Game Theory in the Evolutionary Biology of brood parasites vs hosts
Links: https://doi.org/10.1093/beheco/arr084
Game Theory is not only relevant to human psychology and strategy analysis. It is found throughout nature and the biosphere. Rethinking visual supernormal stimuli in cuckoos: visual modeling of host and parasite signals (shortened as rethinking visual stimuli) is a research paper that examines the effect of visual supernormal stimuli on the feeding patterns of parasitic birds. I will first define a few technical terms and give a brief summary of the paper, then talk about how the ideas presented in the paper is an example of game theory.
Brood parasite is a general term for bird species with a peculiar youth-raising pattern. Instead of building a nest and feed their own young, the birds would lay their eggs in the nest of another specie (the host) and disguise their own child (the parasite) as a child of the host. What are supernormal stimuli? They are a source of stimulus that elicits an extraordinary response when the stimulus is magnitudes bigger than those for which it evolved (Tanaka 2011). In birds for example, usually a parent would prefer larger eggs as those have a higher likelihood to survive. However, an Ostrich egg is clearly too large an egg for a duck to lay, yet the duck would care for the egg as if it is her own. In Rethinking visual stimuli, the authors conducted a study involving Horsfield’s hawk-cuckoo (Cuculus fugax) as the parasite and red-flanked bluetail (Tarsiger cyanurus) as the host. The bright orange spot on the wing of the bird induces a supernormal stimulus on the host, and the host would feed the parasitic bird with a significantly larger spot more than its own children. The size of the bird is not a factor in this study. However, some researchers have also shown that the size of the bird is a significant source of supernormal stimuli and larger parasitic birds tend to receive greater feed time than smaller host birds. In those studies, the bright orange spot and other visual stimuli did not elicit a supernormal response (Grim 2001).
The idea of supernormal stimuli and what triggers it can form a game similar to the attack and defense game explained in class. The strategy of the parasite can be one of the two: visual stimulus involving a bright orange spot or size stimulus by growing very large. Since birds have limited energy for reproduction, only one evolutionary strategy may be chosen. The strategy of the host can also be one of the two: feed those with bright orange spots (visual) or feed those that are large (size). If the strategy of the host and parasite lines up, then the parasite receives a greater payoff and if their strategy did not match, then the host’s children receives a greater payoff (and hence the host, since its children are now stronger and more likely to pass down its genes). The payoff matrix is below. Note that for each ordered pair (x, y), x stands for the payoff for the parasite while y stands for the payoff for the host. All payoffs are estimated.
| Host visual | Host size | |
| Parasite visual | 2, 1 | 1, 2 |
| Parasite size | 1.5, 2 | 2, 1.5 |
Note that there are no dominant strategies for either player or a pure strategy Nash Equilibrium. A mixed strategy Nash Equilibrium will involve the different parties choosing size and visual strategy a certain percentage of times. This may account for the different experimental results shown in different studies on brood parasites.
Bibliography:
Tanaka, K. D., Morimoto, G., Stevens, M., & Ueda, K. (2011). Rethinking visual supernormal stimuli in cuckoos: visual modeling of host and parasite signals. Behavioral Ecology, 22(5), 1012–1019. https://doi.org/10.1093/beheco/arr084
Grim, T., & Honza, M. (2001). Does supernormal stimulus influence parental behaviour of the cuckoo’s host? Behavioral Ecology and Sociobiology, 49(4), 322–329. https://doi.org/10.1007/s002650000295