Bumblebee learning in the lab and performance in the wild

A Bombus terrestris foraging in the wild. Photo my own.

This week's paper is in Scientific Reports, co-lead authored by Lisa Evans and Karen Smith in collaboration with Nigel Raine (Guelph and Royal Holloway). They have tackled one of my favorite topics, which is the link between learning, or cognitive abilities, and fitness. To do this they used colonies of a European bumblebee species, Bombus terrestris, studied learning in assays in the lab and then let those same bees forage in the wild and assessed their foraging performance. 

What they did 

The authors used five bumblebee colonies which they split in half using a piece of mesh screen. Half of the colony was connected to a foraging arena where they conducted learning experiments, and the other half had access to the outside through a window. Individual bees that emerged on the arena side were given colored, numbered tags, and tested in a learning assay. The learning assay was an array of yellow and blue flowers, with nectar in the yellow flowers and the blue flowers empty. Bumblebees generally (although often not!) innately prefer blue flowers. The assay examined how long it took bumblebees to learn to only visit yellow flowers. After a bumblebee had completed the learning assay it was then tagged with an RFID tag and moved to the side of the colony connected to the outside. As these bees then foraged outside they had to pass over a scale that weighed them as they exited and re-entered and recorded their individual ID from the RFID tag. By weighing the bees before and after foraging the authors were able to estimate the amount of nectar brought back to the colony by each bee, and they also visually estimated the amount of pollen bees were carrying.

What they found

First off, they found substantial variation in how long it took individual bees to learn to only go to yellow flowers. Bees that learned faster in the assay did not collect more nectar or pollen than bees that learned slower in the assay. But, as bees continued to forage in the wild, the amount of nectar they brought back over successive trips increased. Bees that learned faster in the assay, however, foraged for fewer days over their lifespan in comparison to slower learners. Because there is no difference in the amount of resources fast and slow learning bees brought back to the colony, slow learning bees that foraged for more days collected more resources for the colony than fast learning bees. 

The takeaway

The authors propose that being a faster learner has physiological costs that manifest as shorter lifespan. This would explain why bees that learned faster foraged for less days than bees that learned slower. But we generally think that learning should increase the foraging efficiency of bees. Why did the fast learning bees not bring back more nectar and pollen on their foraging trips than slow learning bees? Well, there are a number of possibilities. It could be that this "learn to avoid blue" assay is just not a relevant measure of learning for how bees use learning in the field. Maybe a test of learning to handle flowers or extract pollen would be a better measure. Another possibility is that the field conditions these bees were foraging in was just not challenging (or variable?) enough for learning abilities to produce variation in nectar and pollen collection. Maybe the pickings are so good that it doesn't matter how fast a learner you are, you can always collect plenty of food. The shortened foraging lifespan of bees that are faster learners is the most intriguing part of this research for me. We expect maintaining brain tissue to be costly, but measures of the costs of cognitive abilities for fitness are few and far between. Additionally, I think any study that measures the learning abilities of an animal using a controlled lab experiment and then ties that to foraging performance in the wild is taking behavioral ecology in the right direction!