B Sinervo and CM Lively. 1996. The rock-paper-scissors game and the evolution of alternative male strategies. Nature vol. 380, pp. 240-243.
(Side note: I don’t want to feature-paper too many Science or Nature papers, since those journals are so high-profile that you’re likely to hear about the work elsewhere, and part of the point of this feature is that papers in “lesser” journals can be awesome too; but this paper is classic and fun, so I’ll make an exception.)
Male side-blotched lizards come in three
flavors colors: orange-throated, blue-throated, and yellow-throated. Orange males are highly aggressive and defend large territories. Blue males are less aggressive, defending smaller territories. Yellow males look like females and don’t defend territories at all. All three colors compete to mate with females and have offspring. Throat color is highly heritable: orange males have orange sons, blue males have blue sons, yellow males have yellow sons.
If you were studying this, you might find it strange that there are three colors at all. After all, it looks like the dominant orange males should out-compete everyone, have lots of orange offspring, and pretty soon there would just be orange males left. Why doesn’t that happen?
It turns out that, essentially, orange males are rock to blue males’ scissors—and yellow males are paper.
For two years, orange males increased in number by taking females away from blue males. But then for three years orange males decreased, because yellow males, those unassuming female-imitators, started taking their females. Quickly yellow males started to decrease again, as blue males took their females; and by five years from the start of the study, orange males were on the rise again.
This cycle happens because in a rock-paper-scissors scenario, whenever one strategy becomes more frequent, the strategy that beats it quickly begins to increase. If I know you’re going to throw scissors 80% of the time, I’ll throw rock, and win a lot. But then someone else who throws paper will beat me because I’m throwing rock… and on and on.
In studying evolution, we often fall into the trap of thinking that species should evolve to follow the single best strategy. In reality, the world is dynamic, and the best strategy may be constantly changing.