The coevolution of mammae number and litter size
Mammals are unique in provisioning their offspring with milk, lactiferous nourishment produced in glandular organs called mammae. Mammae number is hypothesized to coevolve with litter size, acting as a constraint on offspring survival. However, predicted canonical relations between mammae number and litter size (i.e., the one-half and identity rules) are untested across Mammalia. Here we analyze data for 2,301 species and show how these characters coevolve. In Mammalia, mammae number approximates the maximum reported litter size of a species, and mammae number explains more variation in litter size than other species-level traits (mass, gestation length, diet, and seasonality of contemporary geographic distribution). Clades show differences in these patterns, indicating that certain life history strategies might break the rules of mammary evolution. Mammae number is an underappreciated constraint on fecundity that has influenced the radiation of mammals.