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Flowers and humans – a curious love affair

Humans love flowers! We admire their varied colors and shapes, enjoy the way they smell, and (especially on a day like today) give them to those we love.

But why has this affection for flowers evolved in us – given that flowers have certainly not evolved to impress us? In fact, we gain very little benefit (apart from joy) from them.

It is true that some flowers are edible, and that flowers may indicate where an edible part of a plant can be found, or help us distinguish an edible from a toxic plant – but these “uses” of flowers can hardly explain their universal appeal to our species.

One clue is that flowers stimulate the same sensory apparatus that humans use for assessing the quality of fruits. Fruits often have colors similar to flowers, and one theory suggests that trichromatic color vision in primates has evolved to better detect and evaluate edible fruits. From an olfactory perspective, floral volatiles are chemically similar or even identical to those emitted by fruits, and thus smelling a flower may possibly bring to mind a ripe, sweet fruit.

But the allure of floral fragrances is also culturally embedded into human society, as the perfumes we daub or spray on our bodies are often derived from flowers. Indeed, the search for new fragrance compounds from flowers is carried out by specialist perfumers as a form of bio-prospecting. Roman Kaiser, one of the most famous and dedicated of these perfumers, has undertaken a number of epic journeys in search of new fragrances. One of these field trips involved sampling the scent of flowers in the canopy of a rainforest while suspended from a hot air balloon!

On the other extreme, some flowers have a repulsive smell when they imitate decaying matter, dung, or carrion. The flowers deploy these unpleasant smells to attract animals such as carrion flies and dung beetles which unwittingly disperse the pollen as they search for places to lay their eggs. Such flowers elicit repulsion in humans because they stimulate sensory perception that evolved to help us avoid rotten or toxic material. Interestingly, new research shows that fruit flies share our human aversion to faecal odours and use special receptors around their mouth to detect and avoid animal faeces when searching for fruits on which to lay their eggs. The ultimate reason why humans and fruit flies stay clear of these odours is probably because decaying protein-rich organic material is often populated by harmful bacteria.

In general, flowers elicit sensory responses in us that have evolved in a different context, and that make us like (or, rarely, dislike) them. This biological concept is called “receiver bias”, and is applicable not only to humans enchanted by flowers but also to the pollinators which are the real targets of their alluring signals. The flowers of most plant species contain a food reward for their pollinators in order to encourage loyalty, but pollinators are also frequently tricked into believing that flowers are food sources when they are in fact completely unrewarding. This form of deception is particularly common among orchids. There are about 25,000 species in the orchid family and it has been estimated that around 40% of these species offer no rewards to their pollinators. Biologists have often puzzled over why plants would benefit from cheating rather than rewarding their pollinators. One suggestion is that the lack of rewards discourages pollinators from lingering for too long on any one plant, and thus promotes cross-pollination.

Ophrys speculum. Image provided by the authors and reused with their permission.
Ophrys speculum. Image provided by the authors and reused with their permission.

Flowers that don’t offer floral rewards, and even a few that do, often use various forms of mimicry to attract their pollinators. When asked to think of an example of mimicry in nature, most people will think of wing pattern mimicry in butterflies, a classical example of adaptation through natural selection that was discovered by the English naturalist Henry Walter Bates during his travels in South America in the mid-nineteenth century. Yet few are aware that there is also an abundance of examples of floral mimicry in plants. These elaborate natural hoaxes include flowers that attract their insect pollinators by posing as dead animals, or as receptive female wasps, or even as wounded bees.

In some of these cases, the flowers are pretending to offer food and thereby attract hungry pollinators. However, the greatest deception occurs when flowers emit certain signals, often chemical, that trick pollinators into believing that the flowers are mating partners, or sites to lay eggs. In some of these cases, hapless insects will even ejaculate or lay eggs on the flowers. The flowerheads of one African daisy species are even decorated with raised dark spots, giving sex-starved male insects the impression that the flowerheads are occupied by a whole bevy of resting female insects!

Floral mimicry exploits the powerful receiver biases of pollinators. It evolves because it increases the attractiveness of flowers to pollinators, thereby promoting the genes of the mimic through the additional seeds that are produced. Indeed, the tremendous diversity of flowers in plant families such as the orchids, arums, and milkweeds is in part due to the variety of different mimicry strategies that have evolved to deceive insect pollinators.

Nobel Prize winner Konrad Lorenz once said that it is the affection towards a living object that drives our scientific endeavor towards it. This is certainly true of our research on floral biology, so perhaps it’s fair to say that we were already biased towards flowers, and that it is due to this affection that some of their secrets have been uncovered.

Featured image: Daisy by AdrianaAlexaW. CC0 Public domain via Pixabay.

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