Tropical plant knows whose bill is in its flowers

Some plants crave a long bird bill. One tropical plant can even recognize which kind of hummingbird is slurping its nectar by the shape of its bill, scientists report online March 2 in the Proceedings of the National Academy of Sciences.

In Heliconia tortuosa, long-billed hummingbirds can reach in and guzzle more nectar than shorter-billed birds, new experiments show, and that prompts the plant to begin reproducing. The plant accepts pollen only from birds with bills that match the shape of its flowers.

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The research indicates that “the fine tuning of coevolution between plants and pollinators may be greater than we imagined,” says Ethan Temeles, an ecologist at Amherst College in Massachusetts.

H. tortuosa, an understory plant with slender, red tubelike flowers, is visited by many hummingbird species. Some of the birds have short, straight bills while other species have long, curving bills that echo the flowers’ shape. It seems like the long-billed birds and long-flowered plants evolved to fit each other. Yet “there’s still a lot of debate about how that gets going,” says Matthew Betts, a coauthor of the new study and ecologist at Oregon State University in Corvallis. “How do you end up with these nice tight matches when you’ve got tons of different species visiting the plant?”

Betts and colleagues were actually investigating whether H. tortuosa can reproduce in areas where the Costa Rican rainforest has become fragmented. But when the scientists tried their hand at hand-pollination, the plants reproduced less than when left alone outside. “You’d think if you provide unlimited pollen, it’s going to do better,” says Betts.

Betts and his team decided to test whether the plant relies on the animals that pollinate it to set reproduction in motion. The researchers caught 148 pollinators from six hummingbird and one butterfly species, dusted off any pollen the animals were carrying, and released them into aviaries with hand-pollinated flowers. This time, H. tortuosa accepted the hand-delivered pollen, allowing the grains to crack open and sprout tubes reaching into the plants’ ovaries.

Yet not all pollinators were able to entice the plants to permit pollen tube growth. Those with the most success were two hummingbird species, the green hermit (Phaethornis guy)and violet sabrewing (Campylopterus hemileucurus). Both sport long, curved bills. Somehow, the birds with bills that matched the shape of the flowers were triggering the plants’ reproduction.

“Then the question was, what is the cue the plant is using to tell pollinators apart? It doesn’t have a brain and it can’t see, so what is it doing?” asks Betts. “We thought it had to do with how much nectar the hummingbird is able to extract.”

Betts and his team used pipettes to suck nectar out of hand-pollinated flowers. Sure enough, the more nectar the scientists drew out, the more pollen tubes grew.

Recognizing different animals lets the plants reproduce only when species bearing pollen from unrelated H. tortuosa flowers visit. When perusing existing data tracking the movement of tagged hummingbirds, the scientists saw that the long-billed hummingbirds travel farther afield than do the short-billed species, which hang around a set territory. “If the pollen’s from 20 meters away, the likelihood is it’s … from the plant’s own flowers or its brother or sister. But pollen from a long way away is unlikely to be related,” says Betts. Unrelated pollen cuts down on inbreeding and keeps the plants’ gene pool diverse.

Next, the team wants to find out whether other plants have evolved to respond only to select pollinators.