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Cooperative fish, cheating ants

Cooperation is widespread in nature, despite theoretical predictions that "cheating" mutants could displace cooperators over just a few generations of evolution. We don't apply human moral standards to other species, of course, but define cheating as contributing less, while benefiting from activities of others. The evolutionary persistence of cooperation is usually attributed to reciprocity (trading resources or services) or to kin selection: cooperation among relatives, such as parental care, can persist even without reciprocity. Fish that clean parasites from other fish are a standard example of reciprocity -- they get to eat the parasites -- whereas nonreproductive worker ants are a standard example of kin selection. I will briefly discuss one recent paper on each of these.

Redouan Bshary and coauthors report in Nature that "Pairs of cooperating cleaner fish provide better service quality than singletons." Cleaner fish often prefer to eat client mucus (yum!) than client parasites, but clients don't like this and tend to leave. When a male and female cleaner work together, the client fish may leave if either of them takes a bite of mucus. Females, in particular, were less likely to do this when cleaning with their male partner rather than alone. The authors also did an experiment to see whether cleaner fish would eat a less-preferred food (fish flakes, perhaps analogous to client parasites) if eating their more-preferred food (prawns, perhaps analogous to client mucus) resulted in the food plate being taken away. They did, especially the females. This may have been because the male often chased her if she ate a prawn, costing them both the rest of their meal. Overall, pairs appear to provide better service to clients, because they are better-behaved together than alone, especially the female.

Shigeto Dobata and coauthors reported on "Cheater genotypes in the parthenogenetic ant Pristomyrmex punctatas" in Proceedings of the Royal Society. Social insects, such as ants and bees, usually have reproductive queens and nonreproductive workers. Worker genes are transmitted to the next generation by the queen, who is typically the workers' mother and therefore shares most of their genes. Pristomyrmex punctatas is different. An individual ant may reproduce (usually when young) and also work. Some individuals are more like queens, however. These are larger, reproduce more, and do little or no work for the colony. If these nonworking ants were close relatives of the workers, this behavior could perhaps be maintained by kin selection. It could be an example of division of labor for mutual benefit, a less-extreme version of the more-familiar worker/queen division. So Dobata and coauthors analyzed the DNA of hundreds of ants to see how they were related. They found that these nonworking ants were much less closely related to workers than queens usually are. Most of the ants (working or not) reproduced parthenogenetically, essentially cloning themselves without sex. Working hard while unrelated individuals profit from your work doesn't usually work out over the long run, but these nonworking ants have been seen in the field for over 25 years. Is this an evolutionary dead end? A similar situation with Cape honey bees, whose colonies are parasitized by unrelated "pseudoqueens" usually leads to colony extinction. I look forward to reading more about this interesting ant species.

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