Soldier Bees

The social insects-ants, termites, some wasps, and some bees-somehow never fail to surprise me and challenge what I think I know about biology. I can't claim expertise, but here's a whack at some zoology.

Social insects seem to defy our notion of what it means to be an organism (not to mention their other quirks, like ant agriculture). In some senses you can treat individuals merely as appendages of a larger "superorganism." This is because the vast majority of members in a nest, whether it's a beehive, ant colony, etc., are sterile. If you think about it, all of multicellular life is in an analogous position: most cells are dead ends. Only gametes will pass into the next generation; our muscle, liver, and brain cells die with us, to name a few.

Social insects are just a rung higher on the hierarchy; individuals, in addition to cells, give up their reproductive autonomy. Multicellular organisms have responded to their position by investing resources in specialized cells and tissues. In the same way, social insects have specialized individuals. Honeybees are a familiar example and personal favorite since I am a hobbyist beekeeper. In each hive there are many workers, several drones, and a single queen. However, workers don't sub-specialize into castes. Instead they adopt a pre-programmed sequence of roles beginning with taking care of the brood and ending with foraging outside the nest.

Unlike some ants, honeybees don't have workers that sub-specialize at birth. The extent to which some ant species do specialize is remarkable. Below at right is a honeypot ant worker, a member of a specialized caste of nectar storers that hang from the nest wall-fully alive. Honeybees don't do this, since each worker follows the same progression of jobs.

However, specialization of the ant type (though much less extreme) was recently discovered for the first time in a bee species¹. The species is Tetragonisca angustula, known to Brazilians as Jataí, and members of the species are small and stingless. The researchers noticed that, given the short 20-day lifespan of an average worker, guards station themselves at the hive entrance for an unusual amount of time. While honeybee workers guard the entrance for only a single day in their sequence of jobs, Jataí guards stay for at least five. Combined with the observation that guards appear larger than foragers (the other, larger group of bees), this led them to hypothesize that guards are a separate caste.

The authors of the paper tested this hypothesis by measuring comparing mass, head size, and leg length for guards vs. foragers. They found that guards are 30% heavier than foragers and that they also have measurably larger heads and longer legs. The guard bees represent only about 1-2% of the 10,000 total workers in a nest, a ratio decided by the bees that tend brood (caste is determined by the environment). You can see the size difference for yourself in the image at the top of the page. They further discovered that bees responsible for removing waste from the hive were of intermediate size, and so might represent a third distinct caste.

Learning about natural history should be a fun and humbling experience for biologists or biologists-in-training like me. The beauty and complexity of animals really is unequaled.

Image Credits: Honeypot worker: San Diego Zoo; Jataí workers: Grüter et al. (Figure 1B)

References:

1. Grüter, C. et al. A Morphologically Specialized Soldier Caste Improves Colony Defense in a Neotropical Eusocial Bee. PNAS. Published online 9 January 2012.

2. BBC. Scientists Discover Soldier Bees.

Suggested Reading:

1. Delaplane, K. First Lessons in Beekeeping. Dadant & Sons, Inc., 2007.