Mechanism of synchronous metamorphosis: larvae of a rhinoceros beetle alter the timing of pupation depending on maturity of their neighbours

Synchronous development is widespread throughout the animal kingdom, but how synchrony is achieved remains largely unknown. I examined whether (1) a group-living rhinoceros beetle Trypoxylus dichotomus prepares for pupation (i.e. prepupates) synchronously in the field, (2) whether the synchrony occurs through social interactions, and (3) whether the synchrony incurs physiological costs. I found that larvae prepupate synchronously within natural humus sites. Laboratory experiments show that, when pairs of larvae are placed in the same cage, they prepupate on almost the same day, while two larvae chosen from different cages are expected to prepupate at 6-day intervals. I examined the mechanism of synchronous prepupation by inducing maturity asynchrony between two individuals. Less advanced larvae shortened the larval period in the presence of more advanced neighbours, whilst advanced individuals prolonged the larval periods in the presence of less advanced neighbours. However, variations in the prolongation or shortening of the larval period were dependant on the sites from which the larvae were collected, and in no site did both prolongation and shortening occur together. When the larval periods were prolonged or shortened, the body weight of the resulting pupae decreased. These data show that larvae of this species alter the timing of prepupation depending on the maturity of their neighbours. This developmental plasticity is likely to incur physiological costs due to pupation in suboptimal timing. The pupae and prepupae of this species may gain some benefits such as predator avoidance through the synchrony, which outweigh the cost in terms of reduced body weight.