The hawk–dove game in a sexually reproducing species explains a colourful polymorphism of an endangered bird

Abstract

The hawk-dove game famously introduced strategic game theory thinking into biology and forms the basis of arguments for limited aggression in animal populations. However, aggressive 'hawks' and peaceful 'doves', with strategies inherited in a discrete manner, have never been documented in a real animal population. Thus, the applicability of game-theoretic arguments to real populations might be contested. Here, we show that the head-colour polymorphism of red and black Gouldian finches (Erythrura gouldiae) provides a real-life example. The aggressive red morph is behaviourally dominant and successfully invades black populations, but when red 'hawks' become too common, their fitness is severely compromised (via decreased parental ability). We also investigate the effects of real-life deviations, particularly sexual reproduction, from the simple original game, which assumed asexual reproduction. A protected polymorphism requires mate choice to be sufficiently assortative. Assortative mating is adaptive for individuals because of genetic incompatibilities affecting hybrid offspring fitness, but by allowing red 'hawks' to persist, it also leads to significantly reduced population sizes. Because reductions in male contributions to parental care are generally known to lead to lower population productivity in birds, we expect zerosum competition to often have wide ranging population consequences.

Genetic architecture and balancing selection: the life and death of differentiated variants

“…Imbalances between morph frequencies due to habitat destruction, for instance enhancing the frequency of the competitive red morphs which exhibit poor parental care, are predicted to increase population extinction risk (Kokko et al . ).…”

“…In the polymorphic bird Gouldian finch (Erythrura gouldiae), red, black and yellow morphs coexist and are associated with variation in aggressiveness and parental investment differences. Imbalances between morph frequencies due to habitat destruction, for instance enhancing the frequency of the competitive red morphs which exhibit poor parental care, are predicted to increase population extinction risk (Kokko et al 2014).…”

Hormonal underpinnings of status conflict: Testosterone and cortisol are related to decisions and satisfaction in the hawk-dove game

“…Major males fight for access to females, but minor males defer status to major males and mate with females by sneaking copulations. Evidence for different social tactics (e.g., hawk versus dove) is found in many other species as well, including earwigs, spiders, salmon, birds, and orangutans (Forslund, 2003;Fromhage and Schneider, 2005;Thomaz et al, 1997;Kokko et al, 2014;Harrison and Chivers, 2007).…”

The evolution of respect for property

“…For example, in showing that deference towards owners can evolve as a convention to reduce costly fighting, Hawk–Dove models neatly explain the widespread observation of respect for property. Moreover, the same models clearly show how stable polymorphisms of aggressive and nonaggressive individuals can evolve, which in turn may help explain the maintenance of bold and shy ‘personality types’ within populations (see, e.g., Dall et al ., ; Sih et al ., ; Wolf & Weissing, ; Kokko et al ., ). More specifically, Hawk–Dove models can help biologists understand a range of intriguing observations in animal contests, such as why minor asymmetries in RHP do not take precedence over current ownership in determining contest outcome, and why contests are more likely to be protracted when an existing owner and a former owner meet in staged competitions.…”