Rock, paper, survival?

Rock-paper-scissors: a classic way of resolving petty disputes or making small decisions.

That is, if you’re a human.

For some species, the rock-paper-scissors theory determines survival or reproductive success. It occurs when three equally strong variants coexist in a population. The three variants interact “non-transitively”, meaning there is not one clear best variant; each variant has an advantage in a different environment. Variants of a species are controlled by genetics, so they are called “genotypes”. This article describes two cases of rock-paper-scissors competition between three genotypes of the same species, and one case between different species.

Alternative mating strategies in side-blotched lizards

The first scientific article to describe a real-life rock-paper-scissors system was published by Sinervo and Lively in 1996. They studied male side-blotched lizards that come in three different throat colours: yellow, orange, and blue. The lizards fight each other for territory and female mates, and the more females a male has on his territory, the more offspring he can have, so the more successful he is. Orange-throated male lizards are the most aggressive and have the most testosterone. Blue-throated males are less aggressive, so they lose fights against the orange-throats and therefore lose their females. Yellow-throated male lizards do not fight for mates, so they lose their females to the fighting blue-throated males. Instead, yellow-throated males pretend to be sexually active females, deceiving another male lizard and mating with his females. Since orange males are more aggressive in finding females, they are more prone to yellow-throated deception, and lose many females to the yellow-throats. So yellow beats orange, orange beats blue, and blue beats yellow: a rock-paper-scissors dynamic.

Sinervo and Lively found that blue-throated males dominated the side-blotched lizard population in 1991, but they were wiped out by the more aggressive orange-throated males in 1992, who themselves were wiped out by the deceiving yellow-throated males in 1993-4. A population dominated by yellow-throats was the ideal environment for blue-throated males to come back in 1995 (Figure 1). Therefore, rock-paper-scissors competition causes a population to cycle between three equally strong genotypes over time.

Colicin dynamics in E. coli

While rock-paper-scissors theory determines reproductive success in side-blotched lizards, the stakes are higher in E. coli bacteria, where the three genotypes fight each other to survive. In this system, there are three strains (i.e. types) of E. coli called colicin-producing (C), colicin-susceptible (S), and colicin-resistant (R). Strain C makes a toxin called colicin, which kills strain S. Strain R outcompetes strain C because not only is it resistant to strain C’s colicin, but it also saves the energy needed to produce it. However, by gaining colicin resistance, strain R loses nutrient uptake abilities. This means strain S, which is better at nutrient uptake, outcompetes strain R. So C kills S, S outcompetes R, and R outcompetes C: a rock-paper-scissors dynamic (Figure 2).

Kerr et al. found that when the three E. coli strains were grown on a solid agar surface, they stayed at equal, constant biomass for one week. If one strain grew too much, another strain in a system would outcompete it and bring the biomass back down. Therefore, rock-paper-scissors competition maintained strain diversity and stability in a genetically heterogeneous E. coli population.

Rock-paper-scissors-lizard-Spock in coral reefs

Expanding rock-paper-scissors theory to between species, rather than within species, creates a complex and diverse ecosystem. ‘The Big Bang Theory’ TV show popularised a five-object version of rock-paper-scissors called rock-paper-scissors-lizard-Spock, which expanded the number of possible scenarios in the game and reduced the chance of a draw. Similarly, a 1979 paper describes dozens of possible interactions between seven species of marine animals off the coast of Jamaica, keeping the marine ecosystem diverse. There were five sponges (S-2 Toxemna sp., S-3 Tenaciella sp., and three previously undescribed sponges S-4, S-5, and S-6), one coral species (C-1 Madracis sp.), and one ascidian (A-1 Didemnum sp.). These species all grow as colonies, in which hundreds of individual animals function as one cohesive unit. However, coral reefs have limited space, so colonies of different species fight over a solid surface to grow on. Scientists noted the outcomes of dozens of interactions where the colony of one species grew over the colony of another, to determine whether the ecosystem is linear/transitive or not. They found many examples of non-transitive interactions; for example, one rock-paper-scissors three-species system, and one system where six species beat each other in a cyclical pattern (Figure 3). Even though species S-4 can outcompete five other species, there is always the risk of species S-2 outcompeting it, so S-4 cannot dominate the ecosystem. Depending on the spatial and temporal organisation of the Jamaican coral reef, any of the seven species can dominate.

Conclusion

Non-transitive interactions lead to high diversity and cyclical patterns over time. Scientists have observed these interactions between and within species, and across ecological scales from small laboratory bacteria to complex coral reef ecosystems. If one variant begins to dominate, that makes another variant perform better, keeping the system balanced. While the three variants co-exist in some situations, like the E. coli colicin dynamics, they take turns being dominant in other situations, like the side-blotched lizards. So next time you’re thinking of using rock-paper-scissors to decide something, maybe try C-S-R E. coli or orange-yellow-blue lizards instead!

Written by Simran Patel

REFERENCES

Sinervo, B. and Lively, C.M. (1996) The rock–paper–scissors game and the evolution of alternative male strategies. Nature, 380(6571), 240–243.

Buss, L.W. and Jackson, J.B.C. (1979) Competitive Networks: Nontransitive Competitive Relationships in Cryptic Coral Reef Environments. The American Naturalist, 113(2), 223–234.

Kerr, B., Riley, M.A., Feldman, M.W. and Bohannan, B.J.M. (2002) Local dispersal promotes biodiversity in a real-life game of rock–paper–scissors. Nature, 418(6894), 171–174.