Campus Life

THE NATURALIST’S NOTEBOOK: The birds, the bees, and the flowers

All you ever wanted to know about pollination

It’s a romantic night. A young male bee, just out of his pupa, is looking for adventure. He spies another bee in the bushes, and from her scent, discovers that she is a female. He falls in love, and within a matter of seconds they are having sex. Then, something strange happens: she hits him on the head with a lump of pollen. Confused, he wanders off, and immediately falls for another beautiful bee. They too make love, and his new partner takes the pollen off of his head.

Congratulations, bee. You have just mated with two very clever flowers. When you thought you were having sex, you were actually enabling your two partners to have sex with each other.

Threesomes in the world of plants

Plants have a problem when it comes to dating. It’s not a problem humans have, or birds, or bees. Plants can’t move. That means that most of them use a middleman to find a mate.

The usual way this works is with a flower. Flowers are bright and showy and they have sweet stuff inside — the nectar. They also have pollen, which is the plant version of sperm. Insects come to drink the nectar and get covered with pollen in the process. They then spread the pollen between different flowers. The pollen fertilizes the flowers and produces seeds. Everyone is happy — the insects got tasty nectar, and the plants got to share DNA with each other.

Flowers became common in the Cretaceous period, just when the dinosaurs were preparing to go extinct. That makes them a relatively recent player in the story of evolution. Until only a hundred million years ago, most plants could get by without flowers. Some still do, in fact. Pines and other conifers spread their pollen with the help of the wind. Mosses don’t have pollen at all, but reproduce asexually with spores. Ferns have a dreadfully complicated reproductive cycle, involving both pollen and spores.

For plants that do have flowers, it’s important to attract the right kind of pollinator. It would be a waste of energy for a rose to attract a butterfly, if the butterfly never went to another rose to share its pollen. So flowers are generally set up to encourage certain pollinators and deter others. This means that butterflies go to butterfly flowers, bees go to bee flowers, and so on.

The shape of a flower helps determine what pollinates it. Bees are heavy and like flowers with a sturdy landing platform, where they can sit while they drink nectar. Long tubes are generally set up for butterflies, moths, or hummingbirds, which have long tongues. At the extreme is an orchid from Madagascar, which keeps its nectar at the bottom of a tube one foot long. The only pollinator that can reach the nectar is a species of moth with an extremely long proboscis. The moth seeks out these orchids in the tops of trees, unrolls its proboscis, and drinks the nectar while hovering.

The color of a flower means something too. Bees prefer yellow flowers, and moths like white flowers that are visible at night. Insects can’t see red, so red flowers are often set up for hummingbirds, or else they have bright UV colors that humans can’t see.

Bats also pollinate flowers in many parts of the world. They like big, white flowers and generally eat some of the pollen as well as the nectar. Because of this, bat flowers pack extra protein into their pollen so the bats get a balanced diet. It takes a lot of energy to cater to bats, and generally only big plants like trees can afford it. But there is an advantage: bats can travel long distances to spread pollen between trees all over a forest.

Some flowers don’t follow the rules of pollination, like those orchids that trick bees and wasps into mating with them. This strategy lets the flowers exchange pollen without paying any nectar in return. But most of the time, flowers aren’t about deception. They are about cooperation, telling the story of a plant and an animal working together in a way that helps both of them. It’s a cheerful message for the springtime.