Performance of cyprinids in non-reversing mirrors versus regular mirrors in tests of aggressiveness

Mirror tests have been used to test aggressiveness because they reduce the variability of the real opponent and allows for repeated measurements. Regular mirrors have also several disadvantages due to the inability to display a head-to-tail position during lateralization. Recently, a method using a non-reversing mirror was developed, which eliminated this disadvantage, but was tested in a single species only. The present study reflects the need for validating the non-reversing mirror test in other species, represented by chub (Squalius cephalus L.) in the present study, due to species-specific manifestations of aggressiveness. The prediction that a non-reversing mirror would lead to more aggressive interactions than a regular mirror in chub was confirmed. The durations of aggressive interactions were longer in fish exposed to the regular mirror test with specific significance in case of the frontal and the lateral displays. The individuals maintained the same level of aggressiveness throughout both tests, suggesting that both tests are valid for testing the aggressiveness. Nevertheless, due to higher number of interactions performed, a non-reversing mirror may offer a stronger stimulus and could represent a real contest better than the regular mirror. Therefore, the non-reversing mirror test should be recommended for use in future studies.