The drive to have sex is greater for many organisms than life itself. Innumerable fish swim up rivers to spawn, eusocial (hive) insects take to the air and spiders brave the wrath of the female, all dying in one way or another from mating. Individuals do not pass on their genes by living the longest but by reproduction, which causes natural selection to encourage the most rampant to prevail – and plants are no exception.
When you’re the stay-at-home-type type though like a plant, you have to be inventive in how you have sex. In temperate zones all matter of flora release clouds of pollen in the spring and summer to waft through the air, to the irritation of hay fever sufferers. These, like sperm, are the male gametes, but in the tropics they strangely are amiss from the air. So profound is this fact that some hay fever sufferers move to the jungles to alleviate their symptoms.
The reason is that this scattershot strategy of wind pollination only works if the plant has many of it’s specie in the local area. In the temperate climes you will find entire forests of only a dozen species of tree, where as in the tropics the first dozen you come across alone will be different. In any given localised area, the rule is that the common are rare and the rare are common. With exception such as epiphytes, tropical stay-at-homes rely on intermediaries to have sex.
Flowers entice animals to act as pollinators, offering nectar, fragrances and oils. When we think of pollinating animals we tend to think of bees and butterflies, but these are but a few of the agents of procreation. Hummingbirds have a thirst for nectar, which is more of an addiction than a necessity as most of their nutrition comes from insects. Except one specie in North America they exist only in the neotropics, with over one-hundred species found in Ecuador alone. Bats are also a major player as pollinators as they too are allured by the rich offerings of flowers.
Enlisting the help of animal pollinators only solves part of the problem though, for the tremendous diversity in the rainforest may mean the animal will not visit another plant of the same specie to fertilise it. So plants have adapted not only to entice pollinators but to ensure that they visit other conspecific plants. To do this, most neotropical flowers have evolved to be mutualistic with specific pollinators and to discourage others. The following is a broad view of the characteristics of flowers that attract pollinators from particular families of the animal kingdom.
Butterflies: These flowers are red or orange in colour, platform shaped with a mild odour and recessed nectaries, particularly common in tree fall areas. Butterflies are one of the few insects that can see well on the red end of the spectrum, requiring the warmth of the sun and a flat perch to drink nectar which these flowers provide with consideration.
Beetles & Gnats: These critters often like rank-smelling flowers, which can even stench of carrion to attract scavenger insects. Some orchids mimic fungi, luring fungus gnats with their shape and smell.
Hummingbirds: Are attracted to vibrant colours such as red and orange, to the point that wearing a bright shirt in the jungle will entice their curiosity. Flowers take advantage of the little birds ability to hover by suspending their flowers away from branches, out of reach of other animals. They are tubular in shape and often droop all as further deterrent.
Bats: Unlike insectivorous bats with their sophisticated sonar, fruit and nectar bats have a good sense of smell and keen eyesight, even though they are colourblind. These flowers tend to be whitish and large, making them stand out in the dark. They will typically have a bushy mass with protruding anthers that rub pollen onto the face and chest of the feeding bat. Although bat-flowers produce ample nectar, they have a musty fermented odour unappealing to humans. The flowers form on the bark or on large branches away from foliage to give the bats the best access. These evolutionary designs would allow other unwanted animals to feed upon them so these flowers only open at dark and often just for one night, so that the excellent senses of the bat can locate and take advantage of them before others can. The agave plant used to produce tequila is an example and is solely pollinated by bats. A particular bat known as Anoura fistulata (a mouse-sized bat of South America) has an 8.5cm long tongue to reach deep recessive necatr. This is 150% of it’s body length which would be the equivalent of a human’s tongue being 9 feet!
Moths: These flowers bloom at night and smell reminiscent of jasmine, such as the dragon fruit flowers. The hawkmoth being large of size has one of the longest reaches with a proboscis that can unfurl six inches, enabling these flowers to have highly recessive tubulars. Other moths, such as the golden silk moth, are lured by the red bract of the hooker’s lips. They are a nocturnal equivalent to hummingbirds for they can hover and there are even some diurnal (day time) hawkmoths that utilise hummingbird flowers.
Even different species of the same genus attract different pollinators, such as the Borrachero. These beautiful trees are known for making the date rape drug Brugmansia (used by cartel hitmen for assassinations) and have hanging white flowers. These flowers first made me question why they would seem to make it so difficult for pollinators to do what they need to do. The answer is that the moths can get to them fine whilst the rest cannot. There is even a red specie of this drooping flower which, you might guess, are pollinated by hummingbirds instead.
It does not always go according to plan however, such as with some bees that cannot get into hummingbird flowers will chew a hole at the base of the flower to get to the nectar. When this happens the bee adds further insult by avoiding being coated in pollen whatsoever. Hummingbirds, although useful to some plants, may evict other species (even non-competing ones) from their territory, inhibiting pollination of those plants.
Whilst some animals have their tricks to circumvent adaptions, some plants are adapted with tricks, orchids being the masters of deception. As nectar is a large investment of energy and only exist to entice pollinators, some orchid lure pollinators in with a pigment that looks like nectar. Other orchids have hairs that resemble pollen-rich anthers but are actually a disappointing ruse and others have structures that trap bees to force them against pollen receptors without rewards. Some species even mimic tachinid fly females which, when the males try to copulate, pollinate the flower instead. The most deceiving of all orchid though is from one that has adapted to sway in a way that looks to a male Centris bee as a territorial challenge from another bee. When the bee goes to fight off the intruder he is perplexed by the imposter and coated in pollen.
Beautiful orchid bees, unlike other bees, are not eusocial. These males visit orchids, not for nectar, but for fragrance to entice females. They fly between different orchids trying to concoct the most appealing perfume in specially evolved pockets on their hind legs – and orchid perfume arouses the males as well. So refined is the evolution of the orchid that it seems to outcompete female bees for the males’ attention. An article by the New Scientist suggests that males bees are five times more likely to be attracted by an orchid bouquet than an actual female, even causing the bee to ejaculate on the flower in excitement.
Often temperate plants all bloom in synchronicity, but with the less profound seasonal changes and the tropical plants all have these unique populations of animals to maintain. For this reason, when you walk through a jungle, you will not encounter copious flowers in bloom at any one time like the May bluebells of England, but a few flowers all year around.
These symbiotic relationships are marvellous at casual glance but utterly astounding when you cue on to the subtleties of the species interactions. You can walk amongst closed red flowers during the day, knowing that they will bloom for moths at night. You can observe tiny chewed holes at the base of a flower and smile at the revelation that a frustrated bee has been victorious. We marvel at the adaption of individual species but it is the interspecies relationships that are astounding and why biodiversity is so important to the health of the planet.