Colour Cues for Leaf Food Selection by Long-Tailed Macaques (Macaca fascicularis) with a New Suggestion for the Evolution of Trichromatic Colour Vision

Abstract

Leaf colour, size and toughness were investigated in five plant species important in the diet of Macaca fascicularis in Singapore. Leaf colour and size were examined as potential visual cues for food selection, whereas toughness mirrored fibre content, the inverse of food quality. As leaves matured, they changed colour and toughened. Leaf lightness and yellowness were strongly negatively correlated with toughness, but variation in both the red-green axis of the CIE Lab colour space and leaf size were not. Leaves selected as food by the macaques were distinguished by being very light, yellow to slightly green. Some leaves were dappled with red. The literature suggests that these leaves are relatively rich in protein without being tough and therefore would be sought after by primates. We argue that leaf colour is an important indicator of the nutritive value of leaves. Trichromatic vision is an important advantage in finding those palatable leaves that are dappled red. These would appear dark to dichromatic primates and be deceptive by making leaves look older (lower in quality) than they actually are. This would decrease the perceived window of feeding opportunity for such primates who would be at a disadvantage in trying to find these leaves. It is possible that trichromatic vision in catarrhine primates may have originally evolved for the detection of red coloration in the leaves of shade-tolerant tropical plants, enabling the better exploitation of a food resource.

Detection of Fruit and the Selection of Primate Visual Pigments for Color Vision

“…Of particular interest is how diet and lifestyle affect selection at this locus. For instance, selection may be strongest during food shortages when primates resort to "fallback" foods, with those that rely on leaves at these times benefiting most from trichromacy (Lucas et al 1998(Lucas et al , 2003Dominy and Lucas 2001;Dominy et al 2003). Conversely, it would be interesting to find a natural situation where dichromat monkeys or humans enjoy any advantage over trichromats.…”

Demonstration of a foraging advantage for trichromatic marmosets ( Callithrix geoffroyi ) dependent on food colour

“…However, these investigations have been limited to measurements of absorption properties of photopigments and/or spectral re£ectance of natural objects such as plant materials (e.g. Nagle & Osorio 1993;Osorio & Vorobyev 1996;Regan et al 1996Regan et al , 1998Lucas et al 1998). Here we report the ¢rst data, to our knowledge, to show that marmosets foraging under semi-natural conditions di¡er in their ability to locate a food source depending on the colour of that food and the photopigment genotype of the individual.…”

Uniformity of colour vision in Old World monkeys

“…This idea has garnered support, both from observations of the spectral properties of tropical fruits harvested by monkeys and from recent modelling studies that demonstrate the spectral positioning of the M and L cones in trichromatic non-human primates to be near optimal for the detection and identi¢cation of such fruits (Osorio & Vorobyev 1996;Regan et al 1998). A second version of the feeding hypothesis has a di¡erent focus, suggesting that many monkeys eat leaves as a signi¢cant part of their diet and in so doing preferentially select edible young leaves that can best be detected by those colour cues available to the trichromatic viewer (Lucas et al 1998). …”