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Cheaters always prosper, to a point

Nests with only workers (C) initially produced fewer eggs than cheater-only colonies (D). But eventually the nest becomes neglected and eggs begin to decay. (Image PNAS)

Nests with only workers (C) initially produced fewer eggs than cheater-only colonies (D). But eventually the nest becomes neglected and eggs begin to decay. (Image PNAS)

Is it better to cooperate or cheat? To work for the good of others, or focus on yourself? This classic self vs. group conflict is a common musing of game theorists and experimental economics known as the “public goods dilemma.”

In cooperative groups, individuals work for the benefit of the group at some personal cost, generating a “public good.” But some individuals can use public goods without contributing, effectively free-loading. Within a group cheaters prosper if they are un-policed; but between groups cooperation gives them a competitive edge.

The “dilemma between a part and the whole is inevitable in [all] social life,” says behavioral ecologist Shigeto Dobata.

In a recently published paper in PNAS Early Edition, Dobata and Kazuki Tsuji demonstrate what they believe is the first observed public goods dilemma observed in a non-human and non-microbial system.

By using the social ant Pristomyrmex punctatus, they were able to show the fitness consequences to the colony and track the shifting genetic make-up as cheaters invaded and took hold. Researchers have recently evaluated these questions in systems involving viruses and cells (where cells may secrete protective substances, or self-destruct to form a spore-dispersing stalk) but not in multicellular organisms before. Yet the results are so similar, write Dobata and Tsuji, that they believe universal principles are at play.

P. punctatus is a curious species. The queen caste, morphologically and functionally distinct in most social insects, has been secondarily lost. All workers are involved in both reproduction and cooperative tasks like foraging. There is still a division of labor, among age groups. Young workers take care of inside-nest tasks, which include asexual (thelytokous) reproduction. Older workers ease out of reproduction and shift to tasks outside the nest, like foraging.

But there is a third kind of P. punctatus. A group of cheaters, made of a single intraspecific lineage in the field, that engage in very few tasks, save for reproduction.

The researchers found when these genetic cheaters infect a colony they have better individual fitness than the workers, both in terms of survival and brood production. They reduce worker survival and reproduction, as more young workers shift to tasks outside the nest to effectively pick up the slack. Eventually, the cheater hordes take over. The authors call the cheaters a kind of  “transmissible social cancer.”

In cheater-only colonies, more eggs are initially produced, compared to worker colonies, but they are neglected. Eggs begin to rot and the nest becomes a dirty, unhygienic place. Eventually, the nest dies. For a group, cheating is an evolutionary dead end.

Some social insects have evolved behavior to detect and punish such cheaters, but these ants have not, so far as we can detect. In future work, Dobata plans to look for the conditions that permit the cheater lineage to persist.

He thinks differing migration rates may be key. Worker ants hardly ever migrate between colonies. Cheater ants are far more likely to up and move nests. He has begun a simulation study to evaluate the possible effects of these rates.

Categories: Ecology | Evolution and tagged | | | |
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