Fading to black doesn’t empower fish

Field studies of three-spined stickleback fish dash a textbook example of an evolutionary principle, claims an evolutionary biology team. Males of Gasterosteus aculeatus typically turn red on their bellies and blue-green on top when breeding—unless they live in Washington’s Chehalis River watershed. There, males turn jet-black.

That quirk achieved textbook status as the only documented example of the theory of convergent character displacement, explain Robert J. Scott and Susan A. Foster of Clark University in Worcester, Mass. According to this theory, two species’ traits get either more or less similar when the species share a home.

In the Chehalis system, male sticklebacks and Olympic mudminnows both claim territories. Scientists had held that if a stickleback flashes black, as the mudminnows do during a threat display, the stickleback becomes more of a contender.

That interpretation originally came from lab studies of Chehalis sticklebacks. However, fieldwork doesn’t agree, say Scott and Foster in the March 22 Proceedings of the Royal Society of London B.

The researchers set big cages in the water so they could control the fish mix. The sticklebacks scrapped among themselves but rarely with mudminnows. Also, red-and-green stickleback males that the researchers introduced did just as well as black sticklebacks in claiming territory. Perhaps the most interesting upset of the theory was that 23 out of 24 males turned black not during—but after—staking out territory.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.