Oddity Effect Research Articles

A collective perception model for neighbor selection in groups based on visual attention mechanisms

The mechanisms by which individuals visually perceive and select others within a group to interact with are of fundamental importance to various collective motion behaviors. However, they remain poorly understood theoretically, partially due to the challenges of modeling neurological perception systems. Here, the classic zoom-lens visual attention model is introduced into collective perception, leading to an attention-based model with only a single parameter. Numerical experiments reveal that the proposed model offers a unified mechanism for collective perception, as reflected in three aspects. First, this model describes both conspecific (e.g. flocking) and heterospecific (e.g. predator-prey) collective behaviors. Second, this model unifies the well-known topological and visibility models in the context of neighbor selection in conspecific groups. Third, in the context of prey selection in heterospecific groups, the model can simultaneously replicate the well-known confusion and oddity effects. These findings demonstrate the fundamental role of visual attention underlying a diverse array of collective motion behaviors.

A mixed-species shoal including fewer heterospecifics is preferred by jack mackerel Trachurus japonicus juveniles

Mixed-species shoals of fishes were reported from various environments such as coral reefs and temperate lakes. The driving mechanism of forming mixed-species shoal can be either haphazard or a preferential choice by an individual. Here, we tested if jack mackerel Trachurus japonicus, a temperate marine pelagic fish, could choose a shoal composition using vision based on a choice experiment. Shoal preference of jack mackerel juveniles was evaluated by presenting shoals comprising one to four heterospecifics (half-lined cardinal Ostorhinchus semilineatus) out of four shoal members to the focal fish. When both conspecific and heterospecific shoals were presented to a single jack mackerel, the focal fish showed a preference for the conspecific rather than heterospecific shoal, indicating that they can choose a conspecific shoal based on the visual information. However, when a focal fish encountered both conspecific and mixed-species shoals including fewer heterospecifics (one out of four individuals), the latter was preferred. The result implies that jack mackerel can judge a preferable composition of shoal using visual information. The inclusion of fewer heterospecifics may be exploiting oddity effects, whereby minority species are exposed to relatively high vulnerability to predators, as well as resulting in reduced competition for food while maintaining predator vigilance in a shoal.

Color assortative shoaling in zebrafish, Danio rerio (Cypriniformes: Cyprinidae)

Zebrafish, Danio rerio, form shoals consisting of loosely aggregated individuals swimming in different directions. Shoal-mate composition in fish can be influenced by body size, shape, coloration, and pattern. The effect of transgenic body coloration on shoal-mate composition in wildtype D. rerio was examined. A test arena was divided into three equal sections with a wildtype zebrafish in the center compartment. For control trials, a wildtype shoal of seven individuals was placed in both side compartments. For experimental trials, one of the wildtype shoals was replaced with seven green transgenic fish. The number of visits made to each side and the time spent on each side were recorded. Control fish showed no significant difference (NSD) in the average number of visits to the left versus right side and NSD in the average time spent with either shoal. Experimental fish exhibited NSD between the average number of visits to the transgenic versus the wildtype shoal, but they spent significantly more time shoaling with the wildtype. Wildtype test fish consistently visited each side, which would enable assessment of shoal composition, but spent more time shoaling with phenotypically similar individuals, as predicted by the oddity effect. Previous studies show no significant preference by wildtype D. rerio for wildtype versus red transgenic conspecifics and indicate a more prominent role of striping pattern. This generates further questions about the influence of body coloration on shoal-mate preference. Future research could examine the effect of other transgenic colors on shoal-mate preference.

PECULIARITIES OF ASSOCIATION OF REPRESENTATIVES OF THE SUPERCLASS PISCES INTO SOCIAL GROUPS

The article presents the results of the analysis of modern literary sources on the identification of common features that contribute to the maintenance of representatives of different species of the superclass Pisces together, ensuring the social organization of individuals in a school. The article considers a «school» as a group of fish that is kept together by social attraction. The social benefits of such social association are considered, namely: providing protection from predators through mechanisms that include increased detection of predators, reduced chances of capture and entanglement of predators; increasing the ability of fish to find prey, i.e., increased success in finding food; increased probability of finding a sexual partner; hydrodynamic efficiency.
 The article describes and analyzesthe scientific evidence for the existence of phenotypic homogeneity (color, shape, size) between individuals in a school; the mechanisms by which fish are able to distinguish between group members by choosing to associate with similar.
 The popular theory of the «oddity effect» is characterized. It states that rare, phenotypically distinct individuals within a school are more likely to be targeted by predators. It is confirmed that the homogeneity of individuals in a school in terms of color, odor, body size and shape, both in homogeneous and mixed schools, is due to the «oddity effect» and the «confusion effect» and is explained by an adaptive defense mechanism against predation. It is noted that modern research provides new data that contradict the predictions of the «oddity effect», which indicates the prospects for further research on this scientific issue.
 It is noted that the complex interactive behavior of fish is influenced by differences in the sex ratio of interacting individuals in a school. It is emphasized that the influence of hormonal regulation on the social behavior of fish is significant and has its own characteristic features, which, unlike terrestrial animals, are not sufficiently studied, so this area of research is promising.

Presupposition triggers and (not-)at-issueness: Insights from language acquisition into the soft-hard distinction

Presuppositions are traditionally understood as a set of backgrounded, and thus not-at-issue, projective inferences that are taken for granted by communicators. In the last decades it has been observed that presuppositions behave heterogeneously, which lead to a discussion about the distinction between soft and hard presupposition triggers. In this paper, another property of presuppositions is exploited to test if there is evidence for a soft-hard dichotomy: their reluctance to answer the current Question Under Discussion, as observed by Simons et al. (2010). Using a modified acceptability judgment task, we tested children between 4 and 6 years of age and adult controls. Audio recordings with image stills featured both kinds of presupposition triggers in at-issue and non-at-issue exchanges, with non-restrictive relative clauses as controls, as they are conventionally not-at-issue but usually add new information. The results indicate that, for our adult participants, such backgroundedness violations are worse in the case of hard triggers, whereas soft triggers are markedly less deviant in such cases. Children are also sensitive to the soft-hard distinction but react less strongly than adult counterparts to oddity effects. Additionally, hard triggers pattern with non-restrictive relative clauses in both groups.

Juvenile bonefish (Albula vulpes) show a preference to shoal with mojarra (Eucinostomus spp.) in the presence of conspecifics and another gregarious co-occurring species

Grouping behavior occurs across a wide variety of taxa, both within and between species. While members are thought to obtain foraging and antipredator advantages, they can also experience costs in the forms of competition or increased conspicuity to predators. The mechanisms behind these costs and benefits can vary depending on group composition, ultimately influencing the choice of which groups to join. Mixed-species fish shoals are common in shallow nearshore habitats, where research has shown that juvenile bonefish (Albula vulpes) occur among similarly-sized mojarras (Eucinostomus spp.) at a rate far exceeding that predicted given the fishes overlapping habitat use, suggesting that bonefish may actively select to join mojarras. To evaluate this hypothesis experimentally, we assessed the shoaling preferences of bonefish when presented with shoals of conspecifics, mojarras, or another gregarious co-occurring fish (pilchard; Harengula jaguana) in a laboratory setting. Focal juvenile bonefish (n = 25) were given the choice between: (i) conspecifics or mojarra, and (ii) conspecifics or pilchard, tested in shoal sizes of one, two, four, and eight. Bonefish were also given the choice between a mixed shoal (two conspecifics, two mojarra) as an alternative to single species shoals of either: (iii) four conspecifics, or (iv) four mojarra. Juvenile bonefish exhibited a strong association with mojarra, spending significantly more time with them than conspecifics in all but one treatment. Moreover, focal fish showed no detectable preference between conspecifics or pilchard, regardless of shoal size. In mixed shoal treatments, focal fish spent significantly more time wherever there was a higher proportion of mojarra. These findings imply that the co-occurrence of bonefish and mojarra in nature is largely a product of bonefish shoal choice behavior, and likewise that any costs that bonefish might incur by joining mojarras (i.e., oddity effects) are likely outweighed by the putative benefits of doing so, which potentially include access to social information and/or reduced intraspecific competition.

Picky predators and odd prey: colour and size matter in predator choice and zebrafish’s vulnerability – a refinement of the oddity effect

Many fish species typically assort into homogeneous groups and thus reduce the risk of predators targeting individuals that are phenotypically odd compared to the majority of the group. The confusion and oddity effect hypotheses predict, that phenotypically odd individuals in a group are preferentially targeted by predators. Therefore, heterogeneous groups of prey should be more vulnerable to predator attention and attacks than homogeneous (all prey is the same colour, size, etc.) groups. Predators might use different cues to select a target, depending on their sensory abilities (sound, smell, vision, etc.). Visual-hunting predators that can see colour, for example, may be more likely to select prey based on colour than size oddity of prey. Previous studies on the oddity effect have found support for the colour-oddity and predator response, and to a lesser extent on size oddity. We thus predicted that, given a choice, a group with an odd-coloured individual would be targeted more often by a predator than a group with an odd-size fish but not odd in colour. Heterogeneous groups of six zebrafish (Danio rerio) containing either one odd-coloured (wildtype or red) or one odd-sized (large or small) individual were used as stimulus prey in dichotomous choice experiments examining prey preference in needlefish (Xenentodon cancila). Needlefish preferred heterogeneous groups regardless of the colour of the odd individual and when the odd individual was smaller than the other group members. However, when the odd individual was larger than the rest of the group, the predators preferred homogeneous groups of small fish. These new findings are inconsistent with the predictions of the oddity effect and warrant investigation of alternative hypotheses for size-assorted grouping in fish.

Computerized stimuli for studying oddity effects

Abstract Visually hunting predators must overcome the challenges that prey groups present. One such challenge is the confusion effect where an overburdened visual system means predators are unable to successfully target prey. A strategy to overcome confusion is the targeting of distinct, or odd, individuals (the oddity effect). In live prey experiments, manipulation of group member phenotypes can be challenging and prey may differ on more than the single feature one intends to define as odd. The use of highly controllable computerized stimuli to study predator–prey interactions is increasingly popular in the field of behavioral ecology. However, to our knowledge, the validity of computerized stimuli to study the oddity effect has not been established. Predator choice experiments were conducted using naive stickleback predators to ascertain whether the oddity effect could be demonstrated in the absence of live prey. We found evidence for both the oddity effect and preferential targeting of group edges and low-density regions, as would be predicted if predators targeted prey individuals to minimize confusion. The oddity effect was evident at a low threshold, above which dots were no longer perceived as odd and no longer attacked more often than expected by chance. We conclude that computerized stimuli are an improved, practical method for studying oddity effects while further validating the use of similar methods for studying other aspects of visual predation. In addition to higher control of “prey” appearance, the replacement of live prey animals with digital stimuli is ethically beneficial and reusing code improves experimental efficiency.

Oddity, predation risk and social decisions in aquatic invertebrates

AbstractGroup living is widespread across animal taxa, incurring benefits such as increased foraging efficiency or an enhanced chance of surviving a predator's attack. The chances of escaping a predator are often lower for odd‐looking individuals, as these are detected at a higher rate than uniform looking group members. While this “oddity effect” shall operate in animals differing in any given phenotype, including colour, size or species identity, it has been experimentally tested mainly for odd‐coloured individuals. We examined the oddity effect and swarming preferences in two differently sized species of freshwater planktonic crustaceans (largeDaphnia magnaand smallDaphnia pulex). We experimentally investigated whether odd individuals in a swarm of heterospecificDaphniawere more vulnerable to predation by a common predator, the three‐spined stickleback (Gasterosteus aculeatus). Furthermore,Daphnia'sswarming preference was tested by giving individuals the opportunity to choose between conspecific/heterospecific odour and a neutral control. In contrast to the predictions of the oddity effect, odd individuals were not always preyed on earlier; instead three‐spined stickleback preferentially predated large, more nutritious individuals.Daphniaof both species reacted towards the perception of con‐ and heterospecifics odours. WhileD. pulexgenerally avoided the smell of other daphnids,D. magnaavoided conspecifics but tended to prefer heterospecifics over the neutral control. These findings provide new insights into swarming strategies and social preference of an invertebrate and how this behaviour can influence predation risk.

An Implicature-Based Approach to Disjointness Effects

The generation and distribution of disjointness effects raise a descriptive and explanatory challenge to linguistic theories: what are the conditions under which these effects arise and why do these effects arise in the first place? In this paper, I flesh out an account of these effects that takes up both these challenges at once in showing that these effects are derivable from current approaches to implicature-reasoning. In substance, it is argued that these effects follow from a genuine implicature-based reasoning whose outcome may, upon certain contextual pressures, result in conflicting representations, giving rise then to oddity effects. The account is shown to unify various disjointness effects under one roof and to explain the source of some of the classical conditions on co-reference and binding.

The oddity effect drives prey choice but not necessarily attack time

AbstractThe tendency of predators to preferentially attack phenotypically odd prey in groups (the oddity effect) is a clear example of how predator cognition can impact behaviour and morphology in prey. Through targeting phenotypically odd prey, predators are thought to avoid the cognitive constraints that delay and limit the success of attacks on homogenous prey groups (the confusion effect). In addition to influencing which prey a predator will attack, the confusion and oddity effects would also predict that attacks on odd prey can occur more rapidly than attacking the majority prey type, as odd prey are more easily targeted, but this prediction has yet to be tested. Here, we used kerri tetra fish, Inpaichthys kerri, presented with mixed phenotypic groups of Daphnia dyed red or black to investigate whether odd prey in groups are preferentially attacked and whether these attacks were faster than those on the majority prey type. In agreement with previous work, odd prey were targeted and attacked more often than expected from their frequency in the prey groups, regardless of whether the odd prey was red in a group of black prey or vice versa. However, no difference was found in the time taken to attack odd vs. majority prey items, contrary to our predictions. Our results suggest that the time taken to make an attack is determined by a wider range of factors or is subject to greater variance than the choice of which prey is selectively targeted in a group.

Does the presence of an odd individual affect group choice?

Odd animals are often targeted by predators, but can other group members exploit this “oddity effect” to their advantage by reducing their own predation risk? We found that the strength of the oddity effect varies across coral reef fish species. When the oddity effect was apparent, fish neither joined nor avoided groups with an odd individual, suggesting that possible benefits of associating with a vulnerable group member are reduced by greater predation risk to the group as a whole.

Dazzle camouflage and the confusion effect: the influence of varying speed on target tracking

The formation of groups is a common strategy to avoid predation in animals, and recent research has indicated that there may be interactions between some forms of defensive coloration, notably high-contrast ‘dazzle camouflage’, and one of the proposed benefits of grouping: the confusion effect. However, research into the benefits of dazzle camouflage has largely used targets moving with constant speed. This simplification may not generalize well to real animal systems, where a number of factors influence both within- and between-individual variation in speed. Departure from the speed of your neighbours in a group may be predicted to undermine the confusion effect. This is because individual speed may become a parameter through which the observer can individuate otherwise similar targets: an ‘oddity effect’. However, dazzle camouflage patterns are thought to interfere with predator perception of speed and trajectory. The current experiment investigated the possibility that such patterns could ameliorate the oddity effect caused by within-group differences in prey speed. We found that variation in speed increased the ease with which participants could track targets in all conditions. However, we found no evidence that motion dazzle camouflage patterns reduced oddity effects based on this variation in speed, a result that may be informative about the mechanisms behind this form of defensive coloration. In addition, results from those conditions most similar to those of published studies replicated previous results, indicating that targets with stripes parallel to the direction of motion are harder to track, and that this pattern interacts with the confusion effect to a greater degree than background matching or orthogonal-to-motion striped patterns.

Homeward bound: fish larvae use dispersal corridors when settling on coral reefs

Homeward bound: fish larvae use dispersal corridors when settling on coral reefs

Turbidity weakens selection for assortment in body size in groups

Prey animals commonly associate with similar-looking individuals to reduce predation risk, via a reduction in predator targeting accuracy (the confusion effect) and preferential targeting of distinct individuals (the oddity effect). These effects are mediated by body size, as predators often preferentially select large-bodied individuals, which are therefore at an increased risk within a group. The selection pressure to avoid oddity by associating with similar sized group mates is stronger for large individuals than small. This selection depends on the ability of both predators and prey to accurately assess body size and respond accordingly. In aquatic systems, turbidity degrades the visual environment and negatively impacts on the ability of predators to detect (and consume) prey. We assessed the effect of algal turbidity on predator–prey interactions in the context of the oddity effect from the perspective of both predator and prey. From a predator’s perspective, we find that 9-spined sticklebacks preferentially target larger Daphnia in mixed swarms in clear water, but not in turbid water, although the difference in attack rates is not statistically significant. When making shoaling decisions, large sticklebacks preferentially associate with size-matched individuals in clear water, but not turbid water, whereas small individuals showed no social preference in either clear or turbid water. We suggest that a reduced ability or motivation to discriminate between prey in turbid water relaxes the predation pressure on larger prey individuals allowing greater flexibility in shoaling decisions. Thus, turbidity may play a significant role in predator–prey interactions, by altering predator–prey interactions.

Predation and landscape characteristics independently affect reef fish community organization.

Trophic island biogeography theory predicts that the effects of predators on prey diversity are context dependent in heterogeneous landscapes. Specifically, models predict that the positive effect of habitat area on prey diversity should decline in the presence of predators, and that predators should modify the partitioning of alpha and beta diversity across patchy landscapes. However, experimental tests of the predicted context dependency in top-down control remain limited. Using a factorial field experiment we quantify the effects of a focal predatory fish species (grouper) and habitat characteristics (patch size, fragmentation) on the partitioning of diversity and assembly of coral reef fish communities. We found independent effects of groupers and patch characteristics on prey communities. Groupers reduced prey abundance by 50% and gamma diversity by 45%, with a disproportionate removal of rare species relative to common species (64% and 36% reduction, respectively; an oddity effect). Further, there was a 77% reduction in beta diversity. Null model analysis demonstrated that groupers increased the importance of stochastic community assembly relative to patches without groupers. With regard to patch size, larger patches contained more fishes, but a doubling of patch size led to a modest (36%) increase in prey abundance. Patch size had no effect on prey diversity; however, fragmented patches had 50% higher species richness and modified species composition relative to unfragmented patches. Our findings suggest two different pathways (i.e., habitat or predator shifts) by which natural and/or anthropogenic processes can drive variation in fish biodiversity and community assembly.

Mixed-phenotype grouping: the interaction between oddity and crypsis

Aggregations of different-looking animals are frequently seen in nature, despite well-documented selection pressures on individuals to maintain phenotypically homogenous groups. Two well-known theories, the 'confusion effect' (reduced ability of a predator to accurately target an individual in a group) and the 'oddity effect' (preferential targeting of phenotypically distinct, 'odd', individuals) act together to predict the evolution of behaviours in prey that lead to groups of animals that are homogeneous in appearance. In contrast, a recently proposed mechanism suggests that mixed groups could be maintained if one species in a mixed group is more conspicuous against the habitat than the other, as confusion effects generated by the conspicuous species impede predator targeting of the cryptic species; thus, cryptic species benefit from association with conspicuous ones. We test these contrasting predictions from the perspective of both predators and prey, and show that cryptic individual Daphnia are at reduced risk of predation from three-spine sticklebacks Gasterosteus aculeatus when in mixed-phenotype groups, a risk that is reduced further as the number of conspicuous individuals increases, supporting the hypothesis for the evolution of mixed groups. In contrast, while the preference for associating with colour-matched conspecifics by mollies (Poecilia sphenops) was reduced when they were cryptic, we found no evidence for active association with conspicuous conspecifics. We conclude that prey animals must balance the relative risks of oddity and conspicuousness in their social decisions, and that this could potentially lead to the evolution of mixed-phenotype grouping as a response to predation risk alone.

Balancing the dilution and oddity effects: decisions depend on body size.

BackgroundGrouping behaviour, common across the animal kingdom, is known to reduce an individual's risk of predation; particularly through dilution of individual risk and predator confusion (predator inability to single out an individual for attack). Theory predicts greater risk of predation to individuals more conspicuous to predators by difference in appearance from the group (the ‘oddity’ effect). Thus, animals should choose group mates close in appearance to themselves (eg. similar size), whilst also choosing a large group.Methodology and Principal FindingsWe used the Trinidadian guppy (Poecilia reticulata), a well known model species of group-living freshwater fish, in a series of binary choice trials investigating the outcome of conflict between preferences for large and phenotypically matched groups along a predation risk gradient. We found body-size dependent differences in the resultant social decisions. Large fish preferred shoaling with size-matched individuals, while small fish demonstrated no preference. There was a trend towards reduced preferences for the matched shoal under increased predation risk. Small fish were more active than large fish, moving between shoals more frequently. Activity levels increased as predation risk decreased. We found no effect of unmatched shoal size on preferences or activity.Conclusions and SignificanceOur results suggest that predation risk and individual body size act together to influence shoaling decisions. Oddity was more important for large than small fish, reducing in importance at higher predation risks. Dilution was potentially of limited importance at these shoal sizes. Activity levels may relate to how much sampling of each shoal was needed by the test fish during decision making. Predation pressure may select for better decision makers to survive to larger size, or that older, larger fish have learned to make shoaling decisions more efficiently, and this, combined with their size relative to shoal-mates, and attractiveness as prey items influences shoaling decisions.

Prey jitters; protean behaviour in grouped prey

There have been many descriptive accounts of so-called ‘‘protean’’ or unpredictable escape tactics in prey animals. It is assumed that predators will have difficulty targeting more protean prey either via the prey’s escape trajectory and/or via enhancement of the confusion effect (reduced ability to track an individual target in a group of moving and phenotypically similar prey). However, to date no studies have examined whether protean movements decrease predation risk as predicted. In this paper we test, using humans as model predators, whether ‘‘protean’’ prey that have a higher variance in turning angle (and thus change direction abruptly, rather than move on a smoother trajectory) are more difficult to capture and whether this interacts with the confusion effect (previously found using the same model system). We found that protean prey were significantly more difficult to capture. However, protean behaviour did not enhance the confusion effect. We suggest that the primary purpose of protean movements may simply be to make it more difficult for a predator to chase and capture prey, increasing the likelihood predators will give up chase sooner, and that such protean behaviour need not be contingent on prey aggregation. Key words: confusion effect, escape path, oddity effect, predator, prey, prey tactics. [Behav Ecol]

How perceived threat increases synchronization in collectively moving animal groups

Nature is rich with many different examples of the cohesive motion of animals. Previous attempts to model collective motion have primarily focused on group behaviours of identical individuals. In contrast, we put our emphasis on modelling the contributions of different individual-level characteristics within such groups by using stochastic asynchronous updating of individual positions and orientations. Our model predicts that higher updating frequency, which we relate to perceived threat, leads to more synchronized group movement, with speed and nearest-neighbour distributions becoming more uniform. Experiments with three-spined sticklebacks (Gasterosteus aculeatus) that were exposed to different threat levels provide strong empirical support for our predictions. Our results suggest that the behaviour of fish (at different states of agitation) can be explained by a single parameter in our model: the updating frequency. We postulate a mechanism for collective behavioural changes in different environment-induced contexts, and explain our findings with reference to confusion and oddity effects.