In our target article published in the latest special issue of the Japanese Journal of Animal Psychology, we proposed a new framework of species comparison for studies of animal psychology in which the generalist and evolutionist approaches were incorporated. The 13 open peer commentaries to the target article, the pros and cons, provided valuable criticisms on our proposal. Although generalists were skeptical of scientific validity of the evolutionist approach due to its difficulty of experimental examination, we think that species comparison allows us to test evolutionist hypotheses with regard to adaptive values (ultimate causes) of certain psychological processes in a falsifiable manner. In the present paper, we reemphasized that species comparison has a critical role in studies of animal psychology and that the examination of ultimate cause is as important as that of proximate causes of psychological processes.

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We investigated how squirrel monkeys and humans employ various facial cues to discriminate individual faces of their own and other species. After squirrel monkeys were trained to discriminate two individual faces, they were tested with a variety of modified stimuli. The test stimuli used in experiment 1 preserved specific facial features used in training and the test stimuli in experiment 2 were faces that included facial features not used in training and those used as test stimuli in experiment 1. We compared the results to those obtained in humans (experiment 3). We found that the eyes played a significant role as cues for discrimination of individual faces. Furthermore, unlike the human subjects, monkeys used the outer facial boundary of their own species as a cue for discrimination. We suggest that squirrel monkeys may use their faces for discrimination of individuals and that the information of faces for discrimination of their own species may differ from those of other species.

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The present study developed a concurrent-chain procedure that appropriately satisfied Charnov's marginal value theorem (1976) in order to analyze the foraging behavior of pigeons (Columba livia). In the new procedure, travel between patches (fixed ratio schedule; FR) was only observed in the case of arrival at the new depleting patch (progressive ratio schedule; PR, reset value 1, PR-step 5) where the food density was rich and did not provide the food. Although the present results were similar to those obtained in biological studies, the pigeons tended to reside in the patch for a longer duration than the values predicted by the marginal value theorem. Adding a free parameter (K) to the marginal value theorem made it possible to analyze the differences in the patch residence time between the obtained and predicted values. The present study suggested that the new procedure and the results obtained from the present experiments expanded the applicability of the marginal value theorem to foraging behavior in the depleting patch.

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The present study investigated a dominance hierarchy and its stability in a group of pigeons under a food competing situation. When a trial started, a feeder lamp was lit and one food pellet was delivered into a cone-shaped tray where a pair of pigeons (dyad) competed against each other. A pigeon that ate the food pellet was a winner of that trial. Each pair of pigeons competed in 30 trials. Thirty-five weeks after the first determination of the social ranking, it was re-determined to assess its stability. The results showed that in the first determination, except one pair of pigeons, a linear hierarchy was formed among the group of pigeons. In the second determination, the highest- and lowest-ranking subjects were the same as the first determination, however, the subjects whose social orders were moderate changed their ranking. Nevertheless, the correlation between the first and the second determination was relatively high. In conclusion, the social ranking of the group of pigeons was stable over the period of time. Moreover, the social ranking predicted by the BBS (Batchelder, Bershad, & Simpson) method highly correlated with actual dominance observations. Overall, the BBS method was a suitable procedure to identify an appropriate ranking of a group of pigeons.

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Four pigeons were run in a two-choice discrete-trial situation to determine which of two principles, i.e., reinforcement of functional response unit and momentary maximizing, works in generating response patterns. After two or more responses to one key (right for 2 birds while left for the others; denoted as A) a changeover response to the other key (denoted as B) was reinforced with .30, whereas after two or more B, A was reinforced with .90. In other cases A and B were reinforced with the probabilities of .15 and .05, respectively. Results showed that birds in the present experiment made more changeover responses to B side after two or more (mostly two) As than after a single A, following which they mostly made only one B and returned to A: The birds showed responding of “AABAAB...”, “AAABAAAB...”, “AAAABAAAAB...”, and so on most of the time. This behavior is interpreted as closer to that predicted by momentary maximizing than that predicted by the functional response unit notion.

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Neuroanatomy is a science of form, whereas cognitive science is a science of function. Comparative neuroanatomy and comparative cognition, however, share a common interest, that is, the evolution of form and function. Pigeons and humans show apparently similar higher visual cognition, but their brain mechanisms are different. Functional constraints caused by differences in the brain structures are still unknown. Interpretation of function by form is the traditional way of explanation in biological sciences, and hence the integration of comparative neuroanatomy and comparative cognition should provide such an approach.

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