Size matters to female guppies

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Female guppies are more interested in the size rather than the intensity of sex-related colour spots, according to new research.

When given a choice between males with a large, dull coloured patches vs small, bright coloured patches, females consistently spent more time with the former, suggesting a significant bias towards larger spots.

The basis for the research lies in the importance of male colouration to mate choice, with the orange spot patterns of male fish determining, in part, their attractiveness to females.

It wasn’t clear though whether the size of the coloured area, or its intensity was a more important factor to females.

Kenji Karino and colleagues set up four separate experiments to test preferences for colour and size in a stepwise fashion.

After digitally filming a male guppy, the team manipulated the images to give males with different sized colour spots and different colour intensities.

The combinations used were:

  • Large vs normal area, both of the same colour intensity
  • Normal area, high vs low colour intensity
  • Large area, high intensity vs small area, low intensity
  • Large area low intensity vs small area, high intensity

The results of the first three tests were relatively predictable – females preferred males with larger or more intensely coloured spots.

When faced with having to compromise between area size and colour intensity though, females spent around 40% more time inspecting males with large, dull spots than those with small, intense spots.  

Against a background of guppy life history, the results make perfect sense: spot size may be a more “honest” indicator than colouration.

As colour intensity is related to diet, even the poorest quality males may be able to become bright where food supplies are abundant.

In contrast spot size is determined mainly by genes rather than by environment.

Given that males with larger spots have been shown to have larger sperm loads than their small-spotted conspecifics and are also more adept at evading predators, females can optimise their offspring’s fitness by only choosing large-spotted males.