Evolution finally explains the real story behind the birds and the beesS

For nectar-eating animals like bats, birds and bees, there is only one way to feed: they are either born with a proboscis that sucks up sweet, tasty flower sugar or a tongue for spooning it up. Whatever nectar-acquiring tool these animals are born with ultimately dictates what kind of nectar they can consume: more-sugary (and therefore more viscous) nectar, or less-sugary (and therefore thinner) nectar.

Now a team of researchers at MIT led by applied mathematician John Bush believe the sucking/spooning split may be indicative of a co-evolutionary process between flowers and their pollinators. In other words, flowers may be driving evolutionary change among the birds and the bees.

"Do the flowers want a certain type of bug or bird to pollinate them? And are they offering up the nectar of their preferred pollinator?" asks Bush. "It's an interesting question whether there's a correlation between the morphology of the plant and the morphology of the insect."

Bush's idea stems from the assumption that in nature, efficiency is key. From the perspective of the animal doing the pollinating, the more efficiently it can access a plant's nectar, the less energy it expends competing for resources with other pollinators, and the less chance it has of being picked off by a predator.

From the perspective of the plant being pollinated, ensuring that the nectar you're producing attracts one species of pollinator over another could mean the difference between your genes being passed on and your genes dying off.

In the latest issue of Proceedings of the National Academy of Sciences, Bush's team has shown that the efficiency of feeding in nectar-consuming animals does, in fact, depend on how sugary a flower's nectar is, and whether an animal accesses the nectar by sucking it up, or by "entraining" it (i.e. dipping into it and drawing it back up) with its tongue.

The team found that animals such as bees, which probe at nectar with their tongues in a manner not unlike an anatomical ladle (a class of animals Bush has dubbed "viscous dippers"), feed most efficiently when consuming more sugary, viscous nectar. Conversely, so-called "suction feeders" — like birds and butterflies, which draw nectar up through anatomical tubes — do their best feeding when they're sucking up thinner, less-sugary nectar.

The researchers claim that their findings provide the first rationale for why suction feeders typically pollinate flowers with nectar of lower sugar concentrations than their viscous-dipping counterparts.

Bush plans to extend his investigations to other species, including a desert lizard that absorbs moisture primarily via its skin.

"People are now interested in moving around small volumes of fluid for microfluidic applications," Bush says. "It's clear that nature has been solving these problems for millions of years. Animals have learned how to efficiently navigate, transport and manipulate water. So there's clearly much to learn from them in terms of mechanisms."

Via Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1108642108)

Photograph by Bruno Cordioli