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Sex and the Single Guppy

Intro | Discovery | Hypotheses | Simulation 1 | Analysis 1 | Simulation 2 | Analysis 2 | Conclusion | Gallery

A properly dressed male guppy, with its gaudy blue spots and brilliant splashes of orange, can't help but stand out. But for a fish that spends its life swimming among predators, it seems that good camouflage would have a big advantage over colors that attract attention. If flashiness is a liability, why are these little guys such exhibitionists?

male guppy

To see the range of coloration that guppies exhibit in the wild, and to see their predators and habitats, check out the Guppy Gallery.

Endler's Discovery

When evolutionary biologist John Endler began studying Trinidad's wild guppies in the 1970s, he was struck by the wide variation among guppies from different streams, even among guppies living in different parts of the same stream. Males from one pool sported vivid blue and orange splotches along their sides, while those further downstream carried only modest dots of color near their tails. Endler also observed differences in the distribution of guppy predators, and in the color and size of gravel in different stream locations.

Endler photographed hundreds of guppies and carefully measured their size, color, and the size and placement of their spots. He began to see a strong correlation between where guppies lived in a particular stream and whether the fish were bright or drab. But what was responsible for these trends in coloration? And if bright colors made guppies more conspicuous to predators, why should males be colorful at all? To find out, Endler formed a hypothesis based on his observations, then set out to test it. His results would prove to be one of evolutionary biology's most important discoveries.

Make your own observations of a guppy stream.

Investigate the different pools in this stream more closely. Look for color trends in the guppies. These may vary relative to the presence or absence of specific predator species in the pools.

Pool 1

Pool 1
Number of guppies: 75
Average male coloration:
Brightly multi-colored with large spots

Number of pike cichlids: 0
Number of blue acara: 0
Number of rivulus: 12

Pool 2

Pool 2
Number of guppies: 83
Average male coloration:
Medium coloration on body and tail, with medium-sized spots

Number of pike cichlids: 0
Number of blue acara: 7
Number of rivulus: 15

Pool 3

Pool 3
Number of guppies: 110
Average male coloration:
Drab coloration, very small spots concentrated near the tail

Number of pike cichlids: 17
Number of blue acara: 20
Number of rivulus: 5

-> What causes such dramatic guppy color variation?

Intro | Discovery | Hypotheses | Simulation 1 | Analysis 1 | Simulation 2 | Analysis 2 | Conclusion | Gallery

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