Mirror self-recognition in the bottlenose dolphin: a case of cognitive convergence

Abstract

The ability to recognize oneself in a mirror is an exceedingly rare capacity in the animal kingdom. To date, only humans and great apes have shown convincing evidence of mirror self-recognition. Two dolphins were exposed to reflective surfaces, and both demonstrated responses consistent with the use of the mirror to investigate marked parts of the body. This ability to use a mirror to inspect parts of the body is a striking example of evolutionary convergence with great apes and humans.

Figure 1

(A) The pool in Phase 1. Phase 1 sessions were conducted in a rectangular pool 13 m × 18.5 m × 3.05 m with three reflective glass walls. During a subset of test sessions (n = 5), a narrow Plexiglas mirror, 41.9 cm × 101.6 cm × 0.32 cm, was affixed in a vertical orientation to the exterior of one of the reflective walls (Wall 6). The demarcated area (Wall 1) was more highly reflective than the other glass walls because of a black surface behind it. Thus, the Plexiglas mirror was the optimal mirror when present. In the absence of the Plexiglas mirror, Wall 1 had the most reflective properties. (B) The pool in Phase 2. In Phase 2, by using the same procedures as in Phase 1, testing was conducted in two connected pools, a large oval pool (21 m long × 13 mm × 3.66 m deep) connected to a smaller round pool (8.9 m in diameter × 3.05 m deep) with nonreflective walls. During experimental sessions, the subject was stationed at the far end of the larger pool and a Plexiglas mirror (88.9 cm × 119.4 cm × 0.32 cm) was affixed vertically to an open gate just inside the mouth of the connected smaller pool in 16 out of 30 sessions.

Figure 2

(A) Locations of the nontoxic, temporary mark and the number of times the dolphins were marked in each location in mark and sham-mark sessions. Marks were applied to either side of the body. Subject 1: b, above eye (right, n = 1); c, above and posterior to ear (right, n = 3; left, n = 4); d, between ear and pectoral fin (right, n = 2; left, n = 2); e, above pectoral fin (right, n = 2; left, n = 1); f, posterior to pectoral fin (left, n = 1); g, below dorsal fin (right, n = 3; left, n = 7); h, between pectoral fin (n = 2); i, umbilical (n = 1); j, underside and tip of pectoral fin (right, n = 1). Subject 2: a, on melon (right, n = 1; left, n = 2); e, above pectoral fin (right, n = 5; left, n = 2); g, below dorsal fin (right, n = 2; left, n = 1; umbilical, n = 2); h, between pectoral fin (n = 1). (B) The dolphin marked above the right eye.

Figure 3

(A) Total amount of time (sec) engaged in mark- or sham-directed behaviors during early sham-marking, genuine markings, and late sham-marking. The subject spent a significantly greater cumulative amount of time engaged in Self-directed behaviors at reflective surfaces when marked than when early sham-marked or late sham-marked. (B) In Phase 2, time (sec) at mirror location under different experimental conditions: No mirror–no mark, No mirror–mark, Mirror–no mark, and Mirror–mark. The mean time spent at the mirror location when the subject was marked and the mirror present and uncovered was significantly greater than under any other condition. In the one session in which the subject was sham-marked, he spent a comparatively brief time at the mirror location. (C) Latency from departure by the subject from stationing to the first mirror orientation when marked, sham-marked, or unmarked. The criterion for mirror orientation was that the dolphin came to a stop at the mirror.

Figure 4

(A) Mark-directed behavior by subject to a real mirror immediately after release from being marked. A narrow Plexiglas mirror, 41.9 cm × 101.6 cm × 0.32 cm is affixed in a vertical orientation to the exterior of one of the reflective walls (Wall 6). During this session, the mirror was the best reflective surface in the subject's environment. The faint white line on the wall indicates the location of mirror. (B) The dolphin at Wall 1, the best reflective surface in the session, exhibiting late sham-directed behavior: a continuous and repetitive sequence of 12 dorsal-to-lateral-ventral flips exposing the location of the sham-marked area of his body, the underside and tip of the right pectoral fin, to the reflective surface. This unusual behavioral sequence continued for 32 sec.