Skip to main content
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 19, Issue 4, 2006


Quantity Perception by Adult Humans (Homo sapiens), Chimpanzees (Pan troglodytes), and Rhesus Macaques (Macaca mulatta) as a Function of Stimulus Organization

Adult humans typically overestimate the number of items in regularly arranged stimulus sets compared to randomly arranged sets. This regular-random numerosity illusion (RRNI) was examined comparatively in adult humans, chimpanzees, and rhesus monkeys. Neither nonhuman primate species showed evidence of the illusion when trained to pick the larger of two sets of randomly arranged dots on a computer screen and then shown regularly arranged sets. Adult humans, given the same task and instructed to select the larger set, showed the illusion, although there were individual differences. Experiment 1 used somewhat different methodologies with the human participants compared to the nonhuman animals, but Experiment 2 presented the identical method to naïve human participants and naïve rhesus monkeys with minimized training, equal exposure to the different arrangement types, and very limited instructions to human participants. In that situation, human and monkey performance was very similar and reflected the RRNI. These results demonstrate that nonhuman animals also are susceptible to the RRNI, and they also indicate how methodological differences used during training both within and between species can impact results of comparative assessments.

The Effect of Context and CS Preexposure on Acquisition of the Classically Conditioned Eyeblink Response in Rats

In a previous study, a latent inhibition (LI) effect was found to be dictated by a facilitation of the acquisition of a conditioned eyeblink response in context pre-exposed rabbits as opposed to slower learning in tone preexposed rabbits. In the present experiments, we examined the effects of preexposure to the tone conditional stimulus (CS) using a similar paradigm with rats. In Experiment 1, rats were given four or eight days of context (SIT) or CS preexposure (TONE) followed by eight days of paired training. Unlike rabbits, control and eight day SIT groups learned faster than TONE exposed rats and the four day SIT group. In Experiment 2, we controlled for the context preexposure control rats received during adaptation in Experiment 1 and tested rats given two days of CS preexposure or no preexposure. Again, SIT rats learned faster than TONE rats as well as rats that did not receive any preexposure. In Experiment 3, we tested a frequently-used method for examining LI, whereby paired training began immediately after the last of four sessions of preexposure, but observed no effect. Similar to our previous results, any LI effect produced in the present set of experiments arose from facilitated performance by SIT rats as opposed to deficits in learning in TONE rats. The present results highlight the need for a unifying theory of preexposure effects immune to differences experimental paradigms and parameters in order explain the variety of results obtained in the field.

Formation of a Simple Cognitive Map by Rats

Cognitive mapping implies the development of an internal representation of the spatial relationships among objects in the environment. One can assess the development of a cognitive map by demonstrating that an animal can select, when appropriate, a novel path to reach a goal in the absence of landmarks and when path integration does not provide an adequate account. Rats were trained to find reinforcement in two of three goal boxes in a three-arm maze. In test, the rats were given a choice between two novel paths, one that led to the goal box that had been baited during training, the other that led to the goal box that had been unbaited during training. When they were trained in the absence of distinctive intramaze cues (Experiment 1), no preference was found; however, when distinctive intramaze alley cues were present during training but were unavailable as directional cues during testing (Experiment 2), the rats demonstrated a significant preference for the correct novel path. These results suggest that under appropriate conditions rats are able to form simple cognitive maps of their environment.

Exploration and Habituation in Intact Free Moving Octopus vulgaris

Despite the huge numbers of studies published on the learning of cephalopod mollusks, studies on non-associative learning are scarce. We tested non-associative learning (habituation) and exploration in Octopus vulgaris in two different studies using a prey-shaped object (Study A) and inanimate objects and food objects (Study B). Study A consisted of the repeated presentation of a prey-like stimulus, which 23 subjects could only explore visually. In study B, 14 octopuses were presented two Lego blocks (one black and white with a smooth surface, one a blue "snowflake" with a rough surface) and two food items, one preferred (clams) and one non-preferred (mussels) inside their home tanks. As hunger is a motivational factor for exploratory behavior, different levels of food satiation (feeding 2h or 24 h prior to experiments) were tested. Within trial habituation was clearly documented in both experiments. In study A across trials habituation was found for all animals, whereas it was only significant in 5 animals in Study B.

Spontaneous Object Sharing in Captive Chimpanzees (Pan troglodytes)

Episodes of object transfer were observed among 3 adult and 2 juvenile unrelated females chimpanzees, kept in adjacent cages with bars between them. Only the adult females had small objects available in their cage only juveniles were observed to request them. Although the possessors could easily prevent transfers, 15 episodes of active sharing were recorded, including spontaneous unsolicited donations, sharing preceded by requests, as well as transfers under obnoxious solicitation.