When males matter: The circumstances when males generate a large eco-evolutionary dynamical signature

Background/Question/Methods

Sex structure is commonly omitted from population modelling in favour of mathematical tractability, with the assumption that female-only models accurately capture the dynamics of the total population. This assumption is probably violated in many mammal populations where sexes differ in demographic rates and morphology. Several scenarios exist where female population development can be affected by abundance of males in different ages or stages. The commonest is when male abundance limits female reproduction. This occurs either through a lack of mates so that some females cannot breed, or through a lack of high-quality mates so that some females have fewer or lower-quality offspring. Other scenarios are when male abundance influences female survival or condition, or the offspring sex ratio. Here we present a recently developed two-sex integral projection model (IPM) that incorporates sex differences in demographic rates and morphology, as well as male abundance effects. IPMs track the population-level distribution of quantitative characters like size or body weight (i.e. stage or condition). They project the number of individuals with different character values by using relationships between the character concerned and demographic traits (i.e. survival and reproduction) alongside character transition rates (i.e. development, offspring character). Fundamental quantities underpinning population ecology, evolutionary biology and life-history can be calculated from the model. Furthermore, input functions can be systematically altered to compare unperturbed model predictions with perturbed ones, thus simulating outcomes under changed conditions. Therefore, perturbation analyses of two-sex IPMs allow us to do two things. Firstly, study the circumstances where males matter for population dynamics by manipulating male survival or character development rates, paternal effects on offspring number or phenotype, or mating pair formation. Secondly, we can simulate eco-evolutionary responses to environmental change in species with sex differences in life-history and morphology and intersexual abundance effects. Here we illustrate both applications using results from two mammal populations: Columbian ground squirrels and African lions.

Results/Conclusions

Our model performed well at predicting key population features. For the ground squirrels we found that male abundance and mating patterns influence population growth rate. For lions we show that the population is likely to respond to environmental change with complex eco-evolutionary dynamics. In addition we illustrate how male abundance in lions can affect female infant survival because removal of adult males exacerbates infanticide. We conclude by highlighting the potential of two-sex IPMs as a new approach to address biological questions arising from intersexual effects.