Primates of the family Callitrichidae. From left to right, Callithrix, Leontopithecus, Saguinus, Cebuella, Mico, Callimico.
A Night monkey.
Based off of the photos above, it is clear that there is only a slight, if not any, difference in the appearance of the male and female titi monkeys. This shows that sometimes sexual dimorphism is not as effective on species which practice monogamy.