Chapter 3. Division of labor

3.3. Factors affecting division of labor

1. Plasticity in age polyethism: Despite of the steorotypical nature behavioral development, age polyethism is also highly flexible. Individuals can accelerate, delay, or even reverse their behavioral development in response to changes in the colony's internal and external environment. Workers can start foraging as early as one week old, if a colony is artificially made with only newly emerged bees. These workers are called precocious foragers because of their young age. Precocious foragers can also be observed in natural colonies, although only infrequently. Conversely, workers can become foragers when they are much older than 3 weeks. One extreme example is bees in temperate areas (in the mid-western United States, for example): workers that emerge during October never have a chance to become foragers in the fall. They live for at least five months, because the earliest flowers would only be available in March or April the next year. Delayed development also occurs in a new colony founded by a swarm, because new workers would not emerge until at least 21 days after the queen starts laying eggs, so the youngest nurses around that time would be at least 21 days old. Even more striking is "reversion", when colony age demography is drastically changed. In this case, old workers become physiologically "younger" and engage in behaviors that are normally performed by younger bees. For example, one can remove all the young bees in a colony, and leave only foragers. In this colony, bees that have been observed foraging on the previous day revert back to taking care of queen and brood.

2. Genetic determination of division of labor: Because a typical honey bee colony has only one queen, a honey bee colony can be seen as one large "family", with everyone as the offspring of the queen. However because each queen can mate with up to 25 drones, a honey bee colony also has many subfamilies. Each subfamily shares both mother and father, but across subfamilies, they only share the mother and not the father. The subfamily is also called "patriline" because of this paternal lineage.

Starting about 1988, scientists (Calderone and Page) have discovered that age of first foraging (essentially the rate of behavioral development) and pollen foraging are partially genetically determined. This was then extended to specialized tasks (guarding and undertaking). That is whether a worker will become a guard or undertaker partly depends which drone she came from. To conduct this kind of study, researchers artificially inseminate a queen with 2-3 drones with identifiable genetical markers (such as body color, or allozyme). One can measure the proportion of the 2 or 3 patrilines in a colony and compare the distribution of guards or undertakers among the same patrilines. If a colony is made of 30% of subfamily A and 70% B, but the undertakers are distributed as 80% from A and 20% from B, and this difference is significant, then we say subfamily A is more likely to become undertakers because of the genetics. So far a genetic effect has been found for foraging distance (Oldroyd), dancing characteristics, egg-laying propensity by workers (Ratnieks).

3. Physiological correlates of division of labor: Scientists have been trying to find what "drives" the behavioral changes with age. Fluri and colleagues measured tires of juvenile hormone (JH) in workers and found higher JH in older bees and suggested that JH might play a role in behavioral canges. Around 1974-76 Jaycox injected honey bee workers with a juvenile hormone (JH) or JH analogs and found this stimulated workers to become foragers earlier compared with the control group. From 1985 on, Robinson did a more refined study, showing that JH titers not only increase with age, but also are linked to task-performance: precocious foragers also showed high JH titers even though they are the same as age as nurses, foragers reverted to nurses also dropped their JH titers. He futher showed that JH modulates the response threshold to alarm pheromone, with bees treated with JH showing a better and earlier response to the pheromone. Measuring response of antenna to the pheromone showed no difference, suggesting that it is the central nevous system that is changed with the changes of JH. For a while, JH has been proposed to be the driving factor, pushing workers to become foragers. If this is true, then we would expect some workers would stay home forever if we have a way to getting rid of their JH in the system. One student (J. Sullivan) of Robinson did such a study, he removed the corpora allata, the sole source of JH in bees, and observe their behavior. These bees, with no JH at all, still become foragers, only 2-4 days later than the sham control (operated but CA not removed). This study suggested that JH is only one step in the system and bees might have redundent pathways, IF THERE IS A PHYSIOLOGICAL factor swithing foraging behaviors on and off. More recent studies have shown that biogenic amines (octopamine, dopamine) may have a role in division of labor: foragers and nurses (regardless of age) show different amine amounts in the brain, and also feeding amines to bees induced earlier foraging.

Question for discussion: We already know that if you remove all old bees, younge bees will start foraging earlier, and if you remove all young bees, young bees will "revert" to become nurses. What do these results tell you about the possible mechanisms of regulation? How do you design an experiment to test your hypothesis?

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