As a conspicuous evolutionary mechanism, sexual selection has
received much attention from theorists and empiricists. Although
the importance of the mating system to sexual selection has long
been appreciated, the precise relationship remains obscure. In a
classic experimental study based on parentage assessment using
visible genetic markers, more than 50 years ago A. J. Bateman
proposed that the cause of sexual selection in Drosophila is
'the stronger correlation, in males (relative to females),
between number of mates and fertility (number of progeny)'. Half
a century later, molecular genetic techniques for assigning
parentage now permit mirror-image experimental tests of the
'Bateman gradient' using sex-role-reversed species. Here we show
that, in the male-pregnant pipefish Syngnathus typhle, females
exhibit a stronger positive association between number of mates
and fertility than do males and that this relationship responds
in the predicted fashion to changes in the adult sex ratio.
These findings give e