A. Memory performance by a macaque in the delayed matching-to-sample task of Hampton (2001). The horizontal axis indicates the length of the retention interval before matching could occur. The percentage of trials that received the uncertainty response is shown (solid line). The percentages correct of memory tests completed are also shown, on occasions when the memory test was mandatory (dashed line) or optional and voluntarily selected by the macaque (dotted line). B. Memory performance by a pigeon in the delayed matching-to-sample task of Inman and Shettleworth (1999). From “Animal metacognition: A tale of two comparative psychologies,” by J. D. Smith, J. J. Couchman, & M. J. Beran, Journal of Comparative Psychology, 128, p. 115. Copyright 2014 by the American Psychological Association. Reprinted with permission.