Readers’ wildlife photos

Athayde Tonhasca Júnior is back with an informative photo-and-text story. Click the photos to enlarge them, and Athayde’s text is indented.

Not biting the hand that pollinates you

Scientific reports can be dull and impenetrable, but that’s not the case for the 1878 ethno-botanical account from Madagascar by a Dr Carle Liche. The explorer spoke of a ceremony performed by Malagasy tribesman where a young woman was forced to climb a sacred tree, and he detailed what happened next: “Suddenly I realized what was happening, and I seemed to be paralyzed with horror. The tree, seemingly so dead and motionless a moment before, had come to life. The palpi, so frail looking, had suddenly ceased to quiver, and had coiled themselves about the girl’s head and shoulders, holding her so firmly that all her efforts to free herself remained absolutely useless. The green branches so rigid before began to writhe, and coiled themselves round and round like snakes. Then as that mass struggled there arose a horrible sight I shall never forget the great leaves began to rise slowly, very slowly. Those evil looking thorns were now closing on her with the force of a hydraulic press. As they came together tightly there trickled down the trunk a pinkish mixture, which the maddened natives fought and trod each other down to get one mouthful of the intoxicating fluid from the tree and the blood of the human sacrifice”. You can read Dr Liche’s full story and other terrifying botanical tales in Prior (1939).

Sacrificed to a man-eating plant. American Weekly, 1920 © Biodiversity Heritage Library.

Stories about killer plants have a long tradition, but picked up momentum during Victorian times with the widely circulated spiel about the upa tree (Antiaris toxicaria) from Java. According to a Dutch surgeon named Foersch, the upa tree releases poisonous fumes potent enough to leave the ground covered with skeletons of birds, mammals and people within a 20-km radius. This tosh fired up the public imagination, and authors such as Charles Dickens, Thomas Carlyle and Charlotte Brontë began using the word ‘upa’ as a metaphor for a person or thing with a poisonous, destructive atmosphere (Price, 2013).

Victorian stories about evils of the plant kingdom relied heavily on personal accounts from faraway places, where the likes of Dr Liche and surgeon Foersch could have been influenced by attention-seeking tendencies or overindulgence in mind-altering substances under the tropical sun. But back home, their countrymen could satisfy their curiosity and fascination and get the facts straight from an easily accessible killer flora: carnivorous plants. That’s exactly what Charles Darwin did for 16 years in his glasshouse: his research and observations resulted in the 1875 book Insectivorous Plants. In a letter to Asa Gray, Darwin couldn’t contain his awe towards his favourite carnivorous plant, the common sundew (Drosera rotundifolia): ‘I care more for Drosera than the origin of species … it is a wonderful plant, or rather a most sagacious animal. I will stick up for Drosera to the day of my death.’ (Jones, 1923).

The luring but lethal droplets of sugary mucilage on the tentacles of a common sundew. In his best-selling erotic/botanic poem The Loves of the Plants, Charles’ grandfather Erasmus Darwin (1731-1802) labelled the common sundew the Queen of the Marsh © Wilson44691, Wikimedia Commons.

The common sundew is one of the approximately 630 plant species around the world that attract, trap and consume animals – generally insects and other invertebrates, but occasionally small vertebrates. Most carnivorous plants produce enzymes that digest their prey, but for some species the job is done by symbiotic bacteria. Plant carnivory has evolved independently several times, and species from about 13 families have come up with elaborate stratagems and structures to lure and catch their quarry such as nectar-scented volatiles, colour mimicry, slippery pitchers, sticky appendages, and snap traps – watch the exploits of the Venus flytrap (Dionaea muscipula), a species from the coastal bogs of North and South Carolina in the United States:

Carnivorous plants are usually found in nutrient-poor habitats, so they rely mostly on insect prey to supplement their required nitrogen and other nutrients. But these plants also need insects for their pollination. This dilemma, known as the pollinator–prey conflict, is circumvented in a variety of ways. For some species, morphological features keep food and pollinators apart: their flowers are high above ground to attract flying pollinators while their traps are close to the ground to catch other insects as prey. Separation could be temporal: for some species, flowers and traps develop at different times.

The long flower stalk of a Venus flytrap keeps pollinators safely away for the deadly traps at ground level © Victoria, Wikimedia Commons:

Some carnivorous plants resort to clever trickery to sort out the pollinator–prey conflict. Tropical pitchers or monkey cups (Nepenthes spp.) capture mostly ants that slip while walking on a wet peristome (the ring surrounding the entrance of the trap) and fall to their deaths. But most potential pollinators of tropical pitchers such as flies, moths and wasps are active in dry weather, when the peristome is not slippery and almost no insects are captured (Mithöfer, 2022).

Ants slide to their deaths while walking on the wet edge of a fanged pitcher-plant (N. bicalcarata) trap © Hans Breuer, Wikimedia Commons:

The judicial use of attractants is another mechanism of differentiation, as El-Sayed etal. (2016) demonstrated for fly-pollinated sundew species from Australia and New Zealand. For D. auriculata (D. peltata for some authors), flowers are adjacent to traps, but both structures release different odours. As a result, flowers attract pollinators and prey, while traps attract only prey. There are no attracting scents for D. spatulata and D. arcturi, but flowers and traps for these two plants have different colours and are spatially separated: the combination of these two factors discriminates pollinators from prey.

Flowers and traps of D. auriculata are cheek and jowl, but they attract different insect cohorts © Murray Fagg, Plants of South Eastern New South Wales.

The strategies employed by carnivorous plants to avoid the pollinator–prey conflict seem to work very well because almost no species show a substantial overlap of prey and pollinators. This was demonstrated by Youngsteadt et al. (2018) for the Venus flytrap, one of the better known carnivorous species. The researchers captured insects found on flowers at several sites during the blooming period and checked whether they were carrying Venus flytrap pollen. They also searched traps for their catches. The result: prey items consisted mostly of spiders, ants and beetles, while the sweat bee Augochlorella gratiosa and the checkered beetle Trichodes apivorus, which were the main pollen carriers, together with other potential pollinators, were hardly ever found in the traps.

A male A. gratiosa: this species is not fooled by Venus flytrap trickery © The U.S. Geological Survey Bee Inventory and Monitoring Lab:

All these contrivances employed by carnivorous plants to attract and snare their prey, at the same time protecting their insect pollinators, make up one of the most alluring file cases to demonstrate the power of natural selection. It’s not surprising that Darwin was captivated (his son Francis inherited dad’s enthusiasm, doing his own research of plant carnivory). Carnivorous plants offer much more interesting stories than man-eating trees.

Frank Aubrey’s The Devil-Tree of El Dorado, 1897. Art by Fred Hyland © The Project Gutenberg.