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Lizard Mating Strategies: a Delicate Balance of Rock, Paper, Scissors

In the dating world, there are many ways to win over a mate. In nature, it’s often a little less complicated; out west, males lizards have narrowed it down to three options.

Uta stansburiana, the common side-blotched lizard, comes in a variety of colors. These are not merely decorative, however. They are indicative of different hormone levels, and different mating strategies. Blue-throated males are devoted to their mates, forming strong pair-bonds with females. Orange-throated males are the largest, with the most testosterone. They favor the “love ’em and leave ’em” strategy, acting aggressively toward other males and mating with females in a large territory. An orange-throat can easily drive a blue-throat ¬†away from the blue-throat’s territory. The third type of male is the yellow-throat; they have the lowest testosterone levels, and actually mimic females. This enables them to sneak into orange-throat territory and mate with females while the orange-throat isn’t looking. This doesn’t work on blue-throats, however, who monitor their females much more closely.

What this creates is an evolutionary game of rock-paper-scissors. Orange beats blue, blue beats yellow, and yellow beats orange. Modeling rock-paper scissors in a payoff matrix looks like this:

R P S
R 0,0 -1,1 1,-1
P 1,-1 0,0 -1,1
S -1,1 1,-1 0,0

The mixed Nash equilibrium strategy would clearly be for each player to play all three options with 1/3 possibility. In a repeated number of trials, one would expect to see about as many rocks played as paper and scissors. Similarly, in the lizard populations that have all three mating types, the relative frequency of the three male types is about even in the long-term (although it fluctuates in the short-term).

How is this maintained? Although the rock-paper-scissors analogy is useful, the lizards don’t look at the current situation in their area and decide which form to take. Rather, their appearance and behavior is more-or-less hard-coded into their DNA. This is where evolution comes into play. ‘Winning’ the mating ‘game’ means having offspring, which means more males of your mating type. So any situation which is favorable to a particular mating type will lead to that mating type become more prevalent in the subsequent generation. In this way, balance is maintained. Are there more oranges than balance would indicate, and not enough yellows? Then the yellows will be more successful, since there are more oranges to beat, and the next generation will have more yellows and fewer oranges.

Of course, this balance is more fragile than the rock-paper-scissors analogy. What happens if a fluctuation is strong enough to eliminate one of the groups entirely? Say, in a small population, most of the males in a certain generation are blues, by chance. These blues might prove too numerous for any yellows, who might actually go extinct in that population. Then, the oranges take over the blues. But, there are now no yellows at all to take advantage of the large number of oranges. With the effective loss of one of the strategies, the balance is lost and cannot be regained (barring migration from another population).

Indeed, there are several populations of lizards that have only one mating type. Interestingly, the yellow type is always lost first. This suggests a cause beyond a fluctuation; perhaps an environmental change that makes both blue and orange beat yellow. According to this paper, a loss is more likely to happen when the environment is very homogeneous. In a more fractured environment, it is more easy for, say, a yellow to succeed at mating even when there are a surplus of blues, because all the lizards have less access to each other. With no mating type going extinct, the cycle is maintained, and the lizards continue their fighting, wooing, and sneaking indefinitely.

Most of the general information about the lizards was from this article: http://news.ucsc.edu/2010/02/3544.html

-Red.Tide

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