Darwin hypothesized that extremely long flowers and the long mouthparts of the animals that pollinate them evolved together in a coevolutionary race. While selective pressures for the animal side of such a race are relatively clear (longer tongues allow the animal to reach more nectar), the plant side remains controversial. What selective pressures might favor long corolla tubes? I examined this question for the highly specialized mutualism between the long-tubed flower Centropogon nigricans and the nectar bat Anoura fistulata, which possesses the longest tongue relative to body length of any mammal.  I hypothesized that longer corolla tubes may increase pollen transfer by causing bats to 1) visit the flower longer, or 2) press more forcefully against the anthers/stigma. I videotaped bat visits in flight cages in order to quantify visit duration and visit pressure (by measuring how far the flower was displaced during the visit).  For one set of experiments, bats were presented with flowers whose corollas were artificially shortened or lengthened.

Results/Conclusions

Results showed that visit duration and visit pressure were both significantly greater for long flowers. Furthermore, long male flowers successfully transferred more pollen than short male flowers, and long females received more pollen than short females. For a second set of experiments, bats were presented with tubes of 6 different lengths with a small amount of nectar at the base. Visit duration was positively correlated with tube length, although no significant trend was detected for visit pressure.  Overall, results suggest that Centropogon nigricans flowers face selective pressure for increased flower length as this increases the duration of bat visits, which in turn increases pollen pick-up and deposition. I propose that Anoura fistulata gradually evolved longer tongues in response to such evolutionary increase in corolla length, and in this way a coevolutionary race between plant and pollinator led to the current extraordinary lengths of tongue and flower.