Elsevier

Animal Behaviour

Volume 114, April 2016, Pages 249-260
Animal Behaviour

Opportunity costs resulting from scramble competition within the choosy sex severely impair mate choosiness

https://doi.org/10.1016/j.anbehav.2016.02.019Get rights and content

Highlights

  • •

    Scramble competition for access to a sexual partner involves opportunity costs.

  • •

    Its consequences for best-of-n and threshold decision rules were modelled.

  • •

    The evolutionarily stable strategies were calculated.

  • •

    Even when there are more partners than competitors, the choosiness remains low.

  • •

    Male-biased sex ratio is not sufficient for the evolution of female choosiness.

Studies on mate choice mainly focus on the evolution of signals that would maximize the probability of finding a good-quality partner. Most models of sexual selection rely on the implicit assumption that individuals can freely compare and spot the best mates in a heterogeneous population. Comparatively few studies have investigated the consequences of the mate-sampling process. Several sampling strategies have been studied from theoretical or experimental perspectives. They belong to two families of decision rules: best-of-n strategies (individuals sample n partners before choosing the best one within this pool) or threshold strategies (individuals sequentially sample the available partners and choose the first one whose quality exceeds a threshold criterion). Almost all models studying these strategies neglect the effect of scramble competition. If each paired individual is removed from the population of available partners, the distribution of partner quality dynamically changes as a function of the strategies of the other competitors. By means of simple simulations assuming opportunity costs, to the exclusion of all other costs, we show that scramble competition is a sufficient constraint to severely impair the evolution of choosy decision rules. In most cases, the evolutionarily stable strategy is to have a very low acceptance threshold or to sample two individuals at most in the population. This result may explain some discrepancies between predictions from previous models and their experimental validations. It also emphasizes the importance of considering the pairing process in studies of sexual selection.

Section snippets

The model

Our goal was to assess the ESS in a self-consistent game-theoretic model (Houston and McNamara, 2002, Kokko and Wong, 2007, McNamara, 2013). While most previous studies have assumed a constant distribution of partner quality, we modelled the effect of scramble competition not as a constant cost, but as an emerging property of the other females' strategies. We assumed a simple situation in which females search for sedentary males. The intensity of competition arises directly from the sex ratio.

Results

The sex ratio had a similar effect on the ESS for both the threshold (Fig. 2a) and best-of-n (Fig. 2b) decision rules. When there were fewer males than females, neither of the strategies could outperform random choice. For the best-of-n decision rule, the ESS was best-of-1, i.e. to always accept the first encountered male, which is identical to random choice strategy (Fig. 2b). The calculation of the ESS for the threshold decision rule led to analogous conclusions. The strategies converged to

Discussion

Opportunity costs arising from scramble competition represent a sufficiently strong constraint to severely reduce and, in many cases, almost suppress female choosiness. Importantly, this result holds even when there are more available males (chosen sex) than females (choosy limited sex). More generally, non-negligible opportunity costs are a crucial component of decision making under scramble competition, and they should be observed in all contests for possession, consumption or use of any

Acknowledgments

We thank John McNamara, Tim Fawcett, François Rousset, Alexandre Courtiol, Matthias Galipaud and two anonymous referees for helpful comments about the model or manuscript. Financial support was provided by the Agence Nationale pour la Recherche (program Monogamix ANR-08-BLAN-0214-02) and the Institut Universitaire de France. Calculations were performed using HPC resources from DSI-CCUB (Université de Bourgogne).

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