MANY species exhibit colour polymorphisms associated with alternative
male reproductive strategies, including territorial males and 'sneaker
males' that behave and look like females(1-3). The prevalence of mult
iple morphs is a challenge to evolutionary theory because a single str
ategy should prevail unless morphs have exactly equal fitness(4,5) or
a fitness advantage when rare(6,7). We report here the application of
an evolutionary stable strategy model to a three-morph mating system i
n the side-blotched lizard. Using parameter estimates from field data,
the model predicted oscillations in morph frequency, and the frequenc
ies of the three male morphs were found to oscillate over a six-year p
eriod in the field. The fitnesses of each morph relative to other morp
hs were non-transitive in that each morph could invade another morph w
hen rare, but was itself invadable by another morph when common. Conco
rdance between frequency-dependent selection and the among-year change
s in morph fitnesses suggest that male interactions drive a dynamic 'r
ock-paper-scissors' game(7).