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

Abstract

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 after 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.

Keywords: condition dependence; intra-sexual competition; mate choice; resource manipulation; sexual selection.

Figure 1

Validating the effects of diet manipulations on male condition (mean ± SE) standardized per block. The lowercase letters (a, b, c) in the figure represent the groupings from the post hoc tests. A) Condition factor after diet treatment for males in high and restricted diet treatment in adult and juvenile diet experiments (circle = adult diet experiment, triangle = juvenile diet experiment). B) Condition in the adult diet experiment for high treatment (dashed line) and restricted treatment (filled line) before and after diet treatment.

Figure 2

Effect of diet treatment on male pre-copulatory traits (mean ± SE) standardized per block. Note that there was no difference between the two experiments, only between the two diet treatments. The lower-case letters (a, b) represent the groupings from the post hoc tests, that is, showing that the traits did not differ between experiments, only between the two diet treatments. The adult (circle) and juvenile diet experiment (triangle) are presented separately. A) Beak length (corrected for body size) for the males in the high and restricted treatments in adult and juvenile diet experiment, B) area of red coloration on the beak (coloration PC2) for the males in the high and restricted treatments in adult and juvenile diet experiment, C) Total number of sexual behaviors index for the males in the high and restricted treatments in adult and juvenile diet experiment.

Figure 3

Effect of diet treatment on the testes mass (mean ± SE) as proportion of body mass (standardized per block) between males in the high and restricted treatments in the adult (circle) and juvenile diet experiment (triangle). Post hoc tests showed that males in the restricted diet treatment (a) differed from the males in the high diet treatment (b), but not between experiments.