Moss code: unique vision helps reindeer find their favourite food

New research has suggested that diet may be the reason reindeer can see light in the ultraviolet spectrum

A reindeer faces towards the camera with pink and yellow tags on one ear. A green landscape appears out of focus in the background

New research from Dartmouth College and the University of St Andrews has outlined how the capacity of reindeer to see light in the ultraviolet spectrum helps them to distinguish lichen from snow.

Writing in i-Perception, scientists suggest that the UV sensitive vision of reindeer could help them to detect UV-absorbing lichens against a background of UV-reflecting snow.

The researchers studied reindeer – which are the only ruminant with a lichen-dominated diet – in Cairngorms National Park, Scotland.

C. rangiferina, or ‘reindeer moss,’ is an integral part of the reindeer diet. The authors observed that the off-white plant can be difficult for humans to distinguish from snow and granite.

There are more than 1500 species of lichen in the Cairngorms, but reindeer rely only on C. rangiferina during the winter months.

“A peculiar trait of reindeer is their reliance on this one type of lichen,” Nathaniel Dominy, of Dartmouth College, shared.

“It's unusual for an any animal to subsist so heavily on lichens, let alone such a large mammal,” he added.

Reindeer have a tapetum – a light-enhancing membrane that gives the eyes a shiny appearance – that changes colour between the seasons.

In summer, it is a golden colour that is typical in many animals, but in winter, it transitions to a vivid blue. The blue tapetum enables up to 60% of ultraviolet light to pass through.

Using spectral data from lichen and light filters calibrated to imitate reindeer vision the scientists found that C. rangiferina would appear to reindeer as dark patches against an otherwise bright landscape.

“Reindeer don't want to waste energy wandering around searching for food in a cold, barren environment. If they can see lichens from a distance, that gives them a big advantage, letting them conserve precious calories at a time when food is scarce,” Dominy said.