Readers’ wildlife photos

The photos tank is emptying fast, and so I importune you for the last time to send them in. This feature might disappear, and so might the website itself due to the waning of reader interest and comments.

Today we have a biology tale and photos contributed by Athayde Tonhasca Júnior. The subject is orchids, flowers that have intrigued biologists from Darwin to the present day. As Dawkins and I discussed reently (since it’s in his new book), orchids have evolved to deceive pollinators into copulating with the flowers, thereby spreading the pollen to the next deceptive orchid. The randy pollinators never seem to cotton on to the deception.

Athayde’s notes and IDs are indented, and you can enlarge the photos by clicking on them.

Cheaters and cheated in the pollination game

Somewhere on the southern British coast, an early spider-orchid (Ophrys sphegodes) blooms. It’s time to attract insects to transport its pollen to another flower so that fertilisation happens. But most pollinators are not interested: the orchid produces negligible amounts of nectar, and its pollen grains are stuck together in inconvenient masses known as pollinia, a feature of orchids and of many plants in the milkweed family. On top of that, the flower’s labellum – the lowermost petal – looks like a legless spider, which is not at all an inviting sign.

An early spider-orchid © GkgAlf, Wikipedia.

An early spider-orchid flower © Orchi, Wikimedia Commons.

An Ophrys apifera flower with its pollinia clearly visible © Escuapio, Wikimedia Commons:

The early spider-orchid does not offer the temptations usually found in many flowering plants to attract pollinators. But it has one trick up its sleeve, and it’s an effective one: its flowers release an aroma bouquet comprising dozens alkanes and alkenes. These organic compounds, consisting of carbon and hydrogen atoms, are the main constituents of the natural waxes that help waterproof plants and regulate their water content. For the orchid, these chemicals have another important property: they mimic the scent of virgin female buffish mining bees (Andrena nigroaenea). You can see where this is going: male bees will be very interested in paying a visit.

A male buffish mining bee © gailhampshire, Wikimedia Commons:

About a quarter to a third of the estimated 30,000 orchid species in the world resort to trickery to be pollinated. They may use food deception, imitating features of rewarding species. For example, the narrow-leaved helleborine, aka sword-leaved helleborine (Cephalanthera longifolia), produces a flower with a small section that is coloured yellow-orange. A passing furrow bee (Halictus sp. or Lasioglossum sp.), mistaking the yellow splotch for pollen, zeroes in. The orchid’s pollinium sticks to the hairs on the bee’s back and it is transported to another orchid. Some species release scents similar to nectar volatiles, thus attracting queen bumble bees to their nectar-free flowers.

Narrow-leaved helleborine’s fake pollen. Notice the pollinia on the opposite side © Hectonichus, Wikimedia Commons:

Other orchids – such as the 140 or so species of the genus Ophrys – rely on sexual deception: they produce flowers that look or smell like female insects, luring males to a non-existent receptive partner. Chemical mimicry is just part of this ruse: once the male is persuaded to approach, visual and tactile cues entice him to land and grab the flower. This contact could be fleeting, but some orchid species induce males to a greater amorous display; they try to copulate with the flower. During the deed, known as pseudocopulation, the Lothario accidentally dislodges the pollinia, which get stuck to its head and transported to another flower. You can join David Attenborough in watching this deceitful liaison taking place.

A male Dasyscolia ciliata (a scoliid wasp), pseudocopulating the flower of a mirror orchid (Ophrys speculum) © Pietro Niolu, Wikimedia Commons”

Sexual deception seems like a counterproductive tactic. By putting all its efforts into attracting only male buffish mining bees, the early spider-orchid misses out on every other potential pollinator (this orchid is pollinated by other bees in continental Europe, but the argument is the same). The Orchidaceae is the only plant family engaged in sexual deception, yet the strategy has evolved repeatedly within the group; so it must be advantageous. Scopece et al. (The American Naturalist 175: 98-105, 2010) suggested that pollination efficiency is the answer. By measuring pollen transport of 31 orchid species in Australia and Italy – all known sexually deceptive orchids are from Australia or Europe – they observed that deceived pollinators tend to be reliable; they go from one orchid flower to another of the same species, wasting little time and few pollinia in the process. Orchids with many pollinators had more pollen taken from their flowers, but more of that pollen is accidentally dropped or deposited in flowers of the wrong species.

Sexual deception  works for the orchids, but how about the cheated pollinators? They don’t seem to benefit at all from these encounters, which may interfere with their reproduction. Some orchids induce vigorous pseudocopulation that ends up in ejaculation, which is an energetically costly wastage.

L: A male Darwin wasp (Lissopimpla excelsa) pseudocopulating a Cryptostylis subulata flower. R: A flower after a pollinator visit: the pollinia were removed, pollen was deposited on the stigma, and a blob of ejaculate was left behind © Gaskett et al., 2008. The American Naturalist 171: E206–E212.

Flower visitors learn to avoid deceptive orchids, so perhaps the number of encounters with fake sexual partners is not sufficient to affect males’ overall mating success. Or genetics may help cheated insects counterbalance missed copulating opportunities. Most sexually deceptive orchids are pollinated by bees or wasps, which are haplodiploids (males and females develop from unfertilized and fertilized eggs, respectively). If females don’t mate, they don’t produce female offspring, but male offspring are not affected; and if females are fertilised with insufficient sperm, they usually produce more males than females. Either way, sexual deception may lead to a male-biased sex ratio for the pollinator, which would compensate for those males worn out from overenthusiastic dates with flowers (Gaskett et al., 2008. The American Naturalist 171: E206–E212).

There’s more to the success of sexual deception than specialised pollen carriers and sex ratios. The strategy depends also on synchronisation. As it is the case for many bee species, male buffish mining bees come out of their nests before females. So the interval between male and female emergence is crucial. If the orchid flowers before male emergence, there will be no pollinators around; if after, the orchid will have to compete with the real thing – female bees. Fertilisation rates for sexually deceptive orchids are naturally low; if the phenologies of orchids and bees get out of sync, say for example as a consequence of climate change, pollination rates may plunge.

Orchids’ amazing variety of forms, colours and shapes fascinated Charles Darwin: “it really seems to me incredibly monstrous to look at an orchid as created as we now see it. Every part reveals modification on modification” (letter to American botanist Asa Gray). So it’s not surprising that Darwin chose orchids as the subject of his next book after On the Origin of Species. His observations, field experiments and discussions with a network of botanists, gardeners and commercial growers convinced him that orchids’ diversity was linked to animal pollination, at a time when the prevailing view was that plants mostly self-pollinated: “In my examination of Orchids, hardly any fact has so much struck me as the endless diversity of structure,—the prodigality of resources,—for gaining the very same end, namely, the fertilisation of one flower by the pollen of another.” (On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing, 1862).

Since then we have learned a great deal more about orchids’ shenanigans to get themselves pollinated, which granted them epithets such as bizarre, cunning, devious, ‘masters of deception’, and ‘purveyors of empty promises’. Darwin probably would have smiled knowingly.

Orchis mascula as an example of adaptations for insect pollination: the diagram from Darwin’s book shows all the petals cut away except the labellum, which extends back to form a tubular nectary below the column © Dave souza, Wikimedia Commons.

And lagniappe from Athayde:

I was going to send you this post some time in the future, but I was prompted to do it now after watching your conversation with R. Dawkins last night. The story addresses a point made by both you about cheated insects not learning to avoid sexually deceptive orchids.

I felt unsafe by the explicit nature of the video. A warning was in order! (evidence attached).

I can’t remember what I was demonstrating here. . .