The Evolutionary History of Primate Litter Size

Litter size plays an important role in the life history strategies of all mammalian taxa. It is one of the most important factors determining whether an organism is deemed to have a 'slow' or 'fast' life history strategy. Investigating how the evolution of litter size has influenced human evolution, extant primate biodiversity, and how it relates to other life history traits is crucial to understanding ourselves and our closest relatives. This thesis summarizes a two-pronged investigation into the evolution of litter size: 1) I performed a meta-analysis using 955 taxa within the magnorder Boreoeutheria, and 2) I performed a geometric morphometric pilot study on twinning and singleton-bearing American monkey (Platyrrhini) crania. My findings include negative correlations between litter size and gestation length, age at maturation, and maximum potential lifespan. Phylogenetic signal tests support that selection has played a role in the distribution of mean litter size across this magnorder, as well as in the distribution of other reproductive characters including gestation length and percentage of brain growth accomplished prenatally. Additionally, I present reconstructions of life history and body size for various ancestral organisms within this magnorder. My results support that twinning was common in boreoeutherian evolution and may have been the ancestral primate condition. To better evaluate these results, I compare the reconstructed values for the ancestral boreoeutherian to recent morphology-informed research on fossil mammals including Vincelestes neuguenianus (Theria). Finally, I present the results of the pilot study, discuss how these relate to my current hypothesis regarding litter size and morphological covariation, and discuss how I intend to expand this dataset.