Female choice is often assumed to be based on absolute preference, driven by a threshold value of male attractiveness. However, increasing evidence suggests that females may instead perform a comparative evaluation of prospective mates, possibly incurring in violation of rational decision rules. A prototypical case is the 'asymmetrically dominated decoy' effect where the preference for a target option over a competitor is altered by the addition of an irrelevant alternative. We tested for this effect in the peacock blenny Salaria pavo. Females, faced with two focal options, differing in body size and extension of a yellow spot, strongly preferred one of the options. The addition of a decoy, exhibiting a lower expression than both focal options on one trait, but lying between them on the other trait, shifted female preference according to the initially preferred trait and to the decoy type. Indeed, females modified their preference in presence of the decoy with the intermediate expression of the trait less preferred initially, whereas they did not when the decoy for the preferred trait was added. Thus, females’ evaluation resulted to be context-dependent, even if the violation of rational decision rules occurred only with respect to the initially less preferred trait. This does not exclude that females are in any case using comparative decision rules. When faced with three alternatives they might not be able to discriminate among them, perceiving stimulus absolute magnitude.

       Field observations showed that peacock blenny females assess males in sequence and do not necessarily mate with the last visited one. To evaluate and rank males on a relative basis, females are expected to be able to remember both qualities and locations of previously encountered males. With the aim to investigate the cognitive tools allowing females to compare males, we are currently testing their spatial abilities.