Data from: Dual function and associated costs of a highly exaggerated trait in a cichlid fish

Exaggerated secondary sexual characteristics are apparently costly and
seem to defy natural selection. This conundrum prompted Charles Darwin to
propose the theory of sexual selection. Accordingly, exaggerated secondary
sexual characteristics might be ornaments on which female choice is based
and/or armaments used during male-male competition. Males of many cichlid
fish species, including the adaptive radiation of Nicaraguan Midas
cichlids, develop a highly exaggerated nuchal hump, which is thought to be
a sexually selected trait. To test this hypothesis, we conducted a series
of behavioral assays in F2 hybrids obtained from crossing a species with a
relatively small hump and one with an exaggerated hump. Mate-choice
experiments showed a clear female preference for males with large humps.
In an open-choice experiment with limited territories, couples including
large humped males were more successful in acquiring these territories.
Therefore, nuchal humps appear to serve dual functions as an ornament for
attracting mates and as an armament for direct contest with rivals.
Although being beneficial in terms of sexual selection, this trait also
imposes fitness costs on males possessing disproportionally large nuchal
humps since they exhibit decreased endurance and increased energetic costs
when swimming. We conclude that these costs illustrate trade-offs
associated with large hump size between sexual and natural selection,
which causes the latter to limit further exaggeration of this spectacular
male trait.