Disappearing Traits: What's the Cause?

We know sexually selected traits are important for the reproductive success of many species, so why are so many males losing them? An analysis of studies across different species has revealed possible causes for reductions in female preference for male traits and subsequent male losses.

Possible causes of male trait loss:

• Environmental
• Predation risk - the trait causes males to become highly visible to predators or reduces locomotor performance when trying to escape (e.g. less agile)
• Signal transmission - traits that are too easy or hard to transmit (e.g. environment too dark to see bright colours)
• Nutrients - limited availability so many males are unable to produce required traits (e.g. specific pigments for colouration)
• Random
• Genetic Drift - may occur in small populations if the pressure of sexual selection is weak or genetic drift is strong

What factors might change female preferences?

Social factors are influenced by environmental and random effects.

• High predation pressure on 'choosy' females
• Signal transmission (see above)
• Geographical overlap - Females may change their preferences when the territory of their species overlaps with a closely related species with similar sexually selected traits (e.g. colouration). This may help prevent interbreeding and hybrids
• Selection for different traits - Female choice for male parental care could reduce the need for males to present sexual traits to attract multiple females

The female preference in both the Pied Flycatcher (top) and Collared Flycatcher (bottom) is for black and white colouration. When these species are found in the same area, female preference in the Pied Flycatcher  changes to a dull colouration. 
Image credits: (top) Piotr Jonczyk; (bottom) Garth Peacock

But remember the effect of factors needs to be considered carefully! 

Factors influencing female choice and male sexual traits are likely to work in combination. Even when present, these factors may not always result in the loss of a male trait or changes to female preference. Sexual traits may not disappear simply because environmental pressures have increased.

Increased predation, decreased nutrients or signal transmission may instead highlight the most resourceful and genetically fit males (Good Genes Model) and increase the number of matings that these 'top quality' males receive. Thus, keeping these characteristics in the species and maintaining female preference. Although research shows that female preference is lost before the male trait is, researchers are yet to explain this!

To read about how male robins respond to difficult signal transmission see 'Changing Tune'. 

Source:

• WIENS, J. J. 2001. Widespread loss of sexually selected traits: how the peacock lost its spots. Trends in Ecology & Evolution, 16, 517-523