Evidence for a Numerosity Category that is Based on Abstract Qualities of “Few” vs. “Many” in the Bottlenose Dolphin (Tursiops truncatus)

Sevgi Yaman,Lorenzo Von Fersen,Annette Kilian,Onur Güntürkün

doi:10.3389/fpsyg.2012.00473

Abstract

A previous study (Kilian et al., 2003) had demonstrated that bottlenose dolphins can discriminate visual stimuli differing in numerosity. The aim of the present study was twofold: first, we sought to determine if dolphins are able to use a numerical category based on “few” vs. “many” when discriminating stimuli according to the number of their constituent patterns. Second, we aimed to extend the previously demonstrated range of numbers, thereby testing the limits of the numerical abilities of bottlenose dolphins. To this end, one adult bottlenose dolphin learned to discriminate between two simultaneously presented stimuli which varied in the number of elements they contained. After initial training, several confounding parameters were excluded to render it likely that discrimination performance indeed depended on numerosity. Subsequently, the animal was tested with new stimuli of intermediate as well as higher numbers of elements. Once discrimination had been achieved, a reversal-training on a subset of stimuli was initiated. Afterward, the subject generalized the reversal successful to new and unreinforced stimuli. Our results reveal two main findings: firstly, our data strongly suggest a magnitude and a distance effect. Thus, coding of numerical information in dolphins might follow logarithmic scaling as postulated by the Weber-Fechner law. Secondly, after learning a reversal of contingencies, the dolphin generalized the reversal successful to new and unreinforced stimuli. Thus, within the limits of a study that was conducted with a single individual, our results suggest that dolphins are able to learn and use a numerical category that is based on abstract qualities of “few” vs. “many.”

Highlights

The visual world comes in a bewildering variety of shapes and colors
Our results clearly replicate Kilian et al (2003) and demonstrate that numerical competence is in the reach of bottlenose dolphins
In the study by Mitchell et al (1985), numerosity was confounded by the amount of food, and the subject could just have perceived the objects as representing hedonic values rather than members of an ordinal series

Summary

Introduction

Since it is impossible to learn the relevant properties of each object one by one, humans and other animals have developed the ability to group stimuli along several dimensions (e.g., Herrnstein and Loveland, 1964; Delius et al, 2000; Makino and Jitsumori, 2007). Members of a category are grouped on the basis of physical similarities. A category is defined by an ability to generalize within a class of stimuli and to discriminate between classes (Keller and Schoenfeld, 1950), as well as to extrapolate the categorical knowledge to new members of the stimulus class (Wasserman et al, 1988). A large number of demonstrations of successful categorizations in nonhuman animals have been published. In most of these studies performance could be based on “categorization by rote” (Vaughan and Greene, 1984; Yamazaki et al, 2007) without requiring an understanding of the abstract relation between the categorized stimuli

Objectives

Results

Conclusion