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Open Access Publications from the University of California


The International Journal of Comparative Psychology is sponsored by the International Society for Comparative Psychology. It is a peer-reviewed open-access digital journal that publishes studies on the evolution and development of behavior in all animal species. It accepts research articles and reviews, letters and audiovisual submissions.

Volume 25, Issue 2, 2012


Perception of Scary Halloween Masks by Zoo Animals and Humans

Zoo animals were tested to see if they perceived the scary nature of Halloween masks, using a procedure that measured the avoidance response latency to take food from a masked human experimenter. Human perception of the masks was also assessed using a rating scale, with results showing that a Bill Clinton mask was rated not scary, while a Vampire mask was rated very scary. Animal results showed that primate latencies correlated significantly with the human ratings, while non-primate latencies did not. Taken together, these results indicate that human perception of scary faces does not depend upon human-specific cultural factors, e.g., belief in the supernatural. Rather, it has a more biological basis, shared specifically with other primates, by which scary faces are perceived as predators or threatening conspecifics.

Human-Socialized Wolves Follow Diverse Human Gestures… And They May Not Be Alone

Many studies document the domestic dogs’ responsiveness to human gestures. Reports of success on human guided tasks have led to evolutionary hypotheses that set dogs’ skills apart from other species, including other canids, in terms of their social cognition and comprehension of human communicative stimuli. However, until recently the range of other species tested and the availability of studies using equivalent testing methods between different species and groups have been limited, making it difficult to interpret cross-species comparisons. Here we demonstrate that human-socialized wolves are not only capable of responding to points made with the arm and hand, but are sensitive to a wide range of human gestures when given the opportunity to utilize such gestures in an object-choice task. Claims that domestic dogs are unique in their ability to respond to diverse novel stimuli may be in part due to the absence of data for the same range of gestures in other species. We also provide the first evidence that human-socialized coyotes have the capacity to utilize a human point to locate a target; further demonstrating that domestication is not a prerequisite for canid responsiveness to human actions, and that socialization and life experience are likely more important predictors of success

A Free Choice Task Evaluating Chimpanzees’ Preference for Photographic Images of Sex Swellings: Effects of Color, Size, and Symmetry

Exaggerated sex swellings characterized by increased size of skin surrounding the anogenital region occur in female chimpanzees in response to ovarian hormone fluctuations and are associated with increased likelihood of ovulation and female receptivity. The swellings generate heightened sexual interest from males and evoke increased male competitive behavior. To determine potential attractiveness of specific visual characteristics of these signals to males, a free choice task using push-button methodology that allowed subjects to choose one stimulus in a pair by pressing a button corresponding to the stimulus choice was developed. Initially, preferences for specific food items were first determined based on animals’ selection of actual items, followed by selection of same items from photographs and subsequently selection of preferred food utilizing push-button responses to photographs. We found that when allowed to choose between photographs of sex swellings, novel objects, and other body parts, male chimpanzees preferred images of both sex swellings and other body parts over images of objects, and showed a significant preference for sex swellings over other body parts. However, chimpanzees showed no preference in sex swelling color or symmetry. Only one subject displayed a preference for normal sized swellings rather than enlarged swellings. Overall these results suggest that when considered individually, visual characteristics alone are not sufficient to provoke preference responding.

Responses of Human-Habituated Wild Atlantic Spotted Dolphins to Play Behaviors Using a Two-Way Human/Dolphin Interface

Since 1985, a well-studied community of free ranging Atlantic spotted dolphins, Stenella frontalis, and bottlenose dolphins, Tursiops truncatus, has been observed underwater in the Bahamas. Over this period, the dolphins have become habituated to human swimmers. Long-term underwater observations revealed that some of these dolphins engaged in play behavior using man-made and natural objects in the presence of humans, and allowed humans to interact with them during play. We investigated the dolphins’ play behaviors in response to a more formal two-way communication interface from 1997-2000. Spotted dolphins were exposed to an underwater keyboard in which visual and acoustic symbols represented the objects the dolphins were observed playing with. Objects could be obtained and played with by indicating the appropriate associated symbol. Pointing and triadic gaze between human participants was used to model the system in the presence of dolphins. Our results indicated that human use of the system encouraged the dolphins to attend to activity at the keyboard. Female juveniles, especially six main individuals, were the main players. Dolphins increased their normal levels of associations with certain conspecifics during exposure sessions and also took dominant roles during sessions in the presence of certain conspecifics. Dolphin age class, sex, and levels of synchronization with humans all contributed to the success and level of engagement during exposure sessions between humans and dolphins.

Naturalistic Conditioned Stimuli Facilitate Sexual Conditioning Because of their Similarity with the Unconditioned Stimulus

Previous sexual conditioning studies with male Japanese quail have shown that adding taxidermic female head and neck cues to a conditioned stimulus (CS) facilitates conditioned sexual responding. The present experiment examined CS-US similarity as a possible mechanism for this facilitation. During sexual conditioning trials, artificially colored CSs with or without taxidermic female head cues were paired with copulatory access to artificially colored female quail (the unconditioned stimulus or US) to form similar and dissimilar CS-US combinations. As in previous experiments the presence of taxidermic female cues on the CS enhanced conditioned copulatory responding. Additionally, more conditioned copulatory responding developed when the CS and US colors were similar compared to when they were different. The results indicate that similarity between the CS and US strengthens the sexual conditioning of male quail, and similarity in color is more important than similarity in shape. The results also suggest that CS-US similarity may be one factor responsible for the facilitated conditioned responding that occurs when female cues are added to a CS object. The behavior systems approach is used to interpret the findings.