A defining feature of catarrhine primates is uniform trichromacy – the ability to distinguish red (long; L), green (medium; M), and blue (short; S) wavelengths of light. While the tuning of photoreceptors is conserved, the ratio of L:M cones in the retina is variable within and between species, with human cone ratios differing from other catarrhines. Yet, the sources and structure of variation in cone ratios are poorly understood, precluding a broader understanding of color vision variability. Here, we report a large-scale study of a pedigreed population of macaques. We collected foveal RNA and analysed opsin gene expression using cDNA. We estimated the additive genetic variance of cone ratios. The average L:M ratio and standard error was 1.03:1± 0.02. There was no age effect, and genetic contribution to variation was negligible. We found marginal sex effects with females having larger ratios than males. S cone ratios (0.143: 1± 0.002) had significant genetic variance with a heritability estimate of 43% but did not differ between sexes or age groups. Our results contextualize the derived human condition of L-cone dominance and provide new information about the heritability of cone ratios and variation in primate color vision.

Cone ratio expression was determined through the use of droplet digital PCR, which is an absolute gene expression method. This provided the phenotypic information for color vision effects. 

Pedigree information was acquired from the Caribbean Primate Research Center, which has been managing the population of the Cayo Santiago Rhesus macaques. The pedigree confirms sex, age, and parental information of all macaques involved in the study.

All analyses were done with R and R studio using the following packages: tidyverse (Wickham et al. 2019), reshape2 (Wickham 2020), lans2r (Polerecky et al. 2012), lubridate (Grolemund and Wickham 2011) and ggplot2 (Wilkinson 2011).

Pedigree information and individual identifiers of the monkeys in the study have been changed to anonymous names. All data were analyzed in R, and the code provided is set up for R script running.