Chapter 3. Division of labor

3.4. Models of division of labor

There are currently three main models to explain the age-related division of labor in social insects.

1. Foraging for work: Tofts and Franks published a paper in 1992 that drew a controversy. Their foraging for work model basically works like this. They want to find the most fundmental mechanism, so they purposely ignore physiological and genetic effects on behavior. They wanted to see (through math and modelling) if using simple behavioral rules can generate age-related division of labor, in absence of other factors. Assum that an ant colony is an assembly line, workers emerge from one end (broodnest), through some intermediate tasks, and eventually out foraging. Because the broodnest is now the source of young ants, and the outside nest (foraging) is the "sink" for old foragers (since foragers die at a much higher rate), there is a natural "flow" of workers from the source to the sink. Workers would keep performing the same task(s) unless they become unemployed, when they then seek for other jobs that are nearby. Because of the direction of worker flow, ants tending brood will be replaced by younger ones and move on to the next task in that assembly line. Indeed the model generated weak age polyethism, similar to what is observed in ants. Critics of this model say that this model does not work for honey bees: first the nest is not as linear, second, there is overwhelming evidence of physiological changes with age that can explain more. In fact if one gathers one group of foragers, and one group of day-old bees, randomly mix them, then introduce them another colony (this has been done), one will observe that foragers will keep foraging but young bees will stay in the brood nest. Notice here that one has removed the effect of location (both young and old bees introduced into the same location in the assembly line, if that existed).

2. Response threshold: There are much evidence that workers have variable response thresholds for different chemicals, thus it can be assumed that workers will also have different response thresholds for certain behaviors. Old workers respond to alarm pheromone more than younger bees, even though their isolated antenna respond similarly the chemical. Undertakers appear to be more sensitive to odors emenating from dead bees, so they are the ones that do the dirty job. Page and Robinson suggested that because a colony has many patrilines, each with different distributions of response thresholds, then the whole colony would show division of labor, i.e. generate specialists because some bees have lower threshold to the sttimulus associated with a task, will perform this task, which then reduces the stimulus level, such that other, higher-threshold bees would never have a chance to perform this job. The response threshold model can explain many phenomenon in insect societies, however, it still does not explain why a nurse will eventually become a forager: what changes her threshold such that the forager would ignore brood but go foraging and vice versa?

3. Social inhibition: Recall the question from last session. Clearly the two facts (precocious foraging occurs when a colony is made only of young bees, foragers revert to nurses if a colony is made up entirely of foragers) hinted that the colony is trying to maintain a homoestatic setpoint so that a group of foragers is always available, yet not everyone becomes a forager. How can this be accomplished? A simple negative feedback mechanism would play the trick. What if bees can sense whether there are foragers in the colony? if foragers are absent, young bees become foragers precociously to fill this void. Conversely, if everyone is a forager, do bees need to sense the lack of nurses? Not so according to Huang and Robinson, they proposed a simple inhibition model that can explain both facts with two elements: 1). that each worker will automatically increase their tendency to become a forager, if there is no force retarding this tendency and 2). this retarding force (inhibition) comes from other foragers. They thought that this tendency to become foragers was driven by a chemical (an activator), and thought JH was the activator. The inhibitor comes from foragers, and seems to be chemical in nature (still unidentified). This model can explain nearly all observed data, but the inhibitor remains elusive.

Question for discussion: How do you design one experiment to test which model (foraging for work or social inhibition) is at work in a honey bee society?

The next page "Testing of the social inhibition model" is optional. It details some experiments done by Huang and Robinson to see if social inhibition indeed works in a colony setting.

[previous page] [table of contents] [next page]