Abstract
Mothers that experience different individual or environmental conditions may produce different proportions of male to female offspring. The Trivers‐Willard hypothesis, for instance, suggests that mothers with different qualities (size, health, etc.) will use different sex ratios if maternal quality differentially affects sex‐specific reproductive success. Condition‐dependent, or facultative, sex ratio strategies like these allow multiple sex ratios to coexist within a population. They also create complex population structure due to the presence of multiple maternal conditions. As a result, modeling facultative sex ratio evolution requires not only sex ratio strategies with multiple components, but also two‐sex population models with explicit stage structure. To this end, we combine nonlinear, frequency‐dependent matrix models and multidimensional adaptive dynamics to create a new framework for studying sex ratio evolution. We illustrate the applications of this framework with two case studies where the sex ratios depend one of two possible maternal conditions (age or quality). In these cases, we identify evolutionarily singular sex ratio strategies, find instances where one maternal condition produces exclusively male or female offspring, and show that sex ratio biases depend on the relative reproductive value ratios for each sex.
Highlights
The primary sex ratio can be defined as the proportion of male births in an individual’s offspring production strategy (Trivers 1985)
We identify evolutionarily singular sex ratio strategies, find instances where one maternal condition produces exclusively male or female offspring, and show that sex ratio biases depend on the relative reproductive value ratios for each sex
Because multiple conditions create population structure, and the reproductive advantages of both sexes depends on demographic factors like survival, fecundity, and life span (Leimar 1996; Schwanz et al 2006), an explicitly demographic model is valuable for understanding facultative sex ratio evolution
Summary
The primary sex ratio can be defined as the proportion of male births in an individual’s offspring production strategy (Trivers 1985). Many species have facultative (conditiondependent) sex ratio strategies, where a parent adjusts the primary sex ratio of its offspring depending on some environmental or individual condition (Leimar 1996; West 2009). Our general model is introduced in the section “A two-sex matrix model with multiple maternal conditions’’ and further expanded upon in the “Model” sections of two case studies This framework combines three components that have never (to our knowledge) been simultaneously applied to the problem of facultative sex ratio evolution. In the second case (“Case 2: Maternal quality”), high- and low-quality mothers can evolve different sex ratios, as in the Trivers-Willard hypothesis
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