Cooperation by necessity: condition- and density-dependent reproductive tactics of female house mice

Optimal reproductive strategies evolve from the interplay between an individual's intrinsic state and extrinsic environment, both factors that are rarely fixed over its lifetime. Conditional breeding tactics might be one evolutionary trajectory allowing individuals to maximize fitness. We apply multi-state capture-mark-recapture analysis to a detailed 8-year data set of free-ranging house mice in a growing population to discern causes and fitness consequences of two alternative reproductive tactics in females, communal and solitary breeding. This allows us to integrate natural variation in life-history traits when analysing the expression of two alternative reproductive tactics in females. We find that communal breeding reduces average population fitness, but nevertheless increases over our 8-year study period. The tactic proves to be expressed conditionally dependent on both population density and female body mass - allowing females to breed under subpar conditions, i.e. at high density or when of low body mass. Our results contradict previous laboratory studies and emphasize the importance of studying cooperation under natural conditions, including natural variation in state-dependent survival and breeding probabilities. An 8-year study of house mice revealed an increase in communal breeding despite reduced average population fitness. Breeding tactics were condition- and density- dependent, with only the most competitive females rearing litters solitarily.

Automated summary

Optimal reproductive strategies are influenced by both intrinsic state and extrinsic environment, and conditional breeding tactics can maximize fitness. Through an 8-year study of house mice, it was found that communal breeding reduced average population fitness, but showed an increase over time. The expression of this tactic was conditionally dependent on population density and female body mass.