Two queens?!
Last week, I tweeted that one of my newly installed observation hives had two queens. Unfortunately, I only have these not-so-great-quality-phone-through-Plexiglas photos. But, you can see that one queen has a fading, white paint mark on her back, and the other does not.

How can this be? Each hive is only supposed to have one ruling monarch, right? Usually.
As is typical in nature, there are some exceptions. So, what can lead to a "two queen" hive?
In my case, I probably had an old, waning queen (the one with the fading paint mark) and a young, spry queen (the unmarked one). Sometimes, a new queen is cordial enough to let the old queen spend her final days in the hive. Since the old queen is on her last leg anyway, she’s not a threat. When the old queen gets tired enough, she will leave the new queen to her reign. As of Tuesday, the marked queen was nowhere to be found. The unmarked queen is going strong and laying lots of eggs!
Another way a hive can become doubly-ruled is if the hive is exceptionally tall (or long). Honey bees live in darkness; the main way they communicate is via scent, or pheromones. The queen mandibular pheromone (QMP) is a special pheromone secreted only by the queen (as the name suggests). QMP is detected by the workers and signals “You have a ruler”.
When a hive gets really tall, however,the worker bees struggle to detect their queen. If the queen occupies the top of the hive, the bees at the bottom might not be able to detect her QMP. Or maybe what little QMP the workers can detect is weak. Thinking they’re queenless (no QMP) or that their queen is unhealthy (weak QMP), the bottom-dwelling bees raise a new queen and voila! You’ve got a hive with two queens.
In doing some reading about two queen hives, I found that some beekeepers make their hives into two queen hives on purpose. The rational is: if you have two queens, your hive will have twice as many eggs, which leads to twice as many workers, which can then make twice as much honey. Seasoned beekeeper Michael Bush describes a few different ways to create and keep a hive with two queens.
For the purposes of keeping my research as ecologically-relevant as possible, I think I’ll stick to one queen hives. (Even though the prospect of double the honey is enticing!)
