Bats for two weeks in a row! Huzzah! This week's paper is in Science, lead authored by Vladislav Nachev from York Winter's group at Humboldt University in Berlin. They addressed how animal perception can influence the evolution of another species. In general, nectar feeding animals prefer flower nectar with a higher sugar concentration. Surprisingly, then, plants generally produce nectar with fairly low sugar concentrations. Why are the preferences of pollinators not driving plant evolution of more concentrated nectar in order to receive pollination services?
To answer this question Nachev et al. set up an array of 23 robotic flowers in the jungle at the La Selva Biological Station in Costa Rica. A network of tubing provided each flower with nectar of a predetermined concentration and volume. This array looked pretty much like lots of guinea pig water bottles hanging from the roof of an open pole barn. They captured 16 nectar feeding bats (Glossophaga commissarisi) and tagged them with RFID tags (see chickadee paper I discussed previously). The flowers were equipped with sensors that read the RFID tags and recorded which bat visited them and for how long. Each flower produced the same amount of sugar, but the amount of water added to that sugar (and thus the concentration and volume of the nectar) varied depending on its virtual "genome". Each flower had a diploid set of four virtual genes each with two possible co-dominant alleles (versions of a gene) that controlled the amount of water in nectar. They started with a set of plants of different genotypes, some with high sugar concentrations in nectar (42.2%) and some with low sugar concentrations (17.8%). When a bat visited a two flowers in a row, the computer determined a pollination event and created a "seed" for those two plants. 23 of one night's seeds were randomly selected to be the flower genotypes (with associated nectar concentrations) for the next night. Flowers that received more visits therefore made more seeds, and more of their seeds would be present as flowers the next night. After 10-12 generations (nights) all of the flowers had converged in nectar concentration at around 36%. Why did they not converge at the highest concentration possible? Because there is a trade-off between concentration and volume, and the perceptual biases of bats makes them more selective about volume (see below).
Although different flowers produced different nectar volumes dependent on their genotype, flowers do not fully refill after every visit. Therefore when there are lots of bats foraging, an individual bat may encounter a flower that is partly drained already. This is where the bat perception plays in, in the form of what is called Weber's law. Weber's law has to do with the ability to distinguish amounts. If you have two candies, and somebody adds one, you will probably notice. If, however you have 60 candies and somebody adds one you mostly likely will not notice. That is to say, we are much better at distinguishing proportional differences than real differences, so we are best at distinguishing real differences for very small amounts. This means that when there are lots of bats foraging, so bats are encountering smaller nectar volumes, they are better at distinguishing these nectar volumes, and therefore more selective for higher volumes which results in lower sugar concentrations.
I am really excited about this paper, because it demonstrates how the brain's perception of the world can have consequences for the evolution of other species. Plant-pollinator relationships are an ideal system in which to study this, but the same processes are also occuring for other biological relationships such as predation, competition, mutualism, parasitism etc.