August 2009, Volume 7, Issue 8

An Evolutionary Race

After hanging motionless for a spell, the bat suddenly stretches, catlike, unfurling first one wing, then the other. It yawns widely and extends its tongue. And keeps extending it – longer and longer in a remarkable display. This is a tube-lipped nectar bat, and its tongue, at full-stretch, reaches more than 1½ times its body length.
The bat in a screened-in tent in Ecuador laps sugar-water from the bottom of a plastic test tube, contributing to Nathan Muchhala’s efforts to determine why evolution produced such a spectacular tongue.
Muchhala, now a postdoctoral researcher with the University of Toronto, discovered this unique bat during fieldwork for his Ph.D. dissertation for the University of Miami. He and two Ecuadorian colleagues named the species the tube-lipped nectar bat (Anoura fistulata) because of its distinctive elongated lower lip.
Subsequent research demonstrated that this bat can extend its tongue some 3.3 inches – twice as long as other nectar bats and longer, relative to body length, than any other mammal.
What evolutionary pressures could have brought about such a spectacular tongue? To find out, Muchhala returned to Ecuador with financial support from Bat Conservation International. The BCI grant was made possible by BCI Members Edmund S. and Marie Morgan.
One possible answer to the question was suggested by Charles Darwin to explain the remarkably long tongue of a giant hawk moth in Madagascar. He proposed that the long tongue evolved in a “race of increasing lengths” with the exceptionally long nectar spur of the Malagasy Star Orchid.
Darwin reasoned that, at least for moths, tongues equal to or longer than flower tubes would be required to reach all of the nectar. From the flowers’ perspective, the tubes needed to be longer than moth tongues to ensure that the moth has to push its head down into the flower, and thus pick up (or deposit) the pollen found there. The result of those opposing needs is a repetitive loop: the flower grows longer to ensure pollination, while the moth’s tongue lengthens to reach more nectar, which causes the flower to grow longer, and so on.
The diet of tube-lipped nectar bats includes nectar from the flower Centropogon nigricans, which stores its nectar at the base of 3- to 3½-inch flower tubes. The benefit of increased tongue lengths in nectar bats is clear: it allows the animal to reach more nectar. The value of long tubes for the plant, however, is not as obvious.
Muchhala suspected that although bats will fully insert their heads into any flower, they will push especially hard against flowers with long tubes in an effort to reach that last drop of nectar. This extra force should mean that they pick up and deposit more pollen grains, thus allowing the plant to produce more seeds.
To test this idea, he captured four tube-lipped nectar bats in Ecuador for four days of feeding experiments in separate screened tents. He videorecorded the bats feeding at plastic test tubes in two configurations and varying lengths. Wire “stems” tubes measured the force with which the bats pushed into the tubes to reach sugar-water at the bottom.
After the captive bats had visited first a male-phase and then a female-phase flower, he collected pollen from female-phase flowers and used a microscope to count how many grains had been transferred. Results showed a strong benefit to increased flower length: longer male-phase flowers exported 123 percent more pollen than the shorter flowers, and longer female-phase flowers received 144 percent more pollen.
This research clearly demonstrates that bats are more efficient pollinators of longer flowers. In combination with a length benefit to bats – longer tongues give access to more nectar – this sets up the conditions for a coevolutionary race between plant and bat.
Not surprisingly, Centropogon nigricans is completely dependant on tube-lipped nectar bats for its pollination. The bats, however, are found over a significantly larger range than the plants. That raises a new set of questions, including: Have other flowers coevolved with these bats? Are the tongues of tube-lipped nectar bat populations shorter in forests with shorter flowers, as the coevolutionary scenario would predict? 

BCI Members can read the whole story of Nathan Muchhala’s research and results in the Fall 2009 issue of BATS magazine.

All articles in this issue: