Most tropical plants
rely on animals to transfer pollen from flower to flower. The most important
animal pollinators are bees, but there are many others, including other
insects, bats, and hummingbirds. Since flowers “want” to be pollinated, they
have features that match the physical and sensory abilities of pollinators. To
achieve pollination, flowers provide a reward, advertise it, and are
constructed so that visitors come into contact with their stamens and stigma.
The commonest reward is nectar, but others include pollen and less usual
substances, such as waxes, oils, and perfumes (the latter is used by orchid
bees as sexual attractants).
Nectar varies in the
sugars it contains. Flowers pollinated by hummingbirds, butterflies, hawkmoths,
and many bees secrete nectar rich in sucrose, whereas those pollinated by bats
and by passerine birds (including American orioles and tanagers, Australian
honeyeaters, and Old World sunbirds) have nectar-rich in glucose and fructose,
sugars that are also found in fruits.
The significance of
these intriguing differences is unknown, but they do not seem to matter much to
hummingbirds and bats. Hummingbirds happily feed on leftover nectar in bat
flowers, while bats routinely empty hummingbird feeders full of sucrose
solution. Flowers have evolved a variety of adaptations to attract specific
pollinators. Certain plant families are particularly important for
hummingbirds, some of the most notable being the Heliconiaceae, Bromeliaceae,
Ericaceae, Rubiaceae, Acanthaceae, and Gesneriaceae.
These flowers are
ornithophilous, or bird-loving, and exhibit clear adaptations for pollination by
hummingbirds. Most of them are either red or have red bracts or leaves that
advertise their presence, red being a color that is conspicuous to hummingbirds
but not to most insect competitors. In addition, the flowers are diurnal, have
a tubelike corolla that fits the slender bill of a hummingbird, and lacks any
scent (which might attract insects), and most also lack a landing platform that
would provide easy access for insects.
Flowers aimed at
other pollinators have other characteristics. Those pollinated by hawkmoths,
for example, are nocturnal, white (to show up in the dark), fragrant, and also
tubular (to fit a moth’s proboscis). Bat flowers are nocturnal and usually
pale, with a strong musky odor, and some have “sonar guides,” which bats detect
by echolocation. There are two main types of hummingbird flowers. One sort has
long tubular flowers (mostly 30 to 40 mm long) that secrete copious nectar; these
tend to be scattered and are usually visited by traplining hummingbirds. The
other sort has short tubular flowers (mostly less than 20 mm long); these
contain less nectar but are massed together in numbers great enough to be
worth defending by territorial hummingbirds. These differences have
consequences for the plants.
Trapliners tend to
carry pollen from plant to plant, which results in cross-pollination and
enhanced reproductive success. In contrast, territorial hummingbirds foster
self-pollination. Sometimes a hummingbird visits so many flowers on the same
plant that its face gets covered with white or golden pollen, making it look
like a different species. Hummingbirds that intrude into territories to steal
nectar are probably more useful to a plant than the territorial owner. Since
these filchers visit flowers only briefly before being chased off, they perhaps
deliver pollen to plants of the same species some distance away. Another point
to be considered is why many flowers use hummingbirds as pollinators rather
than insects.
After all, insects
can be attracted with a smaller reward of nectar. The probable reason is that
hummingbirds are more reliable as pollinators when the weather is bad,
particularly at high altitudes. Bees and butterflies remain inactive when it is
very wet or too cold, so flowers dependent on them fail to be pollinated.
Hummingbirds are active in all kinds of weather, so it is no surprise that
there are many more hummingbird-pollinated plants in the highlands than in the
lowlands.
