Birds and Mammals as Agents of Pollination
Investigating Pollen Transfer in Angiosperms
When it comes to pollination, flowering plants have evolved some amazing adaptations. Some types shun showy petals and sepals preferring instead to pump their energies into producing large quantities of pollen, which are then scattered into the wind, to be carried, hopefully, to the stigma of a similar species.
Most flower types, however, have opted to enlist the services of animals to carry their pollen to receptive females. Unlike wind-pollinated flowers, the animal pollinated varieties do not produce huge amounts of pollen. Instead, they pour their energies into attracting and enticing the animals that will ferry their pollen to another plant of the same type.
Birds as Agents of Pollination
On modern earth, birds nearly rival insects as pollinators. To enlist avian envoys, plants have to use different methods than those employed for insects. Birds are almost totally lacking in sense of smell so perfume would be wasted on them. On the other hand, bird eyes are very acute and much more like ours than those of an insect. Red color, nearly invisible to insect eyes, is widely employed by some flowers to attract birds to them.
Birds are much larger than insects and the plants that employ them must make special provision for that fact. Their flowers must be large enough to accommodate the head of a bird as it seeks nectar and coincidentally collects its load of pollen, and the petals must be robust enough to withstand such vigorous treatment. So if a flower is large, robust, red, and lacks smell, birds very likely pollinate it.
Most birds also need a perch to stand on or cling to as they take their drinks. Trees, such as the flame of the forest and the African tulip tree, carry their magnificent scarlet flowers on branches that are more than strong enough to support the weight of bird. However, the Kangaroo-paws of Western Australia grow only a few inches off the ground so there is no place for the birds that pollinate them to perch. Instead, they point their tubular flowers down so that birds have difficulty hopping up to sip the nectar and in the process, get dusted with pollen.
Most bird-pollinated flowers position the stamens so that when a bird thrusts its head inside to reach for nectar, its forehead or its breast are dusted with pollen. But some flowers take no chances and have mechanisms to ensure that their feathery visitors are stabbed, clouted, sprayed, or showered with pollen. A species in the mistletoe family stores its pollen in the roof of its flower. As a bird lands, its weight triggers the chamber so that it opens explosively and showers pollen all over the bird’s forehead.
Some birds recruited by flowers, such as sunbirds, sugarbirds, and honey eaters, are so small that they can perch on the flower itself. In fact, these birds' digestion has now become so adapted to that particular diet that they cannot cope with other foods.
The most highly specialized of all nectar-feeding birds must surely be the hummingbirds of South America. They can beat their wings so swiftly and articulate them so accurately that they can hover in the air in front of a flower. Plants attract them with delicate red flowers, suspended from the ends of long stems and facing outwards so they can only be entered from the air.
Mammals as Agents of Pollination
Mammals are so large and clumsy that they seem ill suited to be pollinators. Even so, a few of them have been pressed into service by plants. In southern Africa, some varieties of proteas are dull colored, largely shrouded by the foliage above, and point downwards. Their nectar is collected at night by small rock mice and shrews. This is only a seasonal treat for the animals, but for the proteas, it is an invaluable service at a crucial time.
The traveler’s palm of Madagascar has such a stout flower that only the local lemurs have the strength to pull them apart to reach the nectar within.
The mammalian equivalents to birds are the bats, not the small ones with sonar for they are insect eaters but rather, the large kind with big eyes. The flowers that attempt to attract them open at night, are pale in color, and release an odor that smells yeasty, musty, rancid, and even reminiscent of urine.
Bats pollinate nearly all wild bananas and cacti. In the dry, hot deserts where cacti grow, very few animals of any kind are active during the oven-hot day. Consequently, the organ-pipe cacti and the cardon cacti open their flowers at dusk and shut them again during the morning. Cacti also arrange their flowering season to correspond with the northward migration of bats from Mexico up to the southern United States.
Of the over 250,000 (and counting) species of angiosperms (flowering plants), most are pollinated by some type of animal.
Some Hummingbirds and Flowers have Coevolved
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