Scientists have long had difficulty establishing the group size that best allows animals to live long lives and raise many surviving offspring. But now a research team that includes Anthropology researchers from Stony Brook University has used 14 years of demographic data on multiple groups of wild geladas (an Old World monkey species) to determine that mid-size group living is best for fitness, essentially optimizing survival and reproduction. Details of the findings are published in the Proceedings of the Royal Society B.