Adult sex ratio as an index for male strategy

Danya Rose, Kristen Hawkes and Peter S. Kim


The adult sex ratio (ASR) is defined as the number of eligible males divided by the number of fertile females in a population. We build an ODE model with minimal age structure, in which males compete for paternities using either a multiple-mating or searching-then-guarding strategy, to investigate the value of ASR as an index for predicting which strategy males will adopt. Parameters in the model characterise aspects of life history and behaviour. Sensitivity analysis on the model parameters informs us that ASR is strongly influenced by parameters characterising life history, while dominant strategy is affected most strongly by the effectiveness of guarding (average length of time a guarded pair persists, and resistance to paternity theft) and moderately by some life history traits. For fixed effectiveness of guarding and other parameters, dominant strategy tends to change from multiple mating to guarding along a curve that aligns well with a contour of constant ASR, under variation of parameters such as longevity and age female fertility ends. This confirms the hypothesis that ASR may be a useful index for predicting the optimal male mating strategy, provided we have some limited information about ecology and behaviour.

Keywords: Population dynamics; Mating strategies; Sexual conflict; Mate guarding; Human evolution.

AMS Subject Classification: Primary 92-02; secondary 92D25, 92B05,.

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Wednesday, March 21, 2018