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Source of photos: BARRY SINERVO |
Not all lizards within one species look or behave the same way--especially when it comes to mating. Among side-blotched lizards (Uta stansburiana), males court their mates according to their own throat-colors, or morphs: Blue-throated males territorially guard their mates to get a shot at reproductive success; orange-throated males aggressively invade the territory of other males in search of females; and yellow-throated males sneak onto other males' turf, often by acting like females themselves.
Scientists have long assumed that these tactics must balance each other out to be evolutionarily stable. After all, if the approach of one type of lizard always won, only that type would be found in the next generation. For side-splotched lizards, the model researchers have used is the game of rock-paper-scissors. Just as a rock crushes--and so beats--scissors in the game, orange-throated lizards out-compete the less aggressive blue-throated males; just as scissors cut paper, protective blue-throated lizards win against sneaky yellow-throated males; and as paper covers a rock, the yellow-throated lizards are successful against roving orange-throated males (see illustration).
Rock-paper-scissors makes for a convenient model, but until now, its predictions had not been tested. Barry Sinervo of the University of California, Santa Cruz and Kelly Zamudio of the University of California, Berkeley report in today's issue of the Proceedings of the National Academy of Sciences that they have accomplished just that. They collected DNA samples from 96 females, 131 putative sires and 458 offspring among a population of lizards living in California during the 1992 breeding season, and ran several different rounds of paternity testing. As expected, they found no significant difference in the total numbers of offspring produced by each male type. "During the 1992 breeding season, each morph successfully used a different tactic to exploit weaknesses of another strategy and a morph's success depended on the close proximity of a vulnerable alternative strategy," the authors write. "Frequency-dependent selection arising from local competition can promote conditions that favor each morph, and thus preserve all three strategies of the rock-paper-scissors cycle in the long term."