Cattle discriminate between familiar and unfamiliar conspecifics by using only head visual cues

Faces have features characteristic of the identity, age and sex of an individual. In the context of social communication and social recognition in various animal species, facial information is relevant for discriminating between familiar and unfamiliar individuals. Here, we present two experiments aimed at testing the ability of cattle (Bos taurus) to visually discriminate between heads (including face views) of familiar and unfamiliar conspecifics represented as 2D images. In the first experiment, we observed the spontaneous behaviour of heifers when images of familiar and unfamiliar conspecifics were simultaneously presented. Our results show that heifers were more attracted towards the image of a familiar conspecific (i.e., it was chosen first, explored more, and given more attention) than towards the image of an unfamiliar one. In the second experiment, the ability to discriminate between images of familiar and unfamiliar conspecifics was tested using a food-rewarded instrumental conditioning procedure. Eight out of the nine heifers succeeded in discriminating between images of familiar and unfamiliar conspecifics and in generalizing on the first trial to a new pair of images of familiar and unfamiliar conspecifics, suggesting a categorization process of familiar versus unfamiliar conspecifics in cattle. Results of the first experiment and the observation of ear postures during the learning process, which was used as an index of the emotional state, provided information on picture processing in cattle and lead us to conclude that images of conspecifics were treated as representations of real individuals.     

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).     

Further Evidence that Social Interaction Reduces Conditioned Taste Aversions in Mice

Social interactions with other conspecifics affect food selection in mammals. A previous study suggested that male mice attenuate conditioned taste aversion (CTA) by lithium chloride after interacting with an unfamiliar male conspecific. The present study was designed to (a) examine whether interactions with a familiar conspecific would attenuate CTA and (b) investigate sex differences in the social attenuation of CTA in mice. Results indicated that interacting with a familiar conspecific of the same sex following poisoning attenuated conditioned taste aversions only in male mice. The results suggest that the familiarity of conspecifics was not a critical factor in the social attenuation of CTA. Discrepancies between present and previous findings are discussed.     

Horses (Equus caballus) discriminate body odour cues from conspecifics

Knowledge about social recognition and memory in animals can help us to determine appropriate management and husbandry techniques. In this study, we used a habituation–discrimination procedure to investigate the ability of horses (Equus caballus) to distinguish between the body odour samples of unfamiliar conspecifics. To pick up body odour, we rubbed material on the coat of horses and presented these unknown body odours to 16 different conspecifics of the same sex and similar age. The test consisted of two successive two-min presentations of a sample from one individual (e.g. individual ‘A’) and a simultaneous presentation of samples from individual ‘A’ and a novel individual (e.g. individual ‘B’) during a final third presentation. The results showed that horses, regardless of sex, decreased the time they spent investigating conspecific body odour across the initial two presentations—demonstrating habituation. In the final presentation, the results demonstrated successful discrimination of the previously experienced odour because horses investigated the novel olfactory sample (‘B’) significantly more than the pre-exposed sample (‘A’). Taken together, these findings suggest, for the first time, that horses are able to discriminate two stimuli derived from body odours of unfamiliar conspecifics over short period of time.     

Hippocampal damage and exploratory preferences in rats: memory for objects, places, and contexts

Rats have a natural tendency to spend more time exploring novel objects than familiar objects, and this preference can be used as an index of object recognition. Rats also show an exploratory preference for objects in locations where they have not previously encountered objects (an index of place memory) and for familiar objects in contexts different from those in which the objects were originally encountered (an index of context memory). In this experiment, rats with cytotoxic lesions of the hippocampal formation were tested on all three versions of the novelty-preference paradigm, with a 5-min retention interval between the familiarization and test phases. Rats with sham lesions displayed a novelty preference on all three trial types, whereas the rats with hippocampal lesions displayed a novelty preference on Object trials but did not discriminate between the objects on Place trials or Context trials. The findings indicate that hippocampal damage impairs memory for contextual or spatial aspects of an experience, whereas memory for objects that were part of the same experience are left relatively intact.     

Retrieval-induced forgetting in rats

“Retrieval-induced forgetting” in rats was evaluated using a modified spontaneous object recognition test. The test consisted of a sample phase, retrieval or interference phase, and a test phase with 60-min delay period inserted between the phases. Rats were randomly assigned to one of three groups (control, retrieval and interference) and allowed to explore the field in which two different objects (A, B) were placed in the sample phase. In the retrieval phase, two identical objects (B, B), which were the same as one of the objects presented in the sample phase, were placed again. In the interference phase, two identical objects (C, C), which were novel for animals, were placed. In the test phase, two different objects (A, D), one of which was identical to that presented in sample phase (familiar object) and the other was novel, were placed and the time spent exploring each object was analyzed. While the exploration of the novel object was significantly longer than that of the familiar object in rats subjected to the interference phase, rats subjected to the retrieval phase could not discriminate between the familiar and the novel objects at the test phase. These results demonstrate the “retrieval-induced forgetting” phenomenon in a spontaneous object recognition test in rats.     

The effect of conspecifics on corticoadrenal response of rats to a novel environment

The effect of housing conditions and the presence of a conspecific on corticoadrenal response to a novel environment was studied in male Sprague-Dawley rats. The response to a novel environment was the same in rats housed in groups and rats housed alone for 20 days before testing, suggesting that isolation during adulthood did not affect corticoadrenal response to a novel environment. The presence of a nonfamiliar rat (proceeding from another home cage) did not modify corticoadrenal response to the novel environment but the presence of a rat proceeding from the same home cage induced a higher corticoadrenal response to this stressful stimuli. These results can be explained considering that rats exhibit greater emotional reactivity, as measured by adrenal function, in situations which combine familiar (the other rat) and novel (the box) elements than those completely unfamiliar.     

You sound familiar: carrion crows can differentiate between the calls of known and unknown heterospecifics

In group-living animals, it is adaptive to recognize conspecifics on the basis of familiarity or group membership as it allows association with preferred social partners and avoidance of competitors. However, animals do not only associate with conspecifics but also with heterospecifics, for example in mixed-species flocks. Consequently, between-species recognition, based either on familiarity or even individual recognition, is likely to be beneficial. The extent to which animals can distinguish between familiar and unfamiliar heterospecifics is currently unclear. In the present study, we investigated the ability of eight carrion crows to differentiate between the voices and calls of familiar and unfamiliar humans and jackdaws. The crows responded significantly more often to unfamiliar than familiar human playbacks and, conversely, responded more to familiar than unfamiliar jackdaw calls. Our results provide the first evidence that birds can discriminate between familiar and unfamiliar heterospecific individuals using auditory stimuli.     

Effects of prior agonistic experience on risk assessment and approach behavior evoked by familiar or unfamiliar conspecific odors

Risk assessment and appetitive behaviors in response to familiar and unfamiliar conspecific odors were measured in mice rendered dominant or subordinate by a series of resident-intruder encounters. Subordinate mice showed elevated levels of risk assessment in response to the odors of both familiar dominant and unfamiliar males. These behaviors were almost totally absent among dominant males exposed to familiar subordinate or unfamiliar male odors. Subordinate mice showed a marginally significant elevation in latencies to approach familiar, but not unfamiliar, dominant odors. Dominant and subordinate mice spent comparable amounts of time in the cage area containing familiar antagonist odors, however, and the durations of subordinates were mildly elevated, rather than decreased, when unfamiliar conspecific odors were present. There were no group differences in any of the appetitive behaviors. These findings suggest that apparent preferences for conspecific odors may arise from quantitatively and/or qualitatively differing emotional states. The inclusion of risk assessment measures is suggested to be a useful adjunct for studies of olfactory preference/rejection. © 1994 Wiley-Liss, Inc.     

Food stealing by young Norway rats (Rattus norvegicus)

Six experiments were undertaken to explore factors affecting young rats' (Rattus norvegicus) frequencies of stealing food from conspecifics when identical food is available in surplus. It was found that (a) rats would walk across a bed of pellets to steal the particular pellet a peer was eating, (b) frequency of stealing within a pair did not decrease over days, (c) rats stole unfamiliar foods more frequently than familiar foods, (d) younger rats stole from older rats more frequently than older rats stole from younger ones, (e) hungry rats stole more frequently than replete rats, and (f) rats that had stolen a pellet of unfamiliar food from an anesthetized conspecific subsequently exhibited an enhanced preference for that food. Results suggest that food stealing is a mode of active seeking of information about what foods to eat.     

Sleep deprivation impairs object recognition in mice

Many studies in animals and humans suggest that sleep facilitates learning, memory consolidation, and retrieval. Moreover, sleep deprivation (SD) incurred after learning, impaired memory in humans, mice, rats, and hamsters. We investigated the importance of sleep and its timing in an object recognition task in OF1 mice subjected to 6h SD either immediately after the acquisition phase (0-6 SD) or 6h later (7-12 SD), and in corresponding undisturbed controls. Motor activity was continuously recorded with infrared sensors. All groups explored two familiar, previously encountered objects to a similar extent, both at the end of the acquisition phase and 24h later during the test phase, indicating intact familiarity detection. During the test phase 0-6 SD mice failed to discriminate between the single novel and the two familiar objects. In contrast, the 7-12 SD group and the two control groups explored the novel object significantly longer than the two familiar objects. Plasma corticosterone levels determined after SD did not differ from time-matched undisturbed controls, but were significantly below the level measured after learning alone. ACTH did not differ between the groups. Therefore, it is unlikely that stress contributed to the memory impairment. We conclude that the loss of sleep and the activities the mice engaged in during the SD, impaired recognition memory retrieval, when they occurred immediately after acquisition. The delayed SD enabled memory consolidation during the 6h when the mice were allowed to sleep, and had no detrimental effect on memory. Neither SD schedule impaired object familiarity processing, suggesting that only specific cognitive abilities were sensitive to the intervention. Sleep may either actively promote memory formation, or alternatively, sleep may provide optimal conditions of non-interference for consolidation.     

Developmental expression of defensive responses during exposure to conspecific adults in preweanling rats (Rattus norvegicus).

I examined preweanling rats' (Rattus norvegicus) expression of ultrasounds and secretion of ACTH when exposed to unfamiliar adult male rats or to their mothers. Pups at 7 days of age produced similar levels of ultrasonic vocalization near both unfamiliar males and mothers. However, these pups could discriminate familiar from unfamiliar adults because ACTH was significantly higher in pups near adult males than in those near mothers. At 14 days of age, pups avoided adult males but not their mothers; therefore, adult males represented a significant threat. Importantly, 14-day-old rats significantly reduced ultrasound production only when near adult males. Pups at 21 days of age no longer emitted ultrasounds when socially isolated or when near conspecific adults. In addition, 14- and 21-day-old rats produced similar elevated ACTH levels across stimulus conditions. Results show significant changes in preweanling rats' responses to conspecific adults.     

Orangutans (Pongo abelii) and a gorilla (Gorilla gorilla gorilla) match features in familiar and unfamiliar individuals

Great apes can perceive images as representative of corresponding real-life objects. Coupled with the potential advantages of identifying specific members of one’s species and mounting evidence for individual recognition in other non-humans, it seems likely that great apes would have the ability to identify conspecifics in photographs. The ability of four orangutans and a gorilla to match images of individuals of their own and a closely related but unfamiliar species was examined here for the first time. First, the subjects matched photographs of familiar conspecifics taken at various time points in a delayed matching-to-sample procedure (Experiment 1). Second, they matched different photographs of unfamiliar individuals of a different species (Experiment 2) at above chance levels. These results suggest that the subjects matched photographs by matching physical features, not necessarily by recognizing the identity of the individuals depicted. However, they also quickly learned to select photographs of familiar individuals when these photographs were paired with photographs of unfamiliar individuals of their own species (Experiment 3), and three subjects showed transfer to novel images of familiar and unfamiliar individuals. Thus, the findings support the idea that subjects attended to physical features to identify individuals that they could categorize on the basis of familiarity.     

Integrated memory for objects, places, and temporal order: evidence for episodic-like memory in mice

Human episodic memory refers to the recollection of an unique past experience in terms of what happened, and where and when it happened. Factoring out the issue of conscious recollection, episodic memory, even at the behavioral level, has been difficult to demonstrate in non-human mammals. Although, it was previously shown that rodents can associate what and when or what and where information given on unique trials, it proved to be difficult to demonstrate memory for what, where, and when simultaneously in mammals, without using extensive training procedures, which might induce semantic rather than episodic memory recall. Towards the goal of an animal model of human episodic memory we designed an three-trial object exploration task in which different versions of the novelty-preference paradigm were combined to subsume (a) object recognition memory, (b) the memory for locations in which objects were explored, and (c) the temporal order memory for object presented at distinct time points. We found that mice spent more time exploring two "old familiar" objects relative to two "recent familiar" objects, reflecting memory for what and when and concomitantly directed more exploration at a spatially displaced "old familiar" object relative to a stationary "old familiar" object, reflecting memory for what and where. These results suggest that during a single test trial the mice were able to (a) recognize previously explored objects, (b) remember the location in which particular objects were previously encountered, and (c) to discriminate the relative recency in which different objects were presented. According to the currently discussed behavioral criteria for episodic-like memory in animals, our results suggest that mice are capable to form such higher order memories.     

Mental representation in visual/haptic crossmodal memory: evidence from interference effects

Two experiments used visual-, verbal-, and haptic-interference tasks during encoding (Experiment 1) and retrieval (Experiment 2) to examine mental representation of familiar and unfamiliar objects in visual/haptic crossmodal memory. Three competing theories are discussed, which variously suggest that these representations are: (a) visual; (b) dual-code—visual for unfamiliar objects but visual and verbal for familiar objects; or (c) amodal. The results suggest that representations of unfamiliar objects are primarily visual but that crossmodal memory for familiar objects may rely on a network of different representations. The pattern of verbal-interference effects suggests that verbal strategies facilitate encoding of unfamiliar objects regardless of modality, but only haptic recognition regardless of familiarity. The results raise further research questions about all three theoretical approaches.     

Mental representation in visual/haptic crossmodal memory: evidence from interference effects

Two experiments used visual-, verbal-, and haptic-interference tasks during encoding (Experiment 1) and retrieval (Experiment 2) to examine mental representation of familiar and unfamiliar objects in visual/haptic crossmodal memory. Three competing theories are discussed, which variously suggest that these representations are: (a) visual; (b) dual-code—visual for unfamiliar objects but visual and verbal for familiar objects; or (c) amodal. The results suggest that representations of unfamiliar objects are primarily visual but that crossmodal memory for familiar objects may rely on a network of different representations. The pattern of verbal-interference effects suggests that verbal strategies facilitate encoding of unfamiliar objects regardless of modality, but only haptic recognition regardless of familiarity. The results raise further research questions about all three theoretical approaches.     

Mediating factors in chick social pecking: 1. Effects of isolation,imprinting, and peer contact

Two experiments examined the influence of periods of social isolation on rates of intersubject pecking in pairs of domestic chicks. Of central interest was the effect of an imprinting condition wherein single birds were reared with either a red or green Styrofoam object. In the first experiment, imprinted subjects were given social pecking tests in the absence of the imprinting object, while in the second study the tests for social pecking were conducted in the presence of one of the imprinting objects. In the latter test the object was familiar to one bud of a pair, but was novel to the other animal. The results of both experiments showed that the rate of intersubject pecking of the imprinted subjects was intermediate to, and statistically different from both the low pecking rate of chicks reared in pairs and the high pecking rate of chicks reared in total isolation. The presence of a familiar object during tests in the second study seemed to serve as a secure base since chicks that had been reared with that object exhibited more social pecking than did chicks that were unfamiliar with the object. However, regardless of the provision of the object the social pecking of the imprinted chicks again fell on a point intermediate to the other groups. Hypotheses concerning a) intolerance of figureground movement and b) sheer stress of isolation were advanced to account for these results.     

Early social experience and responses to visual social stimuli in young monkeys

Juvenile stumptailed macaques (Macaca arctoides) were given preference tests in which they could approach an empty chamber, a mirror, a familiar conspecific, or an unfamiliar conspecific. Control subjects tended to withdraw from the mirror, and threatened the stranger more than did monkeys whose early social experience had occurred exclusively in darkness. Both groups explored the familiar stimulus animal most of all, but the group socialized in the dark showed most positive behavior when in the empty chamber. In a second study peer-reared infants responded more appropriately to slides of conspecifics than did infants reared with a mirror as the main source of social input. Infants reared only with a peer were also strongly attracted to a mirror, whereas infants reared only with mirrors preferred a film of an unfamiliar agemate. The results suggest that early visual social stimulation is important in the development of aggression and other social behaviors, and that novelty and complexity are important aspects of social stimuli that interact with effects of early experience. Study 2 was conducted while the first author was in receipt of a Postgraduate Research Studentship from the U.K. Science Research Council. The DTU tapes were analyzed using a computer program developed with the help of SRC grant B/RG 98910 to A. Chamove. This latter grant also supported the study from which the preference test data were taken.     

Event-related potential (ERP) indices of infants' recognition of familiar and unfamiliar objects in two and three dimensions

We measured infants' recognition of familiar and unfamiliar 3-D objects and their 2-D representations using event-related potentials (ERPs). Infants differentiated familiar from unfamiliar objects when viewing them in both two and three dimensions. However, differentiation between the familiar and novel objects occurred more quickly when infants viewed the object in 3-D than when they viewed 2-D representations. The results are discussed with respect to infants' recognition abilities and their understanding of real objects and representations. This is the first study using 3-D objects in conjunction with ERPs in infants, and it introduces an interesting new methodology for assessing infants' electrophysiological responses to real objects.     

Exploratory behavior as a function of environmental novelty and complexity in male and female rats

Laboratory rats show a positive response to low- and nonstressful novel events. The novel event may involve a number of aspects of the stimulus field. It is usually associated with a change in amount of environmental complexity. Most studies concerning novelty-related behavior involve the introduction of novel objects or the rearrangement of familiar objects. The present purpose was to measure exploratory behavior in response to environments of increased and decreased complexity. Both directions of environmental change are conditions of novelty. A two-way manipulation of increasing and decreasing the complexity of the environment was used. Rats of both sexes showed increased exploration to exposure to novelty, no matter which manipulation was applied; however, female and male rats behaved differently to the two types of novelty. Males responded more to novelty from the introduction of an unfamiliar object. The results indicate novel stimulation, whether of increasing or decreasing complexity, has reward properties. Perhaps the male-specific behavior directed toward unfamiliar objects may serve an adaptive function.