Coevolution of bees and flowers. As the diversity of flowers bloomed during the Cretaceous Period, so too did the wealth of bees. There’s a simple reason for this: Pollen, the male germ cell of plant reproduction, just so happens to be the bee’s favorite food. So rather than rely on the wind to spread their seed, angiosperms (plants with seeds) could now increasingly depend on bees to carry them directly from one flower to the next.
Coevolution of bees and flowers
Their closely interconnected development is one of nature’s preeminent examples of coevolution : a classic win-win situation. For every new type of flower, there evolved a new type of bee. Bees loved the pollen, yes, but they were also suckers for the flower’s tasty, carbohydrate-rich bait: nectar. Producing this sugary treat costs plants, so they endeavor to hide it. Those insects and bees most likely to move pollen evolved in an “I’ll scratch your back if you scratch mine” kind of way.
Today, flowering plants and insects are the two most diverse taxa on earth. The flower come in a kaleidoscope of sizes, shapes, colors, smells, and arrangements, as do the bees who visit them. One might wonder: Why all the variation? Looking closer, it’s clear that most flowers have special features designed to attract one or a small number of mutualistic pollinators.
Through coevolution, flowers and their pollinators have developed uniquely complementary characteristics. Certain flower, for example, have adapted to be pollinated by hummingbirds. Typically, they have bright red, orange, or yellow, often with fused petals, but their nectar has very little scent. Other are attractive to bats; these plants feature pale, often large, bell-shaped, which open only during the night. But by far, the greatest number of flowers have chosen to team up with insects. These plants have brightly colored and odorous, often abundant, prominent flowers. Among all the insects, bees are a flower’s favorites.
Some flowers are generalists, benefiting from a wider array of pollinators. Likewise, some pollinators are generalists, too—the honey bee being the prime example.
Why Make the Best Pollinators
Among insects, bees rule as flower pollinators. But why did bees evolve to become the predominate pollinator? Why not hummingbirds or dragonflies or mosquitos? Research synthesized in the book Crop Pollination by Bees provides us with answers to some of these questions.
All bees are strictly vegetarian. They only eat nectar and pollen, obtaining their food from a single, one-stop foraging trip to a flower. Longer tongues on some bees allow them to access nectar hidden within flowers. Shorter-tonged bees visit flowers in which the nectar is not so hidden.
Hair. Thick, branched hairs located on the bee’s body are well suited to trap and hold pollen. After visiting one flower, pollen clinging on the bee’s body hair may easily rub off on the next flower visited. Flowers have evolved to ensure this happens.
Pollen baskets. Bees have a special bunching of hairs to efficiently transport pollen from the flowers back to their nest. Some have such hairs on hind legs (called pollen baskets ), others underneath the abdomen. A bee must forage a multitude of flowers to collect enough pollen to ensure that their foraging trip is energy-efficient. Before they store pollen in these special hairs, pollen is scattered over their body, increasing the likelihood that some of it will rub off on a different flower.
Bees do not eat until they get home. While bees visit flowers to collect food for developing young, facilitate normal adult glandular function, and maintain their energy, they do not eat at the flower. Rather, they move from one flower to the next before their fuel tank reaches empty, depositing bits of pollen as they go.
Bees exhibit flower constancy. When foraging for food, bees move from one flower to the next of the same kind. They don’t visit a dandelion, then an apple blossom. If they require dietary diversity, they can visit another flower type in a subsequent trip; in the case of social bees, like bumble bees and honey bees, a sister in the same nest might just visit dandelions and another just the apple blossoms. By concentrating on just one flower type, the bee learns how to more efficiently obtain nectar and pollen from flower.
Bees are specialists. Different bees specialize in different, typically visiting only one or a small number of (often related) types of flowering plants. The emergence of the pollinator adult bee from its nest is synchronized for both bee and flower life cycles, boosting the odds that the bee is present when the plant.
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