The evolution of courtship rituals in monogamous species

In this paper we propose an alternative explanation for the evolution of courtship rituals in monogamous species. We demonstrate, using computer simulations, how male courtship might develop as males exploit response biases in females to manipulate the female into starting reproduction before she has been able to assess the male’s intentions. In our coevolutionary simulations, a recurrent, artificial neural network is used to model the female recognition mechanism, while the displaying male is represented by a sequence of signals. Our particular model situation is just one example of how a reproductive conflict could result in the evolution of ritualized displays in monogamous species. Since reproductive conflicts occur even after pair formations, the explanation we propose may also apply to rituals that occur after pair formation. Key words: artificial neural networks, courtship, ritualization, coyness, manipulation, mate choice, monogamy, reproductive conflict, receiver bias, sexual selection. [Behav Ecol 11:405–410 (2000)] A variety of signals and behaviors are used in interactions with the opposite sex. In monogamous species these rituals may occur over an extended period of time. Many of these interactions are identified as courtship and are today often regarded as means of assessing partner quality (Andersson, 1994; Trivers, 1972). For instance, partner quality may be revealed because high-quality males can afford to use more elaborate displays or because a direct relationship exists between quality and performance (see, e.g., Andersson, 1994). Older ethological explanations of courtship rituals have stressed the importance of stimulation and cooperation, rather than choice of partner. For instance, Huxley (1914: 516) proposed that these ritualized displays serve ‘‘to keep the two birds of a pair together, and to keep them constant to each other’’ (see also Armstrong, 1963). A similar idea is that courtship rituals provide necessary stimulation and that they coordinate the reproductive physiology of the male and the female (Bastock, 1967; Lehrman, 1959, 1964). The elaboration of displays was considered to reduce ambiguity (Cullen, 1966). In this paper we describe a new hypothesis for the evolution of rituals within a monogamous pair. Here the courtship display is not informative to the female apart from indicating the presence of a male of her own species. Our hypothesis accords with classical ethology (theories of ritualization; see, e.g., EiblEibesfeldt, 1975) insofar as the emphasis is on stimulation, but it rests on reproductive conflict and manipulation rather than on cooperation. In monogamous species there are a variety of such reproductive conflicts, many concerning parental duties. To illustrate how this can lead to evolution of elaborated display and manipulations, we developed a model. Consider a female that has to find a partner in an environment of both faithful and philandering males. These males differ in the amount of parental care that they provide. Males are eager to court any female and start reproduction as soon as they meet a female. Females, on the other hand, might benefit by staying coy and evaluating the intention of a potential partner before actual reproduction, even if this is associated with some costs (Wachtmeister and Enquist, 1999). Our suggestion is that