Differences in Visual Perception Between Monkeys and Humans Highlighted by Experts

In the realm of New World monkeys (Platyrrhini), there is no universal rule that male monkeys generally have better colour vision than females. Generally, New World monkeys are trichromats, which means they possess three types of colour receptors and have a good ability to distinguish colours.

However, genetic differences between individuals can influence colour vision. For instance, X-chromosomal trichromacy is common, where male monkeys with an X-chromosome may either have normal or weak colour vision, depending on their genetic makeup.

Recent research by Smith suggests that dichromats, who lack the ability to perceive reds and greens, may have an advantage in terms of seeing shapes and forms due to less focus on colours. This is particularly intriguing as among tamarins and spider monkeys, all males are dichromats.

The favourite food of the tamarins is the Abuta fluminum plant. In a fascinating twist, ripe Abuta is orange, but this colour is hard to detect without red-green perception. Smith's preliminary results suggest that trichromat tamarins were 50% more adept at choosing the ripe fruit than their dichromat counterparts.

To test this theory, an experiment was devised in Britain to give tamarins an eye-test, simulating the forest canopy on a wire scaffold decked with large green paper leaves. The small cardboard boxes in the enclosure contained chunks of fudge disguised as ripe and unripe Abuta fruit.

The research also highlights the broad range of vision types in New World monkeys. While every howler monkey is trichromatic, the owl monkey is monochromatic, seeing only in black and white. Some monkeys within the same species can have different forms of sight.

A mystery for the scientists is why trichromacy hasn't replaced dichromacy entirely in the evolution of primates. Nathaniel Dominy, a primatologist at the University of Chicago, stated that Smith's findings are strong and show an advantage to trichromacy in New World monkeys. Gerald Jacobs, an expert on mammalian color vision, concurred, stating that trichromats have an advantage and dichromats have a visual deficit.

In 2001, Dominy published a study on Old World apes and monkeys in Uganda, finding that trichromatic color vision was essential for choosing tender young red leaves. The research on monkey vision was conducted in the Peruvian Amazon and a lab in Scotland.

The study involved guiding saddleback and red-bellied tamarins into the enclosure to forage for the "ripe fruit." The study also involved following tamarins as they jumped from tree to tree in the canopy to measure the colour of the fruit and leaves they feed on. Among females of tamarins and spider monkeys, 60% have three-color vision, while 40% have two-color vision.

The research suggests that various forms of sight may confer survival advantages, adding a new layer to our understanding of primate evolution and behaviour.