Nature 414, 165–166 (2001)

In Fig. 1b of this Brief Communication, the legend and text did not make it clear that two different groups of ten sheep were used in the study to give overall n = 20. A reanalysis of the data using a post-hoc Tukey test (rather than a paired t-test, as originally stated) revealed some small errors that altered the significance values slightly; however, there is no overall change in the results. The maximum retest interval was 801 rather than 800 days, and 100–500 trials were conducted for 1–6 weeks (not 400–500 trials for 4–6 weeks, as published). A revised version of Fig. 1b showing the corrected statistical changes and an expanded legend incorporating further methodological detail are available as Supplementary Information to this Corrigendum.