A test of information use by reproductive bumblebee workers

Abstract

Kin-selected conflict over reproduction in social groups is predicted to occur when group members are unequally related to progeny. Whether kin-selected conflict is expressed may depend on individuals' ability to obtain and use information about their social environment to make decisions affecting their fitness. In some colonies of the bumblebee Bombus terrestris, workers delay laying male eggs until approximately 21 days after the queen starts (at the ‘switch point’) to lay male eggs. This delay is difficult to explain with kin selection theory, because workers are more closely related to their own sons and so should commence reproduction immediately after the switch point. Bourke & Ratnieks (2001, Proceedings of the Royal Society of London, Series B, 268, 347–355) hypothesized that the delay occurs because workers detect the sex only of older larvae (ca. 10 days after emergence from the egg) and hence are delayed in detecting the switch point. We tested this hypothesis by adding male larvae greater or less than 10 days old to each member of nine pairs of B. terrestris colonies. We found no significant difference between the treatments in the timing of the start of worker reproduction, which occurred ca. 19 days after male larvae were added. The results therefore failed to support the hypothesis. We conclude that workers have access to information that would allow them to take a reproductive decision enhancing their inclusive fitness, but for unknown reasons do not act on it. Workers may use other cues to begin reproduction, possibly a queen signal (cessation of pheromone production) to female larvae to start development as queens.