NATURE is rife with cheats. Think of the bumblebee who sucks nectar and darts
away without pollinating the flower. But don鈥檛 judge too quickly, says a US
ecologist. In at least one case, the bee has already fulfilled its part of the
bargain before it takes its reward.
Pollination is a classic example of mutual benefit. Flowers feed bees, and in
return bees are supposed to pollinate flowers. 鈥淏ut it鈥檚 more complicated than
the fairy tale,鈥� says William Morris of Duke University in North Carolina.
In the Wrangell Mountains of south-central Alaska, bumblebees on the alpine
heath seem to cheat on their pollination pact with panicled bluebells. The bees
chew a hole in the base of a flower, sip its nectar, and depart without touching
the flower鈥檚 male or female parts or transferring any pollen.
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To see whether cheating hurts the plants, Morris equipped some flowers with
鈥渁ntitheft devices鈥濃�攃lear plastic collars that protected the base of the
flower. Surprisingly, the collarless plants were pollinated equally well and
bore just as much fruit (Ecology, vol 77, p 1451).
Bluebells avoid the effects of cheating by offering two rewards to the bees,
each wrapped in a differently coloured flower. Despite its name, each bluebell
starts off pink but turns blue within hours. A plant usually has both
pollen-rich pink flowers and nectar-laden blue flowers at the same time.
Though the bees frequently stole nectar from the blue flowers, Morris noticed
they also visited the pink flowers, which produce only pollen. That makes sense,
he says, because bees need both rewards. Pollen from the young, pink flowers
provides the bees with protein. The bees also need nectar for energy and to glue
together the powdery pollen for the flight home.
The plants profit as well, for the bees inadvertently pollinate the pink
flowers when they visit. The bees hang upside down from the bottom of a newly
opened flower and rattle open the pollen sacs with their buzzing wings. A cloud
of pollen falls out, dusting their abdomens. 鈥淎nd since the bees are sloppy,
some of the pollen gets deposited on other flowers,鈥� says Morris.
In fact, most pink flowers received enough pollen for fertilisation. 鈥淭hat
seems paradoxical,鈥� says Morris. Why would a plant bother making nectar to
attract bees if the pink flowers have already received their quota of
pollen?
The blue flowers may provide additional advertising. 鈥淭here鈥檚 an increased
attraction of a big showy floral display,鈥� Morris says. The bluebells lure the
bees with the promise of nectar-rich blue flowers because they may pollinate
nearby pink flowers on the same plant.
Morris believes that the blue, nectar-rich phase could also act as an
insurance policy. If spring is unseasonably cold and wet, the bees might not be
able to visit all the flowers while they are still pink. The blue phase, which
lasts an additional 3 to 5 days, would provide a second chance for pollination.
鈥淚t鈥檚 like buying extra lottery tickets,鈥� says Morris. 鈥淓ach day is another
chance that a pollinator will come and visit.鈥�
And by not skimping on the nectar, the bluebells may be keeping abreast of
their own competitors. 鈥淚t could be that if the plant didn鈥檛 provide a nectar
reward, the bees would be tempted away from the bluebell toward another flower,鈥�
says Morris.
It鈥檚 not clear whether the bluebells鈥� bait evolved for its present role, or
if the nectar was originally produced to attract another now-vanished
pollinator. For now, robbery helps bind the bluebells and bees together in a
mutually beneficial pact. 鈥淭hey鈥檙e both getting a pretty good deal,鈥� says
Morris.