What about male quality? A model to predict male investment in a single female

Monogyny—where a male invests all his reproductive effort in a single female—is a rare and poorly understood mating system. It contradicts conventional sex roles, in which males typically seek multiple mates while females may mate repeatedly. Previous theoretical work has shown that monogyny is most likely to evolve under male-biased sex ratios, as often occurs in species with extreme female-biased sexual size dimorphism (eSSD). While previous models mainly considered the species level, potential individual strategies have received little attention. Here, we used a game theory approach, considering the interplay between male and female decisions, to address how variation in male quality (e.g., size or body condition) affects the payoff of a monogynous strategy. The model is based on the biology of widow spiders, where males exhibit extreme mating investment including prolonged courtship, mate guarding, genital damage, and even self-sacrifice, but can be relevant to other species with eSSD. The results of the model confirm previous predictions and point to a novel, testable, and somewhat surprising prediction: unless females are extremely rare relative to males, and/or male survival prospects are very slim, the highest quality males would avoid extreme investment and rather set out to look for additional females to mate with. This is because high-quality males face a lower risk that the female will not use their sperm, and they are also likely to be accepted by subsequent (even already mated) females. Future studies could address this hypothesis experimentally by comparing the tendency of males of varying qualities to invest in a current mate under different demographic and ecological conditions.