International Journal of Comparative Psychology

Two experiments with rats studied the relationship between reinforcer magnitude and frequency, and extinction rate in a consummatory situation with rats. In Experiment 1, groups received access to either a 2% or a 32% sucrose solution during twenty 5-min sessions and were subsequently shifted to extinction (access to an empty sipper tube). Goal tracking time (time spent near the sipper tube) was the dependent measure. Extinction was faster after training with 2% solution than with 32% solution. In Experiment 2, extinction was faster after training with 50% partial reinforcement than with continuous reinforcement. In both experiments, extinction was gradual and rats exhibited spontaneous recovery of goal-tracking behavior. Results are discussed in the context of evidence pointing to a dissociation of consummatory and instrumental behavior.

During play fighting, rats often assume a pinning configuration, where one animal stands over its supine partner. The on-top partner can stand on the ground or on its supine partner with its hind paws. When standing on the ground, the rat is more stable and is better able to respond to its partner’s actions. The frequency of this more stable pattern of standing during pinning (called anchoring) is higher following puberty than during the juvenile phase. Two hypotheses explaining this developmental change in anchoring were tested. The first hypothesis maintains that the lower level of anchoring in juvenile rats reflects an immature sensorimotor capability. The second hypothesis suggests that, since rats are more playful as juveniles, such heightened levels of play may interfere with movements otherwise used to maintain the stable anchored position at this age. Neither hypothesis was supported: infants have similar anchoring levels to postpubertal rats, and juveniles have a relatively low level of anchoring irrespective of how frequently they play fight. Therefore, the lower level of anchoring in the juvenile phase appears to be a developmental peculiarity of this age. These findings support the view that play fighting in the juvenile phase may be organized in a manner to increase the occurrence of the experiences that are developmentally beneficial in this activity.

Hare et al. (2000) reported that when placed into competitive feeding situations chimpanzees exhibit evidence of reasoning about what each other can see. Subjects were reported to select food items that were hidden behind barriers and therefore were not visible to a rival chimpanzee. We report eight experiments that attempted to replicate these findings and test alternative interpretations of them. Although we robustly replicated the finding that subjects obtained more hidden than visible food items, we consistently did not replicate the more relevant result of subjects' food choice. Further studies revealed that even those subjects who showed a tendency to approach the hidden food first did not differentiate between barriers which did and did not obscure the rival’s view. Overall, the results support the idea that although chimpanzees may use a variety of competitive strategies in such tests, no predictive power is added by postulating that they are reasoning about what each other can or cannot see.

The impairment in responding to a secondly trained association because of the prior training of another (i.e., proactive interference) is a well-established effect in human and animal research, and it has been demonstrated in many paradigms. However, learning theories have been concerned with proactive interference only when the competing stimuli have been presented in compound at some moment of the training phase. In this experiment we investigated the possibility of proactive interference between elementally-trained stimuli at the acquisition and at the retrieval stages in a behavioral task with humans. After training a cue-outcome association we observed retardation in the acquisition of an association between another cue and the same outcome. Moreover, after asymptotic acquisition of the secondly trained association, impairment of retrieval of this secondly trained association was also observed. This finding of proactive interference between elementally-trained cues suggests that interference in predictive learning and other traditional interference effects could be integrated into a common framework.

In the present study we provide more specific analyses of the responses of a subadult bottlenose dolphin ( Tursiops truncates ) to a mirror from an earlier study. An ethogram was constructed in order to classify specific behaviors as contingency checking, social, and other. This ethogram was used to develop a continuous record of behaviors of the dolphin at a mirror and the durations of those behaviors over nine test sessions dispersed across nine days. The subject spent an increasing proportion of time engaged in contingency checking behaviors, such as repetitive head and body movements, over the nine sessions, a very small proportion of time engaged in social behaviors, and another larger proportion of time engaged in behaviors that were unusual but not strictly classifiable as contingency checking or social. These other behaviors included head orientations and circling at the mirror. These findings will add to the ongoing effort to describe and compare mirror responses in cetaceans and primates.