Tasty is in the eye of the beholder – How different plants market their fruit to consumers

Most modern mammals are dichromats – they have photoreceptors for two different wavelengths of light. Their experience of colour is probably similar to that of people with red-green colour blindness.

Some primates – notably humans – are trichromats. Most readers will be familiar with the trichromat colour-vision experience. Trichromatic colour probably evolved multiple times in primates that were active during the day and that ate lots of fruit and immature leaves. Research has suggested that monkeys with trichromatic vision are more effi cient foragers than monkeys with dichromatic vision.

Red fruit among green leaves will be conspicuous to trichromats but not dichromats. Plants that market to dichromatic mammals tend to produce green, yellow, or brown fruit. This explains why wild apples come in a range of colours – they originate from central Asia and were historically eaten by bears.

Why are some ancestral apples red? The answer is that red pigments in plants started out as sunscreen. They still serve this purpose in foliage and immature fruit – wild apple trees have red pigments in their leaves that show up in autumn. And modern red apple cultivars require sunlight to develop good colour.

Many birds have intricate coloration and ultraviolet markings that are invisible to humans. This helps birds tell other birds apart – especially useful when they are trying to find a mate.

The birds and the bugs

Most birds are tetrachromats. They have four types of photoreceptors and some species can see ultraviolet light. Birds also have other adaptations that increase the number of colours they can perceive. Many birds have intricate coloration and ultraviolet markings that are invisible to humans. This helps birds tell other birds apart – especially useful when they are trying to find a mate.

Birds are important seed dispersers. Birds tend to be attracted by small, fl eshy fruits that ripen to black or red. Red is highly visible to birds but black less so. Plants may increase the profi le of black fruit by displaying them alongside red accessory structures, such as stems or bracts, or red immature fruit or flowers.

Some black fruits develop a bloom or waxy layer during ripening. The waxy layer refl ects blue and ultraviolet light, which would make the fruit easier for birds, but not mammals, to spot.

While plants actively try to attract seeddispersing mammals and birds, insects are not welcome, as they consume fruit without spreading seeds and may prevent the normal development of seeds or fruit. The problem is that insects can see colour at least as well as – and often better than – most vertebrates. Like mammals, insects can also sniff out fruit. Ancestral insects were trichromats with receptors for ultraviolet, blue, and green light. Modern insects from different families can have two, three, four or fi ve types of photoreceptors, and all can see ultraviolet light. Some can even see infrared light.

Insects seem to use scent to track down fruiting plants, and sight to identify fruit at close range. Two of the insect families that contain predators of fruit and seeds – butterflies and moths, and beetles – have photoreceptors for red light. Some insects that are specialist fruit-feeders – such as Mediterranean and Oriental fruit flies – are attracted to the colours of ripe fruit, including red, black, blue and ultraviolet.

So, although plants target their colour marketing at an audience of potential seed dispersers, they can’t control who responds to their advertisements. But for every insect that predates there is another that pollinates, and many plants put a lot of effort into attracting pollinators. Flower colour is closely matched to pollinator preference, and flowers may even have ultraviolet guides on the petals to help insect pollinators find their way.

In the end, colour is a form of botanical body language – allowing plants to communicate with no talking needed.

References

Steyn, W.J. 2009. Prevalence and function of anthocyanins in fruits. In Gould K et al (eds). Anthocyanins. Springer. pp85–105.

Steyn, W.J. 2012. Physiology and functions of fruit pigments: an ecological and horticultural perspective. Horticultural Reviews 39:239–271.