Developmental Canalization in Squirrel Monkey (Saimiri bolivienesis) Body Mass.

Animal growth is often described as self-correcting or canalized, with most vertebrate evidence coming from laboratory rodents, domesticated livestock, and human auxological research. Non-human primate models for developmental canalization are necessary given the long duration and multi-phasic nature of primate growth that limit comparisons with these taxa. We explored patterns of body size growth in captive female and male squirrel monkeys (Saimiri boliviensis) through calculation of age- and sex-specific coefficients of variation (CV). These are further decomposed with quantitative genetic models to estimate the heritability of and maternal effects on mass at each age. We also deployed a recently proposed method by Mitteroecker and Stansfield (2021) to estimate the strength of canalization at each age (θ). We found that canalization changed across ontogeny in squirrel monkeys, but the pattern was largely similar between the sexes. Strong canalization resulting in a reduction of phenotypic variance only appeared entering the juvenile period. Prior to this, growth was more variable. Size differences among neonates became greater during infancy (i.e., "negative canalization"). Quantitative genetic models showed significant additive genetic variation for body mass at all ages (h2, 0.27-0.74), but maternal identity effects were only prominent on neonate mass (m = 0.28). This reduction in maternal variance with age and increased canalization likely results from changes in the hormonal regulation of growth and attainment of nutritional independence late in the infant period.