Cost for a Sexually Selected Trait in Male Guppies

Summary: Jean-Guy J. Godin and Heather E. McDonough of Mount Allison University conducted an experiment to gather empirical evidence on whether predators prefer guppies of a brighter coloration. Results support the idea that the guppies’ sexually selected trait of bright coloration turns out to be a disadvantage in terms of survival. 

In many species males use obvious sexual signals, like bright coloration or large plumage, to attract females. It is well known that such traits are an advantage in sexual selection because they are female preferred but do not translate to an advantage when it comes to natural selection since such traits also attract the attention of predators. However, the direct fitness cost of a sexually selected trait has not been thoroughly proven via experimentation.

Godin and McDonough decided to examine the effects of a sexually selected trait on the rate of mortality in a predator-prey system. They chose to test male guppies and their natural predators, the blue acara cichlid fish. Male guppies are genetically polymorphic. They express patches of different color, hue, number, size, and reflectivity. It is known that the guppies with brighter coloration tend to be more active and bold than their dimmer counterparts. Although it seems that bright males have a mating advantage, evidence exists that fish predators, like the blue acara cichlid, also select prey with conspicuous color patterns so it may be that these bright guppies face higher predation intensity. It was found in one study that populations of male guppies in the waters of Trinidad were less colorful since there was an increased population of their natural diurnal predators.

In the first experiment, two male guppies of the same length but difference in brightness were kept restrained in separate Plexiglas tubes. The tubes were equidistance from the cichlid, which was acclimated to the environment for 24 hours before being exposed to the prey. The second experiment was set up in much the same way. However, a larger aquarium was used and the cichlid was left to assimilate to the environment for 36 hours. The guppies in this case were free-swimming which more accurately replicates the natural interactions between the predator and prey.

The end results of the experiment indicated that the brightness of the male guppies affects the frequency in which they get approached and attacked by predators. For the first experiment the bright guppies were attacked significantly more than the drabber guppies. In the second experiment, bright guppies were once again attacked significantly more than the drab guppies irrespective of distance. The results further support the theory that sexually selected traits have a cost in terms of survival.

The study pertains to what we have been learning in class about the apparent paradoxical relationship between sexual selection and natural selection. It is interesting that nature requires organisms to balance mate preference and predator evasion in order to survive. 

Citation: 

Godin,J.J.,McDonough, H.E. 2003. Predator preference for brightly colored males in the guppy: a viability cost for a sexually selected trait. Behavioral Ecology 14(2): 194-200