Resource-dependent investment in male sexual traits in a viviparous fish

Exaggerated and conspicuous sexually selected traits are often costly to
produce and maintain. Costly traits are expected to show
resource-dependent expression, since limited resources prevent animals
from investing maximally in multiple traits simultaneously. However, there
may be critical periods during an individual’s life where the expression
of traits is altered if resources are limited. Moreover, costly sexual
traits may arise from sexual selection acting both before (pre-copulatory)
and after mating (post-copulatory). Gaining a robust understanding of
resource-dependent trait expression therefore requires an approach that
examines both episodes of sexual selection following resource limitation
during different times in an individual’s life. Yet few studies have taken
such an approach. Here, we examine how resource restriction influences a
set of pre- and post-copulatory traits in male pygmy halfbeaks (Dermogenys
collettei), which invest in sexual ornaments and routinely engage in
male-male contests and sperm competition. Critically, we examined
responses in males when resources were restricted during development and
after reaching sexual maturity. Both pre- and post-copulatory traits are
resource-dependent in male halfbeaks. Body size, beak size, courtship
behavior, and testes size were reduced by diet restriction, while,
unexpectedly, the restricted-diet group had a larger area of red color on
the beak and fins after diet treatment. These patterns were generally
consistent when resources were restricted during development and after
reaching sexual maturity. The study reinforces the role of resource
acquisition in maintaining variation among sexual traits.