Recent research on animal behavior has identified a set of common cognitive and behavioral principles that underlie animal collective decision-making. Although endeavor toward synthesizing knowledge about collective decision-making by different species has already begun, human collective phenomena, which have mainly been investigated by social scientists, are still largely left behind from this scope. However, delineating uniqueness and commonality of human collective decision-making in biological contexts is critically important to understand nature of human collectivity. Toward this end, this paper first defines the concept of collective decision-making from a biological perspective. We then review several recent key findings on collective behavior by several animal species (eusocial insects in particular), and argue that a set of fundamental processes such as positive feedback, nonlinear conformity bias and quorum-sensing, commonly underlie human and non-human collective decision-making. We sketch some future research directions to promote cross-fertilizations between the biological and social sciences for a better understanding of collective phenomena by humans and non-human animals.

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Previous studies suggest that nonhuman animals form concepts that integrate information from multiple sensory modalities such as vision and audition. For instance, Adachi, Kuwahata, and Fujita (2007) demonstrated that dogs form auditory-visual cross-modal representation of their owner. However, whether such multi-modal concepts would expand to more abstract, or collective, ones remains unknown. To answer the question, we tested whether dogs were sensitive to congruence of human genders suggested by the voice and the face of an unfamiliar person. We showed to the dogs a photograph of a male or female human face on the monitor after playing a voice of a person either matching or mismatching in gender. Dogs looked at the photograph for longer duration when the auditory stimuli were incongruent than when they were congruent; that is, expectancy violation was suggested. This result suggests that dogs spontaneously associate auditory and visual information to form a cross-modal concept of human gender. This is the first report showing that cross-modal representation in nonhuman animals expands to an abstract social category.

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