler
Steiger and Müller (2008) proposed a less restrictive definition of individual recognition: receivers need only discriminate a signaler from others based on the signaler’s unique characteristics. As long as receivers have a unique phenotype, the behavior is considered individual recognition, even if receivers associate the unique characteristics with class-specific information about the signaler. For example, if a bird has multiple chicks and learns to discriminate their own chicks from among unrelated chicks using the chicks’ unique calls, this behavior would be termed individual recognition. Others would consider this offspring recognition, a type of class-level recognition, rather than individual recognition because the parents treat all their chicks similarly.
A challenge associated with Steiger and Müller’s definition is that it can be difficult to assess whether or not receivers perceive cues as unique if receivers don’t have unique behavioral responses. Cues that appear unique to researchers might not be perceived as unique by the receiver’s sensory system. For example, computer-based acoustic analysis suggests that the alarm calls of meerkats are individually distinctive yet receivers do not respond differently to alarm calls based on the identity of callers (Schibler and Manser 2007). How can we test if meerkats perceive the alarm calls from different individuals as unique without a unique behavioral response?
Others argue for a more restrictive definition of individual recognition that requires receivers to learn and recall multiple, unique individuals (e.g., Thom and Hurst 2004). Under this restrictive definition, king penguins would not have individual recognition because parents only learn and identify one chick at a time.
Introduction
Individual recognition is interesting from a cognitive perspective because it is considered a relatively specific and complex form of recognition. During individual recognition, receivers must learn the unique features of conspecifics, associate the features with individual-specific information, and recall the feature/information link during subsequent interactions. Individual recognition is also interesting from a cognitive perspective because it allows the formation of individually differentiated social relationships. The potential for individually differentiated social relationships influences the type and sophistication of social interactions possible within a society (Platt et al. 2016).
In this entry, individual recognition will be addressed from both the receiver and signaler perspectives. Receivers assess, perceive, and respond to signals. In some cases, receivers have perceptual and cognitive adaptations to facilitate accurate individual recognition. On the other side, signalers have unique phenotypes that are used for recognition. This entry will discuss the contexts in which individual recognition is used, how individual recognition varies across contexts, receiver cognitive and sensory adaptations for individual recognition, and how selection acts on the signaler to facilitate individual recognition.
Social Contexts in which Individual Recognition is Used
Interspecific Variation in Individual Recognition across Contexts
Individual recognition is common in social contexts when there are repeated interactions between the same target individuals and also nontarget individuals. Such situations include territoriality, dominance interactions, cooperation, providing parental care, and pair bonding (Tibbetts and Dale 2007).
Much of the earliest research on individual recognition examined recognition of territorial neighbors (originally termed “dear enemy” recognition). For example, many birds use song to identify the identity and position of their neighbors. Birds largely ignore playbacks of neighbor songs positioned on the correct territorial boundary but responded aggressively to playbacks of neighbor songs positioned on the incorrect territorial boundary and to playbacks of stranger songs (Stoddard 1996). Individual recognition is thought to reduce the energetic costs of territorial defense by enabling territory holders to focus their aggressive efforts on non-territorial floaters instead of their less threatening neighbors (Fig. 2).
Fig. 2
The dear enemy approach to testing individual recognition. White-throated sparrow (Zonotrichia albicollis) is less aggressive to neighbor songs on the correct territorial boundary than neighbor songs on the incorrect territorial boundary, indicating that white-throated sparrow recalls the individual identity and position of neighbors. Speakers played back neighbor and stranger songs from the boundary shared with the neighbor (regular), the center of the territory (center), and the boundary shared with a different neighbor (opposite). The behavioral data are normalized composite scores for the number of flights and the closest approach to the playback speaker (Taken from Stoddard 1996)
Individual recognition is also important during aggressive competition because it can reduce the costs associated with competition and stabilize dominance hierarchies (Barnard and Burk 1979). Individual recognition is likely to be beneficial when there are repeated aggressive interactions between small numbers of individuals. In this situation, learning the individual identity and competitive ability of conspecifics will provide a precise way to assess rivals without multiple fights. For example, some species of fish, including brook trout (Salvelinus fontinalis), individually identify rivals via scent. Trout assess relative dominance ranks based on the outcome of their own aggressive interactions as well as observations of others via transitive inference. If a fish beats rival A in a fight and then observes A win in a contest with B, the fish will correctly infer that it is more dominant than individual B (White and Gowan 2013). Transitive inference was once thought to be a cognitively complex task restricted to few taxa but has now been found in many vertebrate species with individual recognition that form dominance hierarchies (Paz-y-Miño et al. 2004). Individual recognition associated with aggressive competition is widespread in nature, as many species use individual recognition to minimize the costs of conflict (Barnard and Burk 1979).
Although cooperation can occur without individual recognition, there is much evidence that cooperative behaviors such as reciprocity, alliances, social prestige, punishment of non-cooperators, and image scoring are facilitated by individual recognition. As a result, many taxa with complex social interactions rely on individual recognition. Individual recognition of group members is ubiquitous in primates and also occurs in many species that live in cooperative groups, including mammals, fish, and birds (Sharpe et al. 2013). Although social insects have complex social behavior, individual recognition is relatively rare in social insects. Instead, most social insect societies rely on class-level recognition and self-organization to produce apparently complex behavior with little individual cognitive investment.
Individual recognition is involved in multiple aspects of reproduction, including parent-offspring interactions and monogamous pair bonding. Individual recognition during parental care is particularly likely to occur when there is potential for confusion among offspring, such as among organisms that breed in high-density groups with synchronous reproduction and reduced recognition potential based on positional information. For example, the Australian sea lion Neophoca cinerea reproduces in large colonies. Both mothers and pups use individual-specific calls to find each other after the mother’s foraging trips (Charrier et al. 2009). Monogamous species with biparental care use individual recognition to maintain pair bonds and mate guard and defend territories. For example, Chaetodon multicinctus butterfly fish uses visual and olfactory cues to discriminate their mate from non-mates (Boyle and Tricas 2014).
Individual recognition during monogamous pairing and parent-offspring recognition is sometimes considered simple forms of individual recognition because receivers need only learn and recall one individual. Some argue “true” individual recognition requires receivers who can individually discriminate many individuals (Thom and Hurst 2004). As a result, future work that tests whether multiple mates, parents, or offspring can be individually identified will be useful. For example, in taxa with multiple offspring, do parents treat different offspring uniquely? For instance, there is evidence that parents of the songbird black redstart (Phoenicurus ochruros) respond more to the calls of the fledglings that they preferentially fed, suggesting that parents can discriminate between offspring (Draganoiu et al. 2006).
While most examples of individual recognition occur within species, individual recognition is not restricted to conspecifics. Some animals recognize individuals of other species. For example, mockingbirds learn to assess the level of threat posed by different humans. When the same human approaches a mockingbird’s nest for 4 successive days, mockingbirds begin flushing from the nest when the human is a great distance from the nest and also increase alarm calling and attack flights. Mockingbird response to humans who have not threatened the nest does not change (Levey et al. 2009).
Intraspecific Variation in Individual Recognition across Contexts
A large body of research has examined the types of social behavior associated with interspecific variation in individual recognition, but less is known about intraspecific variation in individual recognition. Examining intraspecific variation is important because responses to individual identity signals may vary across the biotic and abiotic environment. For example, a species may use individual recognition during aggressive encounters in certain social or environmental circumstances, but not in other circumstances.
Previous work has shown that there is some intraspecific variation in receiver responses to signals. For example, receiver responses to quality signals often vary with traits like sex, maturity, season, habitat, and social context. Many studies have shown that the biotic and abiotic environment influences whether females choose male partners based on sexual signals or ignore sexual signals during partner choice (Searcy and Nowicki 2005). Such variation occurs because assessing and responding to signals can be costly, so receivers will only pay attention to signals when signal response is sufficiently beneficial. Therefore, intraspecific variation in recognition may occur whenever the benefits of recognition differ across contexts.
A few studies have found intraspecific variation in individual recognition. Notably, pair-bonded male prairie voles learn to recognize individual females, while single males do not learn to recognize females. The authors found no difference in male motivation to contact females across paired and unpaired males, so they speculate that recognition differs because the social relevance of information about females varies across contexts (Blocker and Ophir 2015).
In the paper wasp Polistes fuscatus, the social benefits of individual recognition and receiver capacity for individual recognition vary across castes. Specifically, workers are less able to learn and remember individual conspecifics than foundresses (Tibbetts et al. in press). During social interactions, workers are unable to remember unique individuals following a 6-day separation, though foundresses remember other foundresses for 6 days. The differences in individual recognition across castes are initially surprising because Polistes lack discrete queen/worker castes. Workers and queens are morphologically similar, and workers can take over nests as queens. Differences in recognition may occur because the social benefits of individual recognition differ across foundresses and workers. Workers do not benefit by being individually recognizable on stable queenright nests because worker-worker aggression on queenright nests is low and workers do not reproduce or have unique roles in the colony. In contrast, foundresses benefit by being individually recognizable, as it reduces aggression and stabilizes social interactions among cooperating foundresses (Fig. 3). Individual recognition also allows foundresses to keep track of individual social relationships, including division of aggression, food, and work (Sheehan and Tibbetts 2009).
Fig. 3
Being individually distinctive is beneficial. Faces of Polistes fuscatus foundresses were manipulated so three wasps had a common, indistinguishable appearance while one wasp had a unique appearance (a, lower-right image). Distinctive wasps received the least aggression (1, most; 4, least) in a disproportionate number of trials, indicating that a unique appearance that allows individual recognition is beneficial. Dotted line represents the expected aggression received by chance (b) (Taken from Sheehan and Tibbetts 2009)
Complexity of Individual Recognition
There is enormous variation in the cognitive demands of individual recognition. Learning and remembering a few individuals for a short time may be relatively simple compared with learning many individuals, maintaining memories for long periods, or associating multiple types of information with individual identity. In this section, cognitively challenging aspects of individual recognition are described.
Most early individual recognition research examined how animals recognize others using a single cue in a single sensory modality. However, more recent work has shown that many animals integrate multiple cues from the same or different sensory modalities during individual recognition. For example, golden hamsters (Mesocricetus auratus) have multiple odor glands, and hamsters generalize between different scents from the same individual. The authors suggest that males form integrated representations of conspecifics that incorporate multiple odors (Johnston and Bullock 2001). Other species form cross-modal representations of known individuals by combining unique auditory and visual cues. For example, ring-tailed lemurs (Lemur catta) can produce multisensory representations by matching olfactory and visual signals to individually recognize conspecifics, though the specific proximate mechanisms that are used for sensory integration are not yet clear (Kulahci et al. 2014).
Many animals can remember individual conspecifics after a lengthy separation. For example, fur seal pups (Callorhinus ursinus) remember their mother’s unique calls for at least 4 years (Insley 2000). Dolphins (Tursiops truncatus) may be able to remember unique conspecifics for decades (Bruck 2013). Male hooded warbler birds (Wilsonia citrina) use song to recognize their neighbors individually and are able to retain the memory after being separated for 8 months. During the separation, birds do not sing and migrate to Central America (Godard 1991). Ravens (Corvus corax) remember previous individual calls after up to a 3-year separation (Boeckle and Bugnyar 2012).
Long-term individual memory may be particularly important in societies with fission-fusion dynamics, where individuals meet after unpredictably long periods of separation. Memory is also affected by the context in which the recognition is effective. For example, the mantis shrimp Neogonodactylus bredini can remember conspecifics for 4 weeks, perhaps because this amount of time correlates with their reproductive periodicity because female’s receptivity and male-female pairing is timed with the full moon (Vetter and Caldwell 2015).
The cognitive challenges of individual recognition are also influenced by the number of individuals that are learned. African elephants (Loxodonta africana) have large recognition networks, as adult females can recognize up to 100 other females (McComb et al. 2000). Elephant females can recognize their own family, as well as approximately 14 other families they regularly encounter. Such impressive networks of vocal recognition may be typical of long-lived species that have fission-fusion social systems and long-distance vocal communication.
Some species learn to identify surprisingly few individuals despite apparently high levels of social complexity. For example, gelada baboons (Theropithecus gelada) use vocalizations for individual recognition only within the small social group known as a “unit.” They do not vocally recognize other males, even males from their own band with whom they have extensive social overlap (Bergman 2010).
It is often difficult to assess whether memory constrains individual recognition. Does reduced recognition in geladas occur because males have poor memories, reduced motivation, or little social need for individual recognition? In a creative experiment, Stoddard tested whether memory constrained the ability of song sparrows (Melospiza melodia) to learn neighbor’s songs. Song sparrows can memorize the full song repertoire of new neighbors without a decline in overall learning performance and without forgetting the old songs, even when the number of songs learned approached the number of songs a song sparrow would have to know to make accurate discriminations across neighbors (Stoddard et al. 1992).
Another factor that influences the complexity of individual recognition is the types of information associated with individual identity cues. For example, baboons (Papio hamadryas ursinus) simultaneously classify other females according to dominance rank and matrilineal kinship. Associating both rank and kinship information with the unique calls of conspecifics increases the challenge of individual recognition as well as the functional complexity of social interactions (Bergman et al. 2003). To our knowledge, there is no evidence for this type of two-dimensional classification outside primates, so research in additional taxa will be important.
Some nonhuman primates use individual recognition to keep track of others’ relationships. For example, baboons are more likely to approach and tolerate an opponent if they hear a “reconciliatory” grunt of a close relative of the opponent. However, “reconciliatory” grunts from individuals unrelated to the opponent have no effect on behavior. This behavior means baboons keep track of relatedness between other individuals rather than merely remembering their own relatives. Recognition of others’ relationships will dramatically increase the amount of information individuals need to remember. For example, in a group of 80 individuals, a baboon who keeps track of their own relationship with each group member may only recall two pieces of information about the 79 other group members: are they related or unrelated to the focal individual and are they dominant or subordinate to the focal individual? In that same group of 80 individuals, there are 3160 possible dyad and 82,160 triad combinations (Platt et al. 2016). Current data suggests baboons can identify the relatedness and relative rank in all those potential groupings, a truly outstanding feat. In the future, additional work will be important to understand how well nonhuman species keep track of other’s relationships.
Cognitive and Sensory Adaptations in Receivers to Facilitate Accurate Recognition
Learning and remembering many unique individuals are thought to be cognitively challenging. As a result, some species have cognitive adaptations to facilitate accurate individual recognition. These adaptations fall into three categories: (1) general increase in cognitive capacity in species with individual recognition, (2) specialized increase in cognition related to individual recognition, and (3) sensory adaptations to facilitate assessment of individual identity signals.
First, individual recognition may produce generalized increase in cognitive capacity. The social intelligence hypothesis proposes that the benefits of being able to learn and remember multiple individuals and social relationships have selected for increased cognitive capacity in many social taxa (Dunbar 1998). Between species comparisons have provided consistent support for the social intelligence hypotheses. Across many taxa, species with more complex social behavior have larger brains or brain regions than species with less complex social behavior (e.g., primates, hyenas, lemurs, ungulates, carnivores, cetaceans, birds). For example, a comparative analysis in nonhuman primate species found that relative neocortical volume is linked with social group size, but not ecology, suggesting that individual social relationships are a key factor that shapes brain size evolution (Dunbar 1992).
Other analyses have found no relationship between brain size and social complexity. For example, a comparative study on brain size and social complexity in 29 species of vespids (including solitary and social wasps) showed that brain size does not correlate with colony size, social structure (hierarchies), and type of nest foundation (solitary vs. social) (O’Donnell et al. 2015). The authors hypothesized that insects can perform relatively complex tasks with minor adjustments of neural circuitry. Further, many highly social insects have been selected to have more specialized brains rather than larger brains. More generally, there is a growing appreciation that understanding the relationship between social behavior and cognition requires nuanced assessment of social complexity that is relevant to the taxa of interest (Platt et al. 2016).
Second, some species have specific cognitive adaptations that facilitate individual recognition. Polistes fuscatus paper wasps use individual face recognition and are specialized for face learning; wasps learn to identify conspecific faces more quickly and accurately than other visual information. In contrast, paper wasp species without individual recognition perform poorly at face learning. A closely related wasp species that lacks individual face recognition, P. metricus, lacks face specialization and has trouble learning conspecific faces. The divergence in face learning between closely related wasp species suggests that face specialization in P. fuscatus is an adaptation to facilitate individual recognition (Sheehan and Tibbetts 2011). Face specialization in paper wasps may be associated with some neural changes in the antennal lobe and the mushroom body sub-compartments. However, there appears to be no major optic lobe specialization in species with visual individual recognition, suggesting that the visual processing capabilities of paper wasps might be preadapted to discriminate complex visual features, such that the ability to discriminate facial markings could require relatively small changes in their neuronal substrate (Gronenberg et al. 2007).
Neural specialization for individual recognition has been best studied in primates. Humans are experts at using facial features for individual recognition. Humans can learn and remember many individuals over their lifetime and only require short exposure to form individual memories. Humans also use configural processing to learn faces, as the spatial arrangement of facial features is integrated into a single “face image” rather than recognition being based on the component parts of a face. Finally, humans have neural specialization for face learning; there is a network of distributed neural regions that respond specifically to faces, not other visual stimuli. Damage in specialized area can produce individuals who are unable to recognize individual faces (Tsao and Livingstone 2008). Nonhuman primates also use faces for individual recognition and have some specialization for face learning, including face-selective neural responses in certain regions of the brain. Some primates also use configural processing to discriminate between individual faces, though most primates rely less on configural face processing than do humans (Leopold and Rhodes 2010).
Finally, some animals have sensory adaptations to optimize assessment of individual identity signals. Paper wasps with visual recognition have larger facets in the eye’s acute zone than species without visual recognition. Larger facets improve resolution of small images, such as wasp facial signals. Therefore, the wasps’ sensory systems may evolve to optimize signal assessment (Fig. 4) (Sheehan et al. 2014). In mice, odors (major urinary protein) signal individuality. The neurons in the vomeronasal system (olfactory system) are activated selectively by odors that signal individuality and gender, suggesting an olfactory adaptation for individually recognition of conspecifics (Cheetham et al. 2007).
Signals Versus Cues
Individual recognition can occur via signals or cues. Signals and cues are similar in that both modify the behavior of receivers. The key difference between signals and cues is that signals benefit the signaler, while cues do not (Searcy and Nowicki 2005). For example, human faces are signals that convey information about individual identity to receivers. Humans likely benefit by having unique, easily recognizable faces. As a result, human facial variation has been selected to optimize receiver recognition, and this selection has shaped human facial variation as well as the genetic regions that code for human facial variation (Fig. 5; Sheehan and Nachman 2014). In contrast, human fingerprints are a cue of individual identity. While it is possible to discriminate among individual humans based on fingerprints, variation in fingerprints is a by-product of development. Fingerprints have not been selected to convey information about individual identity, so fingerprints and the regions that code for fingerprint variation will not have signatures of selection to optimize distinctiveness.
Fig. 5
Genomic regions associated with facial morphology show evidence of selection for individual identity signaling. Bars represent the proportion of SNPs within each minor allele frequency (MAF). Genomic regions associated with facial morphology have elevated levels of intermediate-frequency alleles when compared with neutral regions or genomic regions associated with variation in height. The higher genetic diversity in face-associated SNPs is consistent with negative frequency-dependent selection acting on human facial features (Taken from Sheehan and Nachman 2014)
The distinction between signals and cues is important because signals show signatures of adaptive evolution to facilitate receiver responses, while cues do not. The following sections describe the predicted characteristics of individual identity signals.
Negative Frequency-Dependent Selection Acts on Signalers
Individual identity signals are expected to evolve via negative frequency-dependent selection (Dale et al. 2001). Individuals with rare phenotypes are favored by selection because they are less likely to be confused with other individuals than those with common phenotypes. There is experimental evidence in both Polistes paper wasps and guppies, Poecilia reticulata, showing that individuals with a rare appearance are favored relative to individuals with a more common appearance. For example, Polistes fuscatus paper wasps have highly variable facial patterns that are used for individual recognition (Tibbetts 2002). Wasps with unique faces that are easy to discriminate receive less aggression than wasps with common, less distinguishable faces. Receiving aggression is costly, so these results indicate that individuals with unique phenotypes benefit by revealing their identity (Sheehan and Tibbetts 2009; Fig. 3). Field studies have shown that guppies with rare body coloration benefit by receiving more mates than guppies with a common appearance (Hughes et al. 2013). Guppies use body coloration for individual recognition, so the mating benefit may arise through identity signaling if rare guppies are easier to learn and remember than common guppies.
Comparative work provides further evidence that individual recognition selects for increased phenotypic diversity. In paper wasps, mammals, swallows, and bird eggs, individual identity signaling is linked with increased phenotypic diversity (Tibbetts et al. 2017). This relationship is consistent with the hypothesis that social benefits of being individually recognizable favor increased phenotypic diversity in certain social contexts. For example, computer-based model has shown that egg signatures by hosts of the common cuckoo evolve surprisingly fast in the presence of brood parasites. The simulation suggests many hosts evolve unique, highly recognizable egg pattern signatures as a defense against cuckoo egg mimicry (Stoddard et al. 2014).
Effects of Individual Identity Signaling on Phenotypes and Genotypes
Because identity signals evolve under negatively frequency-dependent selection, identity signals are expected to be highly variable and multimodal and have low intercorrelation among identity signaling traits. These predictions are developed in detail in the model by Dale and colleagues (2001). Further, in the taxa that have been tested, there is low intercorrelation among identity signaling traits (Dale 2000; Tibbetts 2002). Finally, reliable identity signals are predicted to be stable over time rather than plastic.
Theoretical and empirical work suggests that individual identity signals will be highly heritable and have low condition dependence. The few individual identity signals studied have high heritability (Dale 2000). Identity signals are also expected to have low costs associated with signal production and maintenance because phenotypes that are not costly will spread to a higher equilibrium frequency than costly phenotypes. For example, the birds ruff sandpipers (Philomachus pugnax) and red-billed queleas (Quelea quelea) live in dense aggregations in which individual recognition is key for territory defense. In both species, plumage coloration is highly heritable and has low production costs (Dale et al. 2001). Similarly, rock hyrax (Procavia capensis) uses the same highly variable calls for individual recognition across multiple years, and these calls are not linked to geographic locations or relatedness (Lee and Eli 2011). Paper wasps use facial patterns for individual recognition. Facial patterns are fixed during adulthood, and facial pattern development is not condition dependent, as larval diet quality has no effect on facial pattern development (Tibbetts and Curtis 2007).
Because individual identity signals are highly heritable traits under negatively frequency-dependent selection, selection for identity signaling may increase genetic diversity. Positive selection for signaling individual identity may have more widespread genetic effects than other cases of negative frequency-dependent selection because identity signaling favors high levels of variation in multiple, uncorrelated traits (Tibbetts et al. 2017). In contrast, other examples of negative frequency-dependent selection typically maintain a limited number of morphs at stable ratios in the population.
The precise effects of individual identity signaling on the genome will differ based on the genetic architecture of identity signals (Tibbetts et al. 2017). For example, negative frequency-dependent selection acting on individual identity signals will have broader genomic consequences if signal development is influenced by many, independently segregating loci than if signal development is influenced by epistatic interactions in a small cluster of genes. Population genetic evidence indicates that negative frequency-dependent selection acting on loci that encode human facial features influences genomic diversity in humans. Specifically, loci that encode human facial features have higher nucleotide diversity than loci associated with variation in height or neutral, intergenic variation, consistent with human facial features experiencing negative frequency-dependent selection (Fig. 5; Sheehan and Nachman 2014). Further, the genomic regions surrounding areas that code for face variation also have increased nucleotide diversity, indicating that selection on human identity signals has broader effects on genomic diversity. In contrast, selection for identity signaling in mice has limited genomic consequences. Mice use major urinary proteins (MUPs) as chemical signals of individual identity. Although variation in MUPs is maintained by negative frequency-dependent selection, identity signaling in mice has little overall effect on the genome because a small number of clustered loci produce the overall MUP signature (Sheehan et al. 2016).
Conclusions
In this entry, we consider the evolution of individual recognition from the perspective of the receiver and signaler. Studies of individual recognition from the receiver’s perspective are very common and have demonstrated that individual recognition is an important aspect of social behavior in a wide range of taxa, including birds, mammals, reptiles, insects, and aquatic invertebrates. Individual recognition is also used in diverse contexts, including territoriality, dominance interactions, cooperation, providing parental care, and pair bonding.
Individual recognition can be cognitively demanding for signal receivers, especially when receivers learn many individuals, maintain those memories for long periods, or associate multiple types of information with individual identity. Complex behaviors involving individual recognition that were once thought to be rare (e.g., transitive inference) have recently been found to be surprisingly common. Further, many taxa have surprisingly sophisticated individual recognition, including long individual memories and the ability to recall many individuals and associate diverse information with individual identities. Future research will likely continue to uncover the diversity and sophistication of individual recognition across taxa.
Individual recognition also has important selective consequences for signalers. In particular, there is growing evidence that signalers are selected to reveal their individual identity by having phenotypes that are memorably different. Individuals with rare phenotypes are favored by selection because they are less likely to be confused with other individuals than those with common phenotypes. Selection on signalers to reveal their individual identity may be an underappreciated mechanism for the origin and maintenance of phenotypic and genetic diversity. Given the growth of genomic data in non-model organisms, future work examining the genetic architecture of individual identity signals as well as the consequences of individual identity signals on animal phenotypes and genotypes will be important.