  Modified versions of Gallup's experiment have also been conducted with non-primate species. Notoriously, Epstein et al. (1981) trained pigeons to peck at a mark on their own bodies that was visible only in a mirror, and they used this to call into question the attribution of “self-awareness” on the basis of the mirror-mark test, preferring an associative learning explanation. Gallup et al. (2002) reject the claimed equivalence, pointing out that chimpanzees were not trained to touch marks before the test was administered. Reiss &amp; Marino (2001) have offered evidence of mirror self-recognition in bottlenose dolphins. Using a modified version of Gallup's procedure that involved no anesthesia, they inferred self-recognition from bodily contortions in front of the mirror (self-touching being anatomically impossible for dolphins). This evidence has been disputed (e.g. Wynne 2004). The mirror-mark test continues to be an area of active investigation in various species including elephants (Plotnik et al. 2006) and magpies (Prior et al. 2008).  Various commentators have pointed out that the mirror test may not be entirely fair for species which depend more heavily on senses other than vision (Mitchell 2002; Bekoff and Burghardt 2002). 