It's not all the same to pigeons: Representations of difference may be shared across species.

Pigeons readily learn and transfer same-different discriminations in a variety of experimental paradigms. However, strategically designed probe tests suggest that they might only represent sameness. Here, we provide the first direct evidence that pigeons also represent difference. We first trained pigeons on a conditional same-different discrimination; then, on probe trials, we replaced either the same-item pair or the different-item pair with a familiar, but ambiguous stimulus. On different-cued probe trials, pigeons' choices were controlled by sameness: they reliably rejected the same-item pair, but they did not reliably select the different-item pair. Conversely, on same-cued probe trials, pigeons' choices were controlled by difference: they reliably rejected the different-item pair, but they did not reliably select the same-item pair. Together, these findings demonstrate that pigeons can represent both sameness and difference, providing an important clue to elucidating the evolutionary origins of same-different conceptualization.