In polygynous mating systems, males compete intensely for mates and may mate several females during a single reproductive season. Accordingly, factors influencing the ability of males to control a larger number of females during the breeding season can provide information on the processes underlying sexual selection. In ungulates, age, body mass, and social rank are considered good predictors of the reproductive success of males, but how male age structure and sex ratio in the population influence mating group (MG) dynamics has received little empirical testing. Between 1996 and 2005, researchers manipulated male age- and sex structure and monitored MG dynamics in a reindeer (Rangifer tarandus) population.⁠ ⁠ _____⁠ ⁠ They investigated the influence of male characteristics, percentage of males, and male age structure on MG size and stability. They found that males with higher social rank (that were also older and heavier) controlled larger MGs (therefore had greater mating opportunitie...