We studied the simultaneous effects of social and environmental factors on the age of sexual maturity to identify the major determinants of reproductive delay in the cooperatively breeding female Cape ground squirrel (Xerus inauris). We verified inhibition of sexual maturity by monitoring reproductive behaviors and hormones (using a noninvasive fecal steroid hormone analysis) in prereproductive and reproductive females from 2 populations of squirrels found in southern Africa. We found that 2 social parameters hold the most influence on the timing of female sexual maturity; specifically, the number of adult breeding female group mates and related adult male group mates were associated with a substantial inhibition of female reproductive maturity. Subadult females in social groups with 0 or 1 adult breeding female became sexually mature significantly earlier than similar-aged females in social groups with ≥2 adult breeding females. Likewise, subadult females in social groups with <2 related adult male group mates became sexually mature significantly earlier than similar-aged females in social groups with ≥2 adult related males. Our results show the influence of multiple social parameters on sexual maturity and are consistent with the predictions of the incomplete control model of skew.