Abstracts (first author)
Operational sex ratios, Bateman gradients, competition, care, and then some nasty interference from the deep evolutionary past
Reviews of sexual selection often emphasize one of two concepts. Strong competition for mates is expected when the operational sex ratio (OSR) is biased towards the focal sex, or when the Bateman gradient is steep for the focal sex. Treatments rarely emphasize that not one but two conditions must be satisfied before one expects selection for a trait that enhances mate acquisition: mating multiply must be beneficial for fitness, and achieving this must be difficult without the trait. Because the former idea is captured by the Bateman gradient and the latter by the OSR, the two concepts are complementary, not competing or mutually exclusive. Combining their insights into a single metric can help understand sex roles, but we also need to know the origin of OSR biases. These biases are often related to the way parental duties are divided or shared. The evolution of care roles is an outcome of a mix of positive and negative feedbacks involving e.g. the population-wide adult sex ratio. Together with the fact that it is easier for a parent to provide further care if it is already associated with its young (e.g., a parent that must physically be present at birth is a more likely caregiver later too), this implies that care evolution can feature strong phylogenetic inertia and multiple stable states. This makes care evolution challenging to analyze, but also offers new avenues for understanding e.g. male-biased care in fishes.