A new study of side-blotched lizards [Uta stansburiana] in California has revealed the genetic underpinnings of altruistic behavior in this common lizard species, providing new insights into the long-standing puzzle of how cooperation and altruism can evolve. The study, led by researchers at the University of California, Santa Cruz, offers the first evidence in vertebrates of an important theoretical concept in evolutionary biology known as "greenbeard" altruism. (source)

From the press release:

The paper describes unrelated male lizards that form cooperative partnerships to protect their territories. These partnerships are often mutually beneficial, enabling both partners to father more offspring than they would on their own. Under some circumstances, however, one male in the pair may have few or no offspring as a result of protecting its partner from the aggressive intrusions of other lizards.

This type of cooperation, in which one individual bears all the costs and another unrelated individual receives the benefits, is called "true altruism." These lizards have an annual life cycle, so this behavior may spell the end of the altruistic male's lineage.

The evolution of cooperation is puzzling because cooperative systems are vulnerable to cheaters or defectors. Cheaters and defectors benefit from the cooperative behavior of others without paying the costs, so they ought to be favored under natural selection. But then cheater genes would spread in the population, and the cooperative system would collapse. The greenbeard effect is one way to get around this.

The concept originated with the British evolutionary theorist W. D. Hamilton and was popularized by Richard Dawkins in his book The Selfish Gene. Dawkins illustrated the concept with a hypothetical example in which a green beard serves as a marker enabling individuals with a gene for cooperation to recognize others with the same gene. The greenbeard gene (or genes) must do three things: establish a signal (the green beard), enable recognition of others that share the signal, and promote cooperative behavior towards other greenbeards.

In side-blotched lizards, the greenbeards have blue throats. This species comes in three different throat colors--orange, yellow, and blue--and throat color corresponds to different territorial behaviors in the males. Blue-throated males form partnerships in which two males cooperate to protect their territories; orange-throated males are highly aggressive and usurp territory from other lizards; and yellow-throated males sneak into the territory of other males to mate with females.

Previous research by [Barry] Sinervo and his collaborators has demonstrated that these strategies result in a kind of "rock-paper-scissors" game in which orange defeats blue, blue defeats yellow, and yellow defeats orange. The lizard populations go through cycles in which one color after another increases its numbers at the expense of the others, but none are able to maintain dominance.

In the new paper, Sinervo's group investigated the genetic basis of the cooperative behavior in blue-throated males. They found that, in addition to the gene that controls throat color, there are at least three other genetic factors that determine the components of the greenbeard effect.

"Our study provides the first comprehensive genetic evidence of a greenbeard in vertebrates," Sinervo said. "Unlike Hamilton's idea and most modern views of greenbeards, we find that many genetic loci across the genome 'cooperate' to establish conditions for the greenbeard behaviors."

The paper is Sinervo et al. (2006) "Self-recognition, color signals, and cycles of greenbeard mutualism and altruism" PNAS published May 1, 2006, 10.1073/pnas.0510260103.