Frequency-dependent survival in natural guppy populations.

The maintenance of genetic variation in traits under natural selection is a long-standing paradox in evolutionary biology. Of the processes capable of maintaining variation, negative frequency-dependent selection (where rare types are favoured by selection) is the most powerful, at least in theory; however, few experimental studies have confirmed that this process operates in nature. One of the most extreme, unexplained genetic polymorphisms is seen in the colour patterns of male guppies (Poecilia reticulata).

Male red-winged blackbirds distrust unreliable and sexually attractive neighbours.

In many species, territorial neighbours fight to establish their mutual border and then develop a truce, known as the dear-enemy phenomenon, characterized by reduced vigilance and aggression along the border. We present evidence that among male red-winged blackbirds (Agelaius phoeniceus) the dear-enemy relationship is a form of reciprocal conditional cooperation that is stabilized, at least in part, by retaliation against cheaters. Simulated intrusions by randomly chosen neighbours were punished by a targeted increase in vigilance and aggression that persists for days.

Cooperative nest defence in red-winged blackbirds: reciprocal altruism, kinship or by-product mutualism?

Male red-winged blackbirds (Agelaius phoeniceus) often cooperate with their neighbours in defending nests against predators. Some studies have suggested that this is an example of by-product mutualism, whereas others have suggested the possibility of reciprocal altruism. No study has addressed the possibility of kin-selected cooperation in nest defence in this species.