Rosenthal's bee hygiene behavior experiment

Rothenbuhler’s honeybee hygienic-behavior inheritance experiment used foul brood, an infectious disease of bee grubs, to show how a complicated behavior could depend on distinct inherited influences. Its central finding was that hygienic removal of infected larvae involved at least two separable steps—uncapping their cells and throwing them out—and that the genes associated with those steps could cooperate in a hive’s behavior while passing independently through generations.

Disease and behavioral variation

Foul brood attacks grubs in their cells. Among domestic bee breeds used by beekeepers, some strains were more susceptible than others, and the difference could be behavioral: hygienic strains located infected grubs, removed the wax caps covering their cells, pulled out the larvae, carried them through the hive entrance, and discarded them. Susceptible strains failed to perform this sequence, allowing infection to persist.

The behavior was therefore more than a single visible response. A worker had to find a diseased grub and uncover it, then continue by removing and disposing of the larva. That sequence gave Rothenbuhler a way to distinguish partial performance from complete hygiene and to investigate how the component actions were inherited.

Crosses and the three outcomes

Because worker bees do not ordinarily reproduce, Rothenbuhler crossed queens from one strain with drones, or males, from another, and assessed the behavior of the daughter workers. All first-generation hybrid hives were non-hygienic. The hygienic behavior of the parent strain appeared to have vanished, but the experiment’s later results indicated that the relevant genes remained present as recessive traits.

Rothenbuhler then back-crossed the first-generation hybrids with a pure hygienic strain. Their daughter hives fell into three behavioral groups: some showed complete hygienic behavior; some showed none; and some uncapped the cells of diseased grubs but did not remove the larvae. That intermediate group revealed that the behavior could be divided into steps rather than treated as a single inherited package.

Rothenbuhler proposed that one gene was associated with uncapping and another with throwing out the larvae. The halfway-performing bees, he inferred, had the uncapping gene in double dose but lacked the throwing-out gene. He further predicted that some bees classified as wholly non-hygienic might carry the throwing-out gene but be unable to express that behavior because they did not first uncover the cells.

The cap-removal test

To test that prediction, Rothenbuhler removed the wax caps himself from diseased-grub cells. Half of the bees in the apparently non-hygienic group then displayed normal throwing-out behavior. The intervention exposed a hidden capacity: without uncapping, the removal response could not be observed, even when the inherited influence associated with throwing out was present.

The result supports a distinction between the observable behavior of an individual hive and the separate genetic influences that can contribute to it. Uncapping and removal were functionally connected, but the experiment showed that they need not be inherited as one indivisible trait.

Significance for gene and behavior

The experiment serves as a model for how genes might influence behavior without being its sole or immediate cause. A gene associated with uncapping need not directly command a worker’s movements; its effect could operate through development or even learning. The account offers a hypothetical possibility: bees might acquire a taste for infected wax, making them more likely to eat the caps and repeat the action. The relevant claim is comparative—that, other things being equal, bees carrying the gene are more likely to uncap than bees carrying its alternative.

The two proposed genes also illustrate cooperation among genes in a shared body. The throwing-out influence is useful only if the cell is opened, while uncapping alone does not complete hygienic removal. Yet Rothenbuhler’s crosses and cap-removal test suggest that the influences remain separable in inheritance. The experiment thus supports a careful formulation of genetic explanations of behavior: genes can make complex actions more likely through interacting developmental pathways, without any one gene single-handedly producing the entire behavior.