THE SCENT OF SEX — but not for you

Diarmuid Breatnach

You sniff the scent of blooms in the air and it pleases you. But that’s not what it’s for – the scent was not manufactured to please you. Scents derived from plants are used by humans to add to their sexual allure and plants also have a sexual function in producing them — but it is not humans the plants are trying to attract.

The scents are there to attract pollinators, insects, mammals or birds that will go to them to collect pollen and/ or nectar, in the course of which they will spread pollen from plant to plant and bring about fertilisation.

There are huge numbers of different chemicals producing scents and combinations of scents and, as the plants have developed them, their pollinators have also developed the ability to distinguish between them and to home in on the appropriate ones. It is likely that inside the pollinators some kind of receptor is created which is keyed into the specific scent, in much the same way as sexual pheromones are keyed into receptors inside so many species, including humans.

Irish Hawthorn (Sceach Gheal) bush or shrub in bloom. Its blooms produce a scent more pleasant to humans at a distance than very close up. (Source photo: AP.Pfeiffer)

Irish Gorse (or Furze) (Aiteann) in bloom. The scent often reminds humans of coconut.
(Source: Environmental Protection Agency)

These pollinators then, quite unintentionally, dive into another bloom somewhere and like some kind of romantic couriers, deliver the pollen to the eager recipient. But unromantic really, to receive fertilisation from an unknown lover, unseen and barely felt. Yet it works, as around 400,000 flowering plants testify (though not all flowers use scent — colour and form suffice for some). And it will continue to work, as long as there are pollinators around (which is turning out to be a problem with large-scale deaths of bees, by far the most active of pollinators). But the animal kingdom? With some exceptions, its members need the presence of both sexual actors or more and stimuli to engage in sexual activity that will lead to procreation: temperature, light, foods, natural odours, shape, touch, erotic imagining, poetry, music, clothing, colour …. and yes, applied plant and artificial fragrances.

Irish Honeysuckle (Féithleann) in bloom; a climbing plant, its flowers produce a scent pleasing to most humans. (Photo source: Internet)

The scent of flowers is composed of a variety of VOCs (volatile organic compounds), most plant scents being composed of many compounds and some of up to several hundred. Collecting, isolating and analysing these compounds present great challenges to scientists but even small insects know what they mean, like an olfactory equivalent of a neon sign: NECTAR AND POLLEN HERE!.

Some plants emit scent throughout daylight and some at specific times of day only. And some not until after the sun has set. At night of course the plants do not have much competition but which pollinators will be active at night? Moths are the big group here but there are others, such as some bats.

There’s a night-scented shrub near my home that made me want to look into this subject and write about it a little. This time of year at night it is producing a strong sweet scent (with, to my nose, just a touch of urine smell in there too). You won’t find moths flying around at this time of year in Ireland and it’s not attracting bats so I guessed that it’s a foreigner and, where it comes from, that this time of year would be perfect. The shrub in question is the “Sweet Box” (Sarcoccoca confusa) and I was not surprised to find that its “native heath” is in eastern and southeastern Asia and the Himalayas.

The Sweet Box ( *Sarcoccoca confusa*), non-native, emits its scent at night (Photo: D.Breatnach)

People like the scent but it’s kind of sad when one thinks more about it, what it’s trying to do, like posting a message of sexual availability and procreation-wish on to a dating page and …. no-one replies.

Blooms and other parts of plants also produce scents which do not appear to have any role in attracting pollinators and scientists believe that these are defensive scents to ward off herbivores. In this case the olfactory sign is saying: “HORRIBLE TASTE! REMEMBER?” Not only that but plants can increase the production of such scents as parts of them are being eaten, to the extent that their attacker feels obliged to desist¹. And some scientists also speculate that scents for pollinators developed first as scents to discourage herbivores.

Hemlock (Moing Mhear), a very poisonous native plant that looks like cow parsley. It has an aroma unpleasant to humans and probably to herbivores.

Given that some animal appropriators of plant products have learned to home in on the repellent scents of some plants this seems quite likely. Imagine a plant exuding a smell to ward off a herbivore and imagine an insect learning that where that particular smell is to be found, so is food such as nectar or pollen. The sign now says: HORRIBLE-TASTING TO HERBIVORES AND PARASITES but is accompanied by another sign that declares: FREE DELICIOUS FOOD FOR OTHERS! And the plants that most reach and “please” the accidental pollinators will naturally be more successful, be visited more often and spread their genes wider. Unless yet other animals learn to follow that scent in order to consume the plant, or lay their eggs on it for the larvae to feed on the plant. Cabbages and carrots for example, both of which have an odour we can identify, have eggs laid on them respectively by the cabbage butterfly and the carrot fly, which are then fed upon by the larvae as they hatch. And the butterfly and carrot fly find the plants by smell.

It is a difficult balance, to attract pollinators and yet repel herbivores and parasites and no doubt the balance is constantly being adjusted through the evolutionary processes of plants, herbivores, pollinators and parasites, in a kind of dance of love and death.

Cut red roses, often purchased from a florist — but do they have a scent?
(Source photo: Internet)

Those volatile compounds to be found in the scents of flowers almost certainly also make them difficult to keep fresh. The cut flowers business is a billion-euro one and growers have now produced blooms that last longer after cutting than they used to – sometimes for weeks. But scent?One of the most delightful scents to the human nose is that of the rose, about which songs have been sung and poems composed. Go into a florist, go to a bunch of roses and try and smell them — the chances are you won’t be able to.

The real thing, form, colour and scent — a pink rose.
(Source photo: Internet)

Shakespeare’s Juliet, who said that “a rose by any other name would smell as sweet”² might say today that “A rose without a scent is but half a rose”. When you do find a rose or another flower that is sweetly scented, breathe deep and enjoy the scent … but remember it’s not made for you. It’s for another flower, through the agency of a courier, a messenger.

It’s all about sex. And it’s a jungle out there.

End.