Color vision variation, feeding behavior, and reproductive output in a group of wild Ateles belzebuth

Color vision among primates is remarkably diverse. Catarrhines and howler monkeys have independently acquired routine trichromacy through duplication of the middle-to-long wavelength-sensitive (M/LWS) opsin gene on the X chromosome. Most platyrrhines and some strepsirrhines, however, possess polymorphic color vision due to allelic variation at a single M/LWS locus; heterozygous females are trichromats, while males and homozygous females are dichromats. Four mechanisms – heterozygote (trichromat) advantage, frequency-dependent selection, mutual benefit of association, and niche divergence – have been proposed to explain the maintenance of the opsin polymorphism, yet the selective advantages of individual phenotypes remain unclear. I reviewed the diversity in platyrrhine color vision as well as the studies that have evaluated each hypothesis. Next, I tested two of these hypotheses in one social group of white-bellied spider monkeys (Ateles belzebuth) that has been monitored at Tiputini Biodiversity Station since 2005. Spider monkeys – a genus of large-bodied frugivorous platyrrhines – have two possible opsin alleles determined by a SNP in exon 5 of the M/LWS locus; in reconstituted pigments they have absorption spectra with peaks at 538 and 553 nm, respectively. Using fecal-extracted DNA, I sequenced exon 5 for 44 group members, identifying 14 male dichromats (seven P538, seven P553), 10 female dichromats (two P538, eight P553), and 20 female trichromats (P538/P553). Then, to investigate the possibility of heterozygote advantage, I compared the interbirth intervals and number of offspring produced for dichromat versus trichromat females. There were no differences in these measures of reproductive output, suggesting that the conditional advantages of trichromacy may not translate to higher fitness. To assess niche divergence, I examined the proportion of total feeding time spent by dichromat versus trichromat adult and subadult males and females on each of three major food types – fruits, new leaves, and flowers – plus all three substrates combined. Additionally, I tested for niche divergence within dichromats by comparing the foraging behavior of P538 versus P553 individuals. Analyses revealed no difference in foraging activity budgets between groups in either case. Future work may consider other aspects of fitness and finer-scale niche differentiation to better understand how multiple visual phenotypes have been maintained in this population.