It was a warm and humid night in September of 2003. In a tropical forest by the coast of Madagascar, Phil Devries, an entomologist and noted nature videographer, swatted mosquitoes hovering around his face. He had been waiting eagerly for a visitor since seven o’clock. As the night transitioned to early morning, without any signs of the visitor, the tension and anxiety in Phil’s mind increased. For the visitor, Phil Devries was inconsequential; the desired object was Darwin’s orchid near which Phil (or the Butterfly Man, as he is popularly known) had parked himself to photograph the orchid’s pollinator.
“Good Heavens what insect can suck it,” Charles Darwin is said to have remarked in reference to the nectar in the long floral tube of Angraecum sesquipedale, now known as the Darwin’s orchid, native of Madagascar.1 Darwin had received the orchid on January 25, 1862, from James Bateman, a businessman, collector of plants, and horticulturist, who grew orchids. Darwin then famously predicted that A. sesquipedale must be pollinated by a hawkmoth with a proboscis that measured at least eleven inches in length.2
In 1903, almost forty years after Darwin intuited its existence, a hawkmoth with long mouth parts was described by Walter Rothschild and Karl Jordan. It was isolated from moth specimens collected on an earlier expedition to Madagascar by Jules Paul Mabille, a French naturalist. Rothschild and Jordan named the species Xanthopan morganii. However, it was not until 1992, a good ninety years later, that Lutz Wasserthal, a German biologist, observed X. morganii visiting the flowers of A. sesquipedale in real life. Only then was the connection between orchid flowers and moths finally confirmed.3
Visits of moths to flowers in the wild are hard to observe. And so, Wasserthal had to use large flight tents to photograph the two partners engaged in the mutually beneficial relationship. Finally, in 2003, after spending several nights in the Madagascar forest, Phil Devries was able to photograph the evasive moths visiting the flowers of A. sesquipedale in the wild—at around three o’clock in the morning.4
The correlation between the length of the floral tube and the length of moth’s proboscis led Darwin to infer the process of coevolution, in which natural selection favors reciprocal increases in the length of the floral tube and moth’s proboscis. Heritable variation—in this case, variation in floral tube and the length of proboscis in moths—is the raw material on which natural selection acts. Between Darwin’s original prediction and the eye-witness observation, 130 years had passed. Nothing in science comes easy. Not even for Darwin.
It was Gregor Mendel, an Austrian monk, who proposed the principles of inheritance in 1865, based on his experiments with peas. From Darwin’s orchids to Mendel’s peas, plants have played an important role in the study of evolution. Curiously and coincidentally, both Darwin and Mendel were contemporaries, and although Mendel’s work filled a critical gap in Darwin’s theory of evolution by natural selection, the two men did not know of each other’s work!
While Darwin is noted for his work on evolution, he is much less known as an ardent botanist. He was greatly interested in the reproduction of plants, particularly orchids. He wrote several books on plants: The Power of Movement in Plants, On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects, On the Good Effects of Intercrossing, The Different Forms of Flowers on Plants of the Same Species, and Insectivorous Plants. Plants were critical to the formulation of his ideas both about inherent variation and how natural selection acts on this variation to enable evolution.
Moths and Sex Pheromones
It is March 1974, and I am waiting, at evening time, under a large Luehea speciosa. The tree stands in a dry tropical forest in Guanacaste Province, Costa Rica. The previous day, I had seen its large white flowers start to bloom around eight o’clock in the evening. And so, the next day, under the tree and in the light of the moon, I staked a vantage point and started my watch. At exactly a quarter to eight, and almost like magic, the large white petals start to unfurl. In a quarter of an hour, almost a hundred flowers in my field of view have opened in near-perfect synchronicity. In my five decades of fieldwork in Costa Rica, that night was one of the most memorable and remains permanently etched in my memory.
Plants depend on a wide variety of animals to get cross-pollinated. The diversity of these pollination systems is on full display in tropical evergreen forests, the world’s most species-rich ecological communities. On any given day, at any time during a short walk through the forest, one can encounter flowers of many sizes, shapes, and colors that are pollinated by insects—largely bees, butterflies, and flies—and, at times, birds. For a different set of plant species that start to open their flowers around dusk and at night, insects (beetles and moths) and mammals (primarily bats) take over the role of major pollinators.
All across the globe, but mostly in the tropics, tens of thousands of plant species are pollinated by an equally large number of moth species at night. Moth-pollinated flowers are almost always white and tubular, with nectar at the base of the tube. They blossom in the evening, soon after dusk, and the blooms last for one or two nights. During this time, the moths visit them frequently, making multiple forays throughout the hours of the night.
Insect pollinators visit flowers for food, but, to them, flowers are more than a food source. They are also sites of mating and, often, a source of compounds that play an important role in facilitating these sexual encounters. Flowers produce a variety of volatile compounds to attract insects, such as moths. Smell plays an important role in attracting insects from afar, especially at night, when visual cues can only function once the pollinators approach the flower closely.
Female moths use volatile compounds produced by flowers to synthesize sex pheromones, which they release to attract males. In some cases, the volatiles associated with the floral smell simply induce female moths to produce large amounts of sex pheromones, but in others, the female moths can absorb or ingest the volatiles and convert the compounds directly into pheromones. The males are not left behind. In some species of moths, males sequester pyrrolizidine alkaloids from flowers to use them as precursors for the synthesis of pheromones. Sometimes, the males even transfer the alkaloids to the female during mating, for the defense of eggs against predators.5 Thus, flowers play a critical role not only in the provision of food and nutrition but also in the mating and reproduction of pollinators.
Evening Fragrances and Romantic Nights
Thirty years later, I am in Bangalore, the techno-hub of South India. It is again late evening, and I am passing through a small market buzzing with people. Walking in front of vegetable and food stores, I am overpowered with fragrances emanating from buds and flowers of jasmine (Jasminum) strung together for hair adornments. And indeed, I see many women walking around with their long hair arranged in many different styles and adorned with strings of fragrant jasmine.
Throughout remembered history, and for millennia, flowers have been a part of daily life in India, as adornments for gods and humans. The Hindu epic Raˉmaˉyana about the life of Ram, one of the most celebrated gods of Hindus, includes references to Sita, Ram’s wife, decorating her hair with floral arrangements. And in a well-known epic poem written in the fourth century CE, the playwright Kaˉlidaˉsa included a verse in which sensuality and pollination merge:
in summer love
from fragile petals
while wild bees
kiss them gently6
Anthologies of classical Tamil, written between 100 BCE and 250 CE, describe the flowers that women bear as those of jasmine. For men, too, flowers have been a bedtime adornment for ages, and the exchange of flowers between individuals has always carried unspoken and covert sexual connotations.7
From trees in Costa Rica that use flowers to attract moths to women in India who use flowers for adornment, the fundamental motives of life are the same irrespective of geographies, gender, or species. But the enchantment of union does not stop there. The collision of these seemingly different worlds gets closer and more intimate. Jasminium sumbac and other species of Jasminum are native to South India and other parts of tropical Asia. Jasmine flowers are highly fragrant, pollinated by moths, and here, too, the maximum production of aromatic compounds is between seven and eight o’clock in the evening!8
Moon and Sex
Back in Costa Rica and on another moonlit night, I am driving to my campsite after a full day of fieldwork in the dry deciduous forest. There is little traffic on the Pan-American Highway, which means that I can easily observe the star-studded trees of Bombacopsis quinata, a relative of the silk cotton tree, on both sides of the road. Under the full moon, it is a beautiful sight, with a tree coming into view every few minutes. The “stars,” indeed, are large, white, moth-pollinated flowers, perched high in the leafless crowns of these very large trees. For the past several evenings, I have been passing by these trees in flower, but this time, the number of flowers on the trees appears to be unusually large. Flowers in this species last for a single night, but individual trees flower over many weeks, with a new batch opening every night. It seemed that the intensity of flowering was associated with lunar cycles, with the largest number of flowers opening on nights with the full moon.
While, on this evening drive, I cannot confirm the correlation between the intensity of flowering and phases of the moon, researchers would later document such trends for other species. Moths are known to be more active on moonlit nights, and pollination can be more intense during a full moon for moth-pollinated species, as, for example, in Ephedra foeminea, a gymnosperm. In contrast to most gymnosperms, which are wind-pollinated, this species attracts moths by secreting a pollination drop from its cones. Individual plants produce their maximum amount of pollination drops during full moons. Meanwhile, a related species of Ephedra is wind-pollinated, and in that case, there is no connection between pollination and lunar cycles.9
Is there a general correlation between lunar cycles and pollination intensity for the thousands of night-blooming plant species? We do not know. Recently, researchers have shown that a desert cactus (Cereus peruvianus), presumably pollinated by bats, puts on its largest display of flowers around the full moon. The species flowers over a few months with the number of flowers going up and down with the lunar cycles.10
The moon has always been associated with romance in our own human cultures. Surprisingly, there is insufficient data to establish a link between sexual activity with lunar cycles. Interestingly, though, research has shown that a larger proportion of females demonstrate ovulation during the full moon, and all genders experience higher aggression levels and less sleep.11
Intertwined in the Web of Life
It is evening again, and the sex lives of plants, moths, and humans intertwine. All of these organisms use the same compounds to attract mates: smell is a main stimulant for each. Plants, indeed, cannot smell, yet floral volatiles are a major incentive for moths to visit flowers.
Among the three partners, plants reign supreme. They seem to dictate the terms of the relationships. Moths, in fact, are held in bondage. They cannot attract mates without pheromones for which the plants hold the precursors. Humans also seem to be dependent on plants as intermediaries, although they, of course, can do without them.
For those who study life on earth, the interconnections among plants, moths, and humans are not surprising. We are a part of the web of life that has celestial connections with other planets. These connections are vital for maintaining all lives, especially ours. We should celebrate and value these connections that enrich our lives by ceasing our assault on nature.
1 Arditti, J., Elliott, J., Kitching I. J., and Wasserthal, L. T. 2012. “Good Heavens what insect can suck it”—Charles Darwin, Angraecum sesquipedale and Xanthopan morganii praedicta. Botanical Journal of the Linnean Society, 169: 403–432. https://doi.org/10.1111/j.1095-8339.2012.01250.x
2 Netz, C. and Renner, S. S. 2017. Long-spurred Angraecum orchids and long-tongued sphingid moths on Madagascar: A time frame for Darwin’s predicted Xanthopan/Angraecum coevolution. Biological Journal of the Linnean Society, 122(2): 469–478. https://doi.org/10.1093/biolinnean/blx086
3 Wasserthal, L. T. 1997. The pollinators of the Malagasy star orchids Angraecum sesquipedale, A. sororium and A. compactum and the evolution of extremely long spurs by pollinator shift. Botanica Acta, 110(5): 343–359. https://doi.org/10.1111/j.1438-8677.1997.tb00650.x
4 See video in: Tartaglia, E. 2015. Year of the Sphingidae—Co-evolution. National Moth Week. https://nationalmothweek.org/2015/07/17/year-of-the-sphingidae-co-evolution/
5 Stökl, J. and Steiger, S. 2017. Evolutionary origin of insect pheromones. Current Opinion in Insect Science, 24: 36–42. https://doi.org/10.1016/j.cois.2017.09.004
6 Miller, B. S. 1984. Theater of memory: The plays of Kaˉlidaˉsa. New York: Columbia University Press.
7 Goody, J. 1993. The culture of flowers. Cambridge: Cambridge University Press, p. 323–324.
8 Braun, N. A. and Sim, S. 2012. Jasminum sambac flower absolutes from India and China—Geographic variations. Natural Product Communications, 7(5): 645–650. https://doi.org/10.1177/1934578×1200700526
9 Rydin, C. and Bolinder, K. 2015. Moonlight pollination in the gymnosperm Ephedra (Gnetales). Biology Letters, 11(4): 10–13. https://doi.org/10.1098/rsbl.2014.0993
10 Ben-Attia, M., Reinberg, A., Smolensky, M. H., Gadacha, W., Khedaier, A., Sani, M.,… Boughamni, N. G. 2016. Blooming rhythms of cactus Cereus peruvianus with nocturnal peak at full moon during seasons of prolonged daytime photoperiod. Chronobiology International, 33(4): 419–430. https://doi.org/10.3109/07420528.2016.1157082
11 Moore, B. 2019. The effect of the lunar cycle on the female reproductive system. South Carolina Junior Academy of Science. https://scholarexchange.furman.edu/scjas/2019/all/242/
I thank my wife, Tshering Bawa, for encouraging me to write this manuscript when I first discussed the idea with her almost twenty-five years ago. A series of discussions with Rohini Nilekani about Brahma Kamal (Epiphyllum oxypetalum), a nocturnal blooming cactus from Mexico and South America, but widely naturalized in Asia, was another source of inspiration. Meena Narayanswamy suggested several improvements in the manuscript.
Kamaljit S. Bawa is president of the Ashoka Trust for Research in Ecology and the Environment (ATREE), Bengaluru, India, and distinguished professor emeritus at the University of Massachusetts, Boston.
Citation: Bawa, K.S. 2021. The intertwined attractions of plants, moths, and people. Arnoldia, 78(5-6): 62–67.