Brood stimulation controls the phasic reproductive cycle of the parthenogenetic ant Cerapachys biroi

Abstract.

Several groups of ants display a reproductive cycle in which two phases of adult activity alternate in synchrony with the brood instars. The brood stimulation hypothesis (Schneirla, 1957) was developed for ecitonine army ants to explain the proximate control of such biphasic cycles. According to it, onsets of cyclic activities are triggered by social stimulations arising from the developing brood, rather than by innate pace-makers inbuilt in adult ants. While it seemed to provide an acceptable explanation, this hypothesis failed to be experimentally demonstrated, in spite of numerous field observations. We used colonies of thelytokous populations of the phasic ant Cerapachys biroi as a model in order to test the brood stimulation theory. Brood removal and substitution experiments allowed us to confirm, first, that the periodicity of the cycle is not controlled by an endogenous rhythm in adults. Moreover, we could also characterise the influence of each brood instar on the activity of adult ants. Although we confirmed the existence of a brood stimulation involved in the control of the cycle, experiments revealed that it was not performed accordingly to Schneirla’s hypothesis. In effect, our study suggests a primacy of larval influence: the foraging phase was triggered and sustained by larvae- induced excitement rather than by stimulation from the newly-emerged callows.