American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“The size, symmetry, and color saturation of a male guppy’s ornaments forecast his resistance to parasites”

Posted on

Jessica F. Stephenson, Martin Stevens, Jolyon Troscianko, and Jukka Jokela (Nov 2020)

Male guppy ornaments indicate parasite resistance before infection, but males may ‘fake’ quality with dynamic traits!

Read the Article

Males that are more resistant to parasites are sexier, even when they’re not infected

Females across animal species are very good at recognizing disease in their mates – these males typically have duller, smaller color patches, for example. Mating instead with males with brighter, larger color patches means that the female is less likely to become infected during sex, and perhaps that her offspring may be more resistant to parasites thanks to their dad’s genes. However, parasites are usually clumped among their hosts, so that the vast majority of males in a population will not be infected. Can females still use a male’s color patches to assess his resistance to parasites, even if he’s not currently infected? Jessica Stephenson and colleagues set out to test this question in small tropical freshwater fish – guppies. They took photographs of several males to analyze their color patterns before infecting them with a common parasite. They made repeated counts of each male’s parasite load as the parasites reproduced and died, depending on their host’s resistance. Interestingly, males with larger areas of orange and more symmetrical black coloration were more resistant, but males with more saturated orange spots were less resistant to the subsequent infection. The authors attribute this to the fact that orange saturation can change rapidly in response to changes in male condition, whereas orange area and black symmetry do not change during sexual maturity: perhaps this makes orange saturation easier to ‘fake’? Female guppies typically prefer more symmetrical color patterns, and larger, more saturated orange patches. When given the choice they prioritize orange area over saturation, suggesting they can spot the fakers and prefer to mate with more resistant males, even in the absence of infection. This finding broadens our understanding of how parasites impact sexual selection in their hosts, and how sexual selection is involved in host-parasite coevolution.


Sexually selected ornaments range from highly dynamic traits to those that are fixed during development and relatively static throughout sexual maturity. Ornaments along this continuum differ in the information they provide about the qualities of potential mates, such as their parasite resistance. Dynamic ornaments enable real-time assessment of the bearer’s condition: they can reflect an individual’s current infection status, or resistance to recent infections. Static ornaments, however, are not affected by recent infection but may instead indicate an individual’s genetically determined resistance, even in the absence of infection. Given the typically aggregated distribution of parasites among hosts, infection is unlikely to affect the ornaments of the vast majority of individuals in a population: static ornaments may therefore be the more reliable indicators of parasite resistance. To test this hypothesis, we quantified the ornaments of male guppies, Poecilia reticulata, before experimentally infecting them with Gyrodactylus turnbulli. Males with more left-right symmetrical black coloration and those with larger areas of orange coloration, both static ornaments, were more resistant. However, males with more saturated orange coloration, a dynamic ornament, were less resistant. Female guppies often prefer symmetrical males with larger orange ornaments, suggesting parasite-mediated natural and sexual selection act in concert on these traits.