Polymorphism in cryptic prey species may be 'protective', in the sense
of reducing the rare of predation by visual predators. We tested this
supposed function by presenting 80 schoolgirls (aged 12-13) with popu
lations of square 'prey' on backgrounds displayed on computer screens.
The backgrounds were composed of random distributions of small colour
ed squares and the prey were composed of the same small squares, the g
rid of the prey being parallel to (but usually imperfectly aligned wit
h) that of the background. Different 'morphs' of prey were defined by
the spatial distribution of the small squares, and prey of a given mor
ph were always presented in the same orientation. Each subject was pre
sented with a sequence of 40 screens, of which 36 were background cont
aining a single randomly positioned prey, while four were background a
lone. Subjects 'zapped' prey with the mouse, or pressed the space bar
if the screen was judged to be empty, and then moved automatically to
the next screen in the sequence. Each subject received a single 'treat
ment': high or low crypsis combined with one of four levels of polymor
phism (1, 2, 6 or 12 morphs). The results showed that hit ('predation'
) rare tended to be lower on populations with high crypsis and was inv
ersely related to the number of morphs in the population. We thus conc
lude that polymorphism may indeed 'protect'.