Observations on the cave-dwelling amphipod "Gammarus minus" showed active selection against their vestigial eyes. When mating, the males remain on their females for a week or so after fertilisation, guarding them against other males. The eye sizes of paired males were compared with those of unpaired males. The eye sizes of paired males were smaller than the unpaired ones. Obviously, on average, the paired males would have higher fertility than the unpaired males. The selection gradient was estimated as –0.30, which means the selection for small eyes was quite strong. The researchers supposed that the loss of sight freed more of the nervous system to process other sensory inputs.