Rachael E. Bonoan, Ph.D. candidate, Tufts University
When you see a honey bee buzzing from flower to flower, she (the honey bees you see on flowers are female) is searching for food to bring back to the hive. Honey bees get protein and fats from pollen, and sugars from nectar. They also get trace amounts of vitamins and minerals from both sources. When searching for food, taste is an important tool—some types of sugar are toxic to honey bees! Also, honey bees need to collect certain proteins to raise healthy young—they likely find these proteins by taste testing flowers as they collect food.
Unlike humans, honey bees are not limited to what they can taste with their tongues. Instead of taste buds like the ones on our tongues, honey bees have small hairs with taste receptors. These hairs are found on a bee’s antennae, mouthparts, and feet. Honey bees can taste with their feet!
In addition to collecting pollen and nectar, honey bees also collect water to bring back to the hive. Water is used to keep the hive cool on hot summer days and is an important ingredient in “brood food”—a mixture of nutrient sources that is fed to the developing young. Beekeepers often put out a clean dish of water for their honey bees, only to find that their bees prefer drinking water from the nearby swimming pool, the neighbor’s compost pile, or the stagnant birdbath. To us, these water sources are “dirty”; beekeepers are often afraid that these dirty water sources are going to make their bees sick. But, if bees can use taste to determine which sugars to avoid and which proteins to collect, they can probably use taste to figure out which water sources to avoid and which water sources to collect. Still, the question remains: why do honey bees like dirty water?
Since honey bees only get trace amounts of minerals from floral sources (pollen and nectar), and many dirty water sources likely have salts and other organic compounds, I hypothesized that honey bees drink dirty water to obtain minerals that their floral diet may lack. So, with the help of my colleagues, I trained honey bees to feed from artificial feeders in a natural setting. Once the bees were trained, we filled the artificial feeders with different mineral solutions, and let the bees drink from the feeder(s). These “preference assays” were run in both summer and fall. If honey bees drink from dirty water sources to supplement their floral diet, we would expect to see a difference in mineral preferences between the two seasons; floral resources are abundant in the New England summer, but not in the fall.
And that’s just what we found! No matter the season, honey bees preferred water containing sodium over plain deionized water. This makes sense: there is not a lot of sodium in pollen and nectar. When it came to water containing either calcium, potassium, or magnesium, an interesting pattern emerged. In the fall, honey bees preferred these three mineral solutions. In the summer, the bees avoided these three solutions relative to plain deionized water. It turns out that calcium, potassium, and magnesium are the three most prominent minerals found in bee-collected pollen, and their levels vary with season. In the fall, these minerals are found in low levels in pollen while in the summer, these minerals are found in high levels in pollen. The fact that our bees preferred water containing these minerals in the fall when pollen only contains low levels of these minerals, but not in the summer when pollen contains a higher amount of these minerals, suggests that honey bees use mineralized water sources to supplement their floral diet. In other words, dirty water is like Gatorade for honey bees.
You might be wondering, how do honey bees determine which mineral is which? Just like with pollen and nectar, our observations suggest that honey bees might be able to discriminate between mineral water sources based on taste. If you watch this video of one of our preference assays and focus on the bee who starts at the tube in the top right corner, you will see that she samples each tube in that column until she decides and settles on the tube in the bottom right corner. If it seems surprising how quickly she makes her decision before moving on to the next tube, remember that she can taste with her feet!
Since our mineral solutions were only contained 1% of each mineral, it is surprising that honey bees can distinguish between them. Genome sequencing tells us that honey bees have relatively few taste receptor genes—only 10. For comparison, humans have over 50 taste receptor genes! Despite their seemingly limited taste repertoire, our recent study shows that honey bees can likely discriminate water sources based on nutritional content, even when those nutrients are at low concentrations. The next time you see a honey bee drinking from what you’d consider a dirty water source, don’t be alarmed: she knows what she’s drinking!