Olfactory self-recognition in a cichlid fish

Animal self-cognizance might be of importance in different contexts like territoriality, self-referent mate-choice or kin recognition. We investigated whether the cichlid fish Pelvicachromis taeniatus is able to recognize own olfactory cues. P. taeniatus is a cave breeding fish with pronounced brood care and social behavior. In the experiments we gave male cave owners the choice between two caves in which we introduced scented water. In a first experiment males preferred caves with their own odor over caves with the odor of an unfamiliar, unrelated male. To examine whether self-recognition is based rather on individual or on family cues we conducted two further experiments in which males could choose between their own odor and the odor of a familiar brother and between the odor of a familiar brother and an unfamiliar, unrelated male, respectively. Males preferred their own odor over that of a familiar brother suggesting individual self-referencing. Interestingly, males (at least outbred ones) preferred the odor of an unfamiliar, unrelated male over that of a familiar brother, maybe to avoid competition with kin. We discuss the results in the context of animal self-cognizance. All experiments were conducted with in- and outbred fish. Inbreeding did not negatively affect self-recognition.     

Cross-modal recognition of human individuals in domestic horses (Equus caballus)

This study has shown that domestic horses are capable of cross-modal recognition of familiar humans. It was demonstrated that horses are able to discriminate between the voices of a familiar and an unfamiliar human without seeing or smelling them at the same moment. Conversely, they were able to discriminate the same persons when only exposed to their visual and olfactory cues, without being stimulated by their voices. A cross-modal expectancy violation setup was employed; subjects were exposed both to trials with incongruent auditory and visual/olfactory identity cues and trials with congruent cues. It was found that subjects responded more quickly, longer and more often in incongruent trials, exhibiting heightened interest in unmatched cues of identity. This suggests that the equine brain is able to integrate multisensory identity cues from a familiar human into a person representation that allows the brain, when deprived of one or two senses, to maintain recognition of this person.     

Possible olfaction-based mechanisms in human kin recognition and inbreeding avoidance

Three studies explored kin recognition through olfaction. In Study I, adults (N=22) were tested for ability to identify the odors of themselves; their mother; their father; a sister; a brother; a familiar, unrelated individual; and a stranger. Acquaintances were identified as accurately as biological kin, implicating an association mechanism. However, biological kin were often confused, implicating phenotypic matching. Same-sex kin were confused more than opposite-sex kin, but mainly when same-sex kin had odors of similar intensity. Study II implicated phenotypic matching. Mothers (N=18) could identify their biological children but not their stepchildren. The preadolescent children (N=37) identified their full siblings but not half-siblings or stepsiblings. Thus, olfactory cues may help mediate favoritism of blood relatives. In Study III, mutual olfactory aversion occurred only in the father-daughter and brother-sister nuclear family relationships. Recognition occurred between opposite-sex siblings but not same-sex siblings. Thus, olfaction may help mediate the development of incest avoidance during childhood (the Westermarck effect).     

The discrimination of dog odours by humans

The domestic dog is able to identify both individual conspecifics, i.e. other dogs, and individual interspecifics, e.g. humans, by smell. Whilst humans can recognise individual members of their own species using olfactory information, it is unknown whether they can identify members of any other species in this manner. We examined the ability of humans to identify individual dogs by smell. Twenty-six dog owners were required to smell two blankets, one impregnated with the odour of their own dog, the other impregnated with the odour of an unfamiliar dog. Participants were required to indicate which of the odours smelt the strongest, which smelt the most pleasant, and which of the odours belonged to their own dog. Most of the participants (88.5%) were able to recognise the odour of their own dog. They showed no significant bias, however, in responding which of the odours they thought smelt the strongest or most pleasant. The results indicate that dogs produce odours that are individually distinctive to their owners, and highlight the fact that humans can recognise members of another species using olfactory cues--an ability presumably acquired without conscious effort.     

Sex Differences in Novelty-related Location Preferences of Hooded Rats

In a square exploration box comprising a familiar and a novel half, hooded rats of both sexes were confronted with brightness differences between the two halves, brightness changes in the novel half, and attenuation of odour cues. Only males showed a significant preference for occupying the novel half when it had been visible but physically inaccessible during prior confinement to the familiar half. Although the brightness differences and changes did not affect the rats' novelty-related location preferences, preferences were lower whenever the novel half was white. This obvious brightness aversion was accentuated by a change from black to white. Attenuation of odour cues by washing the apparatus resulted in males (as well as females) failing to show a preference for the novel half. It was concluded that, although disparities in the specific type of visual information provided did not affect preferences, female rats appeared able to familiarize themselves with the future novel half through being able to see into it, whereas males may have required odour information for this to occur.     

Organization of behavior in unfamiliar female rabbits

Under natural conditions, female European rabbits usually spend the whole of their life in their natal groups, establishing a network of stable relationships with their group-mates, particularly with other females. Indeed, female social interactions with groupmates are ultimately responsible for the dimension, structure, and cohesion of the group itself. Despite their importance in social organization, non-reproductive female behaviors are poorly known. This study aims to provide information on the nature and distribution of these behaviors in familiar and unfamiliar milieux. For this purpose, female rabbits unfamiliar with each other were experimentally grouped, housed in outdoor enclosures new to them, and observed as they gradually familiarized with their environment and group-mates. When females were unfamiliar with both environment and group-mates, they would often mark objects (but never conspecifics) and engage in agonistic activities, preceded by approach and olfactory investigation. Once females had become familiar with their environment and group-mates, they spent a lot of time eating and self-grooming. Marking frequency decreased, and agonistic behavior became rare and appeared no longer related to introductory behaviors, which under these conditions led to amiable interactions, such as lying side by side with group-mates or grooming them. Under both unfamiliar and familiar conditions, high-ranking females appeared more active and interactive than low-ranking ones.     

The potential hedonic role of olfaction in sexual selection and its dominance in visual cross-modal interactions

Perfumes are commonly used to cover body odour, or to provide a positive, attracting, and interesting impact, or a smell that belongs to a social group. A role in sexual communication of such non-pheromonal olfactory cues has been suggested in the literature. However, there remain the questions whether these stimuli are involved in human chemosexual communication and, if so, at what level, and whether they interact with other sensorial modalities, in particular vision. To answer these, we investigated the influence of male and female perfumes as nonconscious stimulation during visual assessments of a range of facial qualities across and within the sexes. The female subjects were in their ovulatory phase, to avoid changes in perception across the menstrual cycle. Our data indicate that non-pheromonal olfactory cues are potentially involved in mate choice and may elicit strong hedonic responses that can dominate visual signs, with a cross-modal interaction.     

Object segmentation by top-down processes

In cluttered scenes, some object boundaries may not be marked by image cues. In such cases, the boundaries must be defined top-down as a result of object recognition. Here we ask if observers can retain the boundaries of several recognized objects in order to segment an unfamiliar object. We generated scenes consisting of neatly stacked objects, and the objects themselves consisted of neatly stacked coloured blocks. Because the blocks were stacked the same way within and across objects, there were no visual cues indicating which blocks belonged to which objects. Observers were trained to recognize several objects and we tested whether they could segment a novel object when it was surrounded by these familiar, studied objects. The observer's task was to count the number of blocks comprising the target object. We found that observers were able to accurately count the target blocks when the target was surrounded by up to four familiar objects. These results indicate that observers can use the boundaries of recognized objects in order to accurately segment, top-down, a novel object.     

The sibling familiarity effect: Is within-person facial variability shared across siblings?

Humans are experts at familiar face recognition, but poor at unfamiliar face recognition. Familiarity is created when a face is encountered across varied conditions, but the way in which a person’s appearance varies is identity-specific, so familiarity with one identity does not benefit recognition of other individuals. However, the faces of biological siblings share structural similarities, so we explored whether the benefits of familiarity are shared across siblings. Results show that familiarity with one half of a sibling pair improves kin detection (experiment 1), and that unfamiliar face matching is more accurate when targets are the siblings of familiar versus unfamiliar individuals (experiment 2). PCA applied to facial images of celebrities and their siblings demonstrates that faces are generally better reconstructed in the principal components of a same-sex sibling than those of an unrelated individual. When we encounter the unfamiliar sibling of someone we already know, our pre-existing representation of their familiar relation may usefully inform processing of the unfamiliar face. This can benefit both kin detection and identity processing, but the benefits are constrained by the degree to which facial variability is shared.     

An attention account of neural priming

Repetition priming of familiar stimuli (e.g., objects) produces a decrease in visual cortical activity for repeated versus novel items, which has been attributed to more fluent processing for repeated items. By contrast, priming of unfamiliar stimuli (e.g., abstract shapes) produces an increase in visual cortical activity. The mechanism for priming-related increases in activity for repeated unfamiliar stimuli is unknown. We hypothesised that such increases in activity may reflect attentional allocation to these items. We tested this hypothesis using a priming-spatial attention paradigm. During Phase 1 of Experiment 1, participants viewed unfamiliar abstract shapes and familiar objects. During Phase 2, participants identified target letters (S or H). Each target letter was preceded by a non-informative shape or object cue that was repeated (from Phase 1) or novel in the same (valid) or opposite (invalid) hemifield. In Experiment 2, we manipulated shape familiarity by presenting shapes once or six times during Phase 1. For both experiments, at valid locations, target identification accuracy was higher following repeated versus novel unfamiliar item cues and lower following repeated versus novel familiar item cues. These findings support our hypothesis that priming-related increases in visual cortical activity for repeated unfamiliar items may, in part, reflect attentional allocation.     

Nestling recognition via direct cues by parental male bluegill sunfish (<Emphasis Type="Italic">Lepomis macrochirus</Emphasis>)

Parental care can be costly to a parent in terms of both time and energy invested in the young. In species with cuckoldry or brood parasitism not all of the young under a parent's care are necessarily offspring. In such cases, distinguishing between kin and non-kin, and investing only in the former (nepotism), can be advantageous. Bluegill sunfish (Lepomis macrochirus) are characterized by paternal care and cuckoldry, and care-providing males appear to show nepotistic behaviours. Here, we investigated nestling recognition in bluegill, determining whether parental males can differentiate between young from their own nest (familiar and related) and young from non-neighbouring nests (unfamiliar and unrelated) using (1) visual and chemical cues, and (2) chemical cues only. In the first experiment, wild-caught parental males were presented with samples of eggs or fry (newly hatched eggs) collected from their own nest or a foreign nest and placed on opposite sides of an aquarium. The time these parental males spent associating with each sample, and their "pecking" behaviours (indicating cannibalism), were recorded. Parental males showed no preference between eggs from their own nest and eggs from a non-neighbouring nest, but they preferred to associate with fry from their own nest over foreign fry. There also was a positive relationship between male body size and the time spent associated with fry from their own nest. Parental males pecked at foreign fry more than 5 times as often as fry from their own nest, though this difference was not statistically significant. In the second experiment, fry that were collected from the nest of a wild-caught parental male or a non-neighbouring nest were placed in different containers and the water from each was dripped into opposite ends of an aquarium. The time the male spent on each side was recorded. In this case, parental males spent more time near the source of water conditioned by unrelated fry, but there was a positive relationship between male condition (fat reserves) and the time he spent near the source of water conditioned by fry from his own nest. Results confirm that chemicals cue nestling recognition by parental male bluegill. Electronic Publication     

Horses (<Emphasis Type="Italic">Equus caballus</Emphasis>) use human local enhancement cues and adjust to human attention

This study evaluates the horse (Equus caballus) use of human local enhancement cues and reaction to human attention when making feeding decisions. The superior performance of dogs in observing human states of attention suggests this ability evolved with domestication. However, some species show an improved ability to read human cues through socialization and training. We observed 60 horses approach a bucket with feed in a three-way object-choice task when confronted with (a) an unfamiliar or (b) a familiar person in 4 different situations: (1) squatting behind the bucket, facing the horse (2) standing behind the bucket, facing the horse (3) standing behind the bucket in a back-turned position, gazing away from the horse and (4) standing a few meters from the bucket in a distant, back-turned position, again gazing away from the horse. Additionally, postures 1 and 2 were tested both with the person looking permanently at the horse and with the person alternating their gaze between the horse and the bucket. When the person remained behind the correct bucket, it was chosen significantly above chance. However, when the test person was turned and distant from the buckets, the horses’ performance deteriorated. In the turned person situations, the horses approached a familiar person and walked towards their focus of attention significantly more often than with an unfamiliar person. Additionally, in the squatting and standing person situations, some horses approached the person before approaching the correct bucket. This happened more with a familiar person. We therefore conclude that horses can use humans as a local enhancement cue independently of their body posture or gaze consistency when the persons remain close to the food source and that horses seem to orientate on the attention of familiar more than of unfamiliar persons. We suggest that socialization and training improve the ability of horses to read human cues.     

Discrimination of familiar human faces in dogs (Canis familiaris)

Faces are an important visual category for many taxa, and the human face is no exception to this. Because faces differ in subtle ways and possess many idiosyncratic features, they provide a rich source of perceptual cues. A fair amount of those cues are learned through social interactions and are used for future identification of individual humans. These effects of individual experience can be studied particularly well in hetero-specific face perception. Domestic dogs represent a perfect model in this respect, due to their proved ability to extract important information from the human face in socio-communicative interactions. There is also suggestive evidence that dogs can identify their owner or other familiar human individuals by using visual information from the face. However, most studies have used only dogs’ looking behavior to examine their visual processing of human faces and it has been demonstrated only that dogs can differentiate between familiar and unknown human faces. Here, we examined the dog's ability to discriminate the faces of two familiar persons by active choice (approach and touch). Furthermore, in successive stages of the experiment we investigated how well dogs discriminate humans in different representations by systematically reducing the informational richness and the quality of the stimuli. We found a huge inter-individual and inter-stage variance in performance, indicating differences across dogs in their learning ability as well as their selection of discriminative cues. On a group level, the performance of dogs significantly decreased when they were presented with pictures of human heads after having learned to discriminate the real heads, and when – after relearning – confronted with the same pictures showing only the inner parts of the heads. However, as two dogs quickly mastered all stages, we conclude that dogs are in principle able to discriminate people on the basis of visual information from their faces and by making active choices.     

Tonkean macaques ( Macaca tonkeana) find food sources from cues conveyed by group-mates

It is possible that non-specialised cues transmitted by conspecifics guide animals' food search provided they have the cognitive abilities needed to read these cues. Macaques often check the mouth of their group-mates by olfactory and/or visual inspection. We investigated whether Tonkean macaques (Macaca tonkeana) can find the location of distant food on the basis of cues conveyed by group-mates. The subjects of the study were two 6-year-old males, who belonged to a social group of Tonkean macaques raised in semi-free-ranging conditions. In a first experiment, we tested whether the subject can choose between two sites after having sniffed a partner who has just eaten food corresponding to one of the sites. We found that both subjects were able to choose the matching site significantly above the chance level. This demonstrated that Tonkean macaques are capable of delayed olfactory matching. They could associate a food location with an odour conveyed by a partner. In a second experiment, the same subjects were allowed to see their partner through a Plexiglas window. Both subjects were still able to choose the matching site, demonstrating they could rely on visual cues alone. Passive recruitment of partners appears possible in macaques. They can improve their foraging performances by finding the location of environmental resources from olfactory or visual cues conveyed by group-mates. Electronic Publication     

A behavioral study of alpha-1b adrenergic receptor knockout mice: increased reaction to novelty and selectively reduced learning capacities

Knockout mice lacking the alpha-1b adrenergic receptor were tested in behavioral experiments. Reaction to novelty was first assessed in a simple test in which the time taken by the knockout mice and their littermate controls to enter a second compartment was compared. Then the mice were tested in an open field to which unknown objects were subsequently added. Special novelty was introduced by moving one of the familiar objects to another location in the open field. Spatial behavior and memory were further studied in a homing board test, and in the water maze. The alpha-1b knockout mice showed an enhanced reactivity to new situations. They were faster to enter the new environment, covered longer paths in the open field, and spent more time exploring the new objects. They reacted like controls to modification inducing spatial novelty. In the homing board test, both the knockout mice and the control mice seemed to use a combination of distant visual and proximal olfactory cues, showing place preference only if the two types of cues were redundant. In the water maze the alpha-1b knockout mice were unable to learn the task, which was confirmed in a probe trial without platform. They were perfectly able, however, to escape in a visible platform procedure. These results confirm previous findings showing that the noradrenergic pathway is important for the modulation of behaviors such as reaction to novelty and exploration, and suggest that this is mediated, at least partly, through the alpha-1b adrenergic receptors. The lack of alpha-1b adrenergic receptors in spatial orientation does not seem important in cue-rich tasks but may interfere with orientation in situations providing distant cues only.     

Food Avoidance Learning in Squirrel Monkeys and Common Marmosets

Using a conditioned food avoidance learning paradigm, six squirrel monkeys (Saimiri sciureus) and six common marmosets (Callithrix jacchus) were tested for their ability to (1) reliably form associations between visual or olfactory cues of a potential food and its palatability and (2) remember such associations over prolonged periods of time. We found (1) that at the group level both species showed one-trial learning with the visual cues color and shape, whereas only the marmosets were able to do so with the olfactory cue, (2) that all individuals from both species learned to reliably avoid the unpalatable food items within 10 trials, (3) a tendency in both species for quicker acquisition of the association with the visual cues compared with the olfactory cue, (4) a tendency for quicker acquisition and higher reliability of the aversion by the marmosets compared with the squirrel monkeys, and (5) that all individuals from both species were able to reliably remember the significance of the visual cues, color and shape, even after 4 months, whereas only the marmosets showed retention of the significance of the olfactory cues for up to 4 weeks. Furthermore, the results suggest that in both species tested, illness is not a necessary prerequisite for food avoidance learning but that the presumably innate rejection responses toward highly concentrated but nontoxic bitter and sour tastants are sufficient to induce robust learning and retention.     

Auditory–visual intermodal matching based on individual recognition in a chimpanzee (Pan troglodytes)

The ability to recognize familiar individuals with different sensory modalities plays an important role in animals living in complex physical and social environments. Individual recognition of familiar individuals was studied in a female chimpanzee named Pan. In previous studies, Pan learned an auditory–visual intermodal matching task (AVIM) consisting of matching vocal samples with the facial pictures of corresponding vocalizers (humans and chimpanzees). The goal of this study was to test whether Pan was able to generalize her AVIM ability to new sets of voice and face stimuli, including those of three infant chimpanzees. Experiment 1 showed that Pan performed intermodal individual recognition of familiar adult chimpanzees and humans very well. However, individual recognition of infant chimpanzees was poorer relative to recognition of adults. A transfer test with new auditory samples (Experiment 2) confirmed the difficulty in recognizing infants. A remaining question was what kind of cues were crucial for the intermodal matching. We tested the effect of visual cues (Experiment 3) by introducing new photographs representing the same chimpanzees in different visual perspectives. Results showed that only the back view was difficult to recognize, suggesting that facial cues can be critical. We also tested the effect of auditory cues (Experiment 4) by shortening the length of auditory stimuli, and results showed that 200 ms vocal segments were the limit for correct recognition. Together, these data demonstrate that auditory–visual intermodal recognition in chimpanzees might be constrained by the degree of exposure to different modalities and limited to specific visual cues and thresholds of auditory cues.     

What if they knocked down the library and nobody noticed? The failure to detect large changes to familiar scenes

The purpose of the current study was to investigate people's ability to detect changes to familiar scenes. College students were asked either to identify what was wrong with a picture of a familiar location on their college campus (e.g., the library had been removed from the scene), or to estimate the difficulty of change detection for a hypothetical cohort performing the same task. Performance in the change-detection condition was extremely poor, even when changes were large. Participants who were familiar with the scenes and those who were unfamiliar with the scenes both overestimated the actual levels of change-detection performance. A follow-up analysis indicated that the participants who were unfamiliar with the scenes produced estimations of difficulty that were highly correlated with the mathematical area of the change, whereas participants who were familiar with the scenes produced estimations of difficulty that were highly correlated with the actual difficulty of change detection. The results indicate that people's visual long-term memory for familiar scenes lacks the precision to be able to effectively identify even large-scale changes, although subjectively people believe this should be relatively easy.     

Famous faces as icons. The illusion of being an expert in the recognition of famous faces

It is a common belief that we are experts in the processing of famous faces. Although our ability to quickly and accurately recognise pictures of famous faces is quite impressive, we might not really process famous faces as faces per se, but as 'icons' or famous still pictures of famous faces. This assumption was tested in two parallel experiments employing a recognition task on famous, but personally unfamiliar, and on personally familiar faces. Both tests included (a) original, 'iconic' pictures, (b) slightly modified versions of familiar pictures, and (c) rather unfamiliar pictures of familiar persons. Participants (n = 70 + 70) indeed recognised original pictures of famous and personally familiar people very accurately, while performing poorly in recognising slightly modified, as well as unfamiliar versions of famous, but not personally familiar persons. These results indicate that the successful processing of famous faces may depend on icons imbued in society but not on the face as such.     

Evidence that rats discriminate between familiar and unfamiliar putative urinary odorants of adult male conspecifics

Adam [1976; Lehman and Adams, 1977] suggested that a resident rat makes an olfactory comparison of cage odor and other rat odor prior to attacking an unfamiliar conspecific intruder. The findings of the present study are consistent with the notion that rats discriminate between familiar and unfamiliar putative urinary odorants Adult male albino rats were tested for preferences between areas treated with familiar urine (11 hours pretest exposure), unfamiliar urine (no pretest exposure), and untreated areas. Subjects (N = 12) preferred areas treated with familiar urine over ones treated with unfamiliar urine (p < 0.05). Also, they (N = 12 per preference-test group) preferred areas treated with either urine over untreated ones (familiar versus clean, p < 0.01; unfamiliar versus clean, p < 0.05).