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Specific mate recognition relies on the chemical senses in most animals, and especially in nocturnal insects. Two signal types mediate premating olfactory communication in terrestrial habitats: sex pheromones, which blend into an atmosphere of plant odorants. We show that host plant volatiles affect the perception of sex pheromone in males of the African cotton leafworm Spodoptera littoralis and that pheromone and plant volatiles are not perceived as independent messages. In clean air, S. littoralis males are attracted to single synthetic pheromone components or even the pheromone of a sibling species, oriental cotton leafworm S. litura. Presence of host plant volatiles, however, reduces the male response to deficient or heterospecific pheromone signals. That plant cues enhance discrimination of sex pheromone quality confirms the idea that specific mate recognition in noctuid moths has evolved in concert with adaptation to host plants. Shifts in either female host preference or sex pheromone biosynthesis give rise to new communication channels that have the potential to initiate or contribute to reproductive isolation.

Genitalia appear to evolve rapidly and divergently in taxa with internal fertilization. The current consensus is that intense directional sexual selection drives the rapid evolution of genitalia. Recent research on the millipede Antichiropus variabilis suggests that the male genitalia are currently experiencing stabilizing selection – a pattern of selection expected for lock‐and‐key structures that enforce mate recognition and reproductive isolation. Here, we investigate how divergence in genital morphology affects reproductive compatibility among isolated populations of A. variabilis. Females from a focal population were mated first to a male from their own population and, second, to a male from one of two populations with divergent genital morphology. We observed variation in mating behavior that might indicate the emergence of precopulatory reproductive barriers: males from one divergent population took significantly longer to recognize females and exhibited mechanical difficulty in genital insertion. Moreover, we observed very low paternity success for extra‐population males who were successful in copulating. Our data suggest that divergence in genital shape may be contributing to reproductive isolation, and incipient speciation among isolated populations of A. variabilis. The shape of male genitalia often varies considerably between different species. We show that small differences in male genital shape among isolated populations of a single species of Australian millipede are sufficient to hamper successful reproduction between those populations when they are brought together. Our results confirm the importance of selection on male genital morphology in speciation.

Mate recognition systems evolve rapidly to reinforce the reproductive boundaries between species, but the underlying neural mechanisms remain enigmatic. Here we leveraged the rapid coevolution of female pheromone production and male pheromone perception in Drosophila 1 , 2 to gain insight into how the architecture of mate recognition circuits facilitates their diversification. While in some Drosophila species females produce unique pheromones that act to arouse their conspecific males, the pheromones of most species are sexually monomorphic such that females possess no distinguishing chemosensory signatures that males can use for mate recognition 3 . We show that Drosophila yakuba males evolved the ability to use a sexually monomorphic pheromone, 7-tricosene, as an excitatory cue to promote courtship. By comparing key nodes in the pheromone circuits across multiple Drosophila species, we reveal that this sensory innovation arises from coordinated peripheral and central circuit adaptations: a distinct subpopulation of sensory neurons has acquired sensitivity to 7-tricosene and, in turn, selectively signals to a distinct subset of P1 neurons in the central brain to trigger courtship. Such a modular circuit organization, in which different sensory inputs can independently couple to parallel courtship control nodes, may facilitate the evolution of mate recognition systems by allowing novel sensory modalities to become linked to male arousal. Together, our findings suggest how peripheral and central circuit adaptations can be flexibly coordinated to underlie the rapid evolution of mate recognition strategies across species. Peripheral and central circuit adaptations can be flexibly coordinated in Drosophila , and such a modular circuit organization may facilitate the evolution of mate recognition systems by allowing novel sensory modalities to become linked to male arousal.

Social recognition represents the foundation of social living. To what extent social recognition is hard-wired by early-life experience or flexible and influenced by social context of later life stages is a crucial question in animal behaviour studies. Social insects have represented classic models to investigate the subject, and the acknowledged idea is that relevant information to create the referent template for nest-mate recognition (NMR) is usually acquired during an early sensitive period in adult life. Experimental evidence, however, highlighted that other processes may also be at work in creating the template and that such a template may be updated during adult life according to social requirements. However, currently, we lack an ad hoc experiment testing the alternative hypotheses at the basis of NMR ontogeny in social insects. Thus, to investigate the mechanisms underlying the ontogeny of NMR in Polistes wasps, a model genus in recognition studies, and their different role in determining recognition abilities, we subjected Polistes dominula workers to different olfactory experiences in different phases of their life before inserting them into the social environment of a novel colony and testing them in recognition bioassays. Our results show that workers develop their NMR abilities based on their social context rather than through pre-imaginal and early learning or self-referencing. Our study demonstrates that the social context represents the major component shaping recognition abilities in a social wasp, therefore shedding new light on the ontogeny of recognition in paper wasps and prompting the reader to rethink about the traditional knowledge at the basis of the recognition in social insects. This article is part of the theme issue ‘Signal detection theory in recognition systems: from evolving models to experimental tests'.

A conflict over male production arises in social insects where workers are able to lay unfertilized male eggs. This happens because each female (queen or worker) is most closely related to her own sons and is thus predicted to reproduce. The conflict is modulated by worker policing where workers prevent each other from reproducing by aggression or egg cannibalism. In this study, we show that in the ant Formica fusca, worker policing occurs by egg cannibalism rather than by overt aggression among workers. Furthermore, we show that, contrary to bees, wasps and other ant species, egg discrimination in F. fusca is not based only on a universal queen signature chemical and that nest mate recognition of eggs occurs.

Many animal signals are brightly colored and convey information about species identity as well as information about individual conspecifics. Colorful bird and lizard signals have received much attention, and many studies have related specific spectral properties of these signals to variation in mating success and territory defense. Far less attention has been given to invertebrates even though there are spectacularly colorful species. The enlarged claw of the male banana fiddler crab Austruca mjoebergi, for example, is bright yellow and contrasts vividly against the mudflat substrate. It is used in waving displays to attract females and in male territory defense and combat. Claw color varies among males in the degree of “yellowness,” ranging very pale yellow to orange. In this study, we examined female responses to claw color variation in two-choice tests using robotic crabs. We found that although females strongly discriminate against colors that fall outside the natural range of intensity, hue, and chroma, they show no consistent preferences for different claw colors within the natural range, and no single component of claw color (hue, chroma, or intensity) independently affected female choices. Using three-choice tests, we also showed that female preferences induce stabilizing selection on male claw color. We conclude that, although claw color is sufficient to facilitate species recognition, it is unlikely to be used in intraspecific mate choice to provide information about male quality.

Many aspects of behaviour depend on recognition, but accurate recognition is difficult because the traits used for recognition often overlap. For example, brood parasitic birds mimic host eggs, so it is challenging for hosts to discriminate between their own eggs and parasitic eggs. Complex signals that occur in multiple sensory modalities or involve multiple signal components are thought to facilitate accurate recognition. However, we lack models that explore the effect of complex signals on the evolution of recognition systems. Here, we use individual-based models with a genetic algorithm to test how complex signals influence recognition thresholds, signaller phenotypes and receiver responses. The model has three main results. First, complex signals lead to more accurate recognition than simple signals. Second, when two signals provide different amounts of information, receivers will rely on the more informative signal to make recognition decisions and may ignore the less informative signal. As a result, the particular traits used for recognition change over evolutionary time as sender and receiver phenotypes evolve. Third, complex signals are more likely to evolve when recognition errors are high cost than when errors are low cost. Overall, redundant, complex signals are an evolutionarily stable mechanism to reduce recognition errors. This article is part of the theme issue ‘Signal detection theory in recognition systems: from evolving models to experimental tests’.

Individual recognition is conceptually complex and computationally intense, leading to the general assumption that this social knowledge is solely present in vertebrates with larger brains, while miniature-brained animals in differentiating societies eschew the evolutionary pressure for individual recognition by evolving computationally less demanding class-level recognition, such as kin, social rank, or mate recognition. Arguably, this social knowledge is restricted to species with a degree of sociality (sensu Wilson, 2000, for a review Gherardi et al., 2012). Here, we show the exception to this rule in a non-social arthropod species, the jumping spider Phidippus regius . Using a habituation-dishabituation paradigm, we visually confronted pairs of spatially separated spiders with each other and measured the ‘interest’ of one spider towards the other. The spiders exhibited high interest upon initial encounter of an individual, reflected in mutual approach behaviour, but adapted towards that individual when it reoccurred in the subsequent trial, indicated by their preference of staying farther apart. In contrast, spiders exhibited a rebound from habituation, reflected in mutual approach behaviour, when a different individual occurred in the subsequent trial, indicating the ability to tell apart spiders’ identities. These results suggest that P. regius is capable of individual recognition based on long-term memory. Recognising familiar individuals helps animals navigate their social lives, including finding mates, avoiding rivals, and caring for their young. This skill, called individual recognition, is often linked to social species with large brains, and it supports higher cognitive functions such as empathy, reputation tracking, and attributing mental states. Recognising an individual requires memory and flexible matching across viewpoints and lighting, and larger brains have more neurons and brain areas that respond strongly to faces. Jumping spiders are tiny hunters with excellent vision, but they are mostly solitary and rarely encounter the same spider twice. Previous studies showed that these animals can distinguish species and sex using colours and patterns. However, it was less known whether a jumping spider can also recognise a specific individual. To find out more, Dahl and Cheng set up face-to-face meetings between regal jumping spiders ( Phidippus regius ) while keeping them safely separated by transparent panels. Overhead video tracked how close the pair chose to be. Mutual distance was treated as a simple readout of interest, while moving closer suggested stronger attention to the other spider. Keeping distance indicated reduced interest or growing familiarity. When a spider first saw another individual, it often approached. If the same pair met again minutes later, they tended to keep a bit more distance, consistent with becoming familiar with that specific spider. But when the next encounter featured a different individual, the spiders moved closer, showing increased interest. Across many controlled pairings, this clear behavioural switch between familiar and new repeated reliably. The effect weakened as the same individuals were shown repeatedly across sessions, even hours later, potentially indicating that the spiders were building up familiarity. Introducing a completely new individual at the end of the experiment – after hours of testing – produced the strongest renewed interest. This late rebound is inconsistent with general fatigue or waning motivation. If spiders were merely tired, an approach would be expected to drop for all stimuli. Instead, this points to sensitivity to individual novelty. In other words, the spiders retain information about who they have seen and retrieve this information after delays – a pattern consistent with encoding and storing individual-specific visual features and comparing new views against that record. These findings invite a shift in perspective. If a tiny, mostly solitary spider can recognise individuals, how much brain is really needed for flexible social memory? We may underrate animals with compact nervous systems because we treat brain size as a stand-in for cognition. What do such abilities imply for debates about animal consciousness? Behaviour cannot prove subjective experience, but it narrows what kinds of minds are plausible. Future work can map individual recognition across animal groups using the same test. It can then test whether the pattern tracks lifestyle and sensory systems more than brain size, and whether the trait evolved many times independently or from an ancient common origin.

Seasonal plasticity in insects is often triggered by temperature and photoperiod changes. When climatic conditions become sub-optimal, insects might undergo reproductive diapause, a form of seasonal plasticity delaying the development of reproductive organs and activities. During the reproductive diapause, the cuticular hydrocarbon (CHC) profile, which covers the insect body surface, might also change to protect insects from desiccation and cold temperature. However, CHCs are often important cues and signals for mate recognition and changes in CHC composition might affect mate recognition. In the present study, we investigated the CHC profile composition and the mating success of Drosophila suzukii in 1- and 5-day-old males and females of summer and winter morphs. CHC compositions differed with age and morphs. However, no significant differences were found between the sexes of the same age and morph. The results of the behavioral assays show that summer morph pairs start to mate earlier in their life, have a shorter mating duration, and have more offspring compared to winter morph pairs. We hypothesize that CHC profiles of winter morphs are adapted to survive winter conditions, potentially at the cost of reduced mate recognition cues.

Animals employ different strategies to establish mating boundaries between closely related species, with sex pheromones often playing a crucial role in identifying conspecific mates. Many of these pheromones have carbon-carbon double bonds, making them vulnerable to oxidation by certain atmospheric oxidant pollutants, including ozone. Here, we investigate whether increased ozone compromises species boundaries in drosophilid flies. We show that short-term exposure to increased levels of ozone degrades pheromones of Drosophila melanogaster , D. simulans , D. mauritiana , as well as D. sechellia , and induces hybridization between some of these species. As many of the resulting hybrids are sterile, this could result in local population declines. However, hybridization between D. simulans and D. mauritiana as well as D. simulans and D. sechellia results in fertile hybrids, of which some female hybrids are even more attractive to the males of the parental species. Our experimental findings indicate that ozone pollution could potentially induce breakdown of species boundaries in insects. Some atmospheric pollutants may disrupt chemical communication in insects. Here, the authors show that exposure to elevated ozone disrupts pheromone-mediated mate recognition and increases hybridization in laboratory colonies of four Drosophila species.