Peace in ground beetle larvae: non-aggressive outcome in Chlaenius spp. larvae interactions

Abstract

Cannibalism reduces the number of competitors and provides high-quality food, but it may have costs related to the risk of injury, disease/parasite transmission and reduction of inclusive fitness. Carabid larvae are usually very aggressive, with very low tolerance of conspecifics. Thus cannibalism is common. We investigated aggressive and cannibalistic behaviour in the larvae of eight ground beetle species, especially Chlaenius velutinus and Chlaenius spoliatus whose larvae have long articulated cerci. Chlaenius species live in dense populations with frequent intra-specific encounters, whereas encounters are rare in the other species. Dyads of conspecific larvae of the eight ground beetles species were tested for aggressive and cannibalistic behaviour. Unlike the other six species, larvae of the two Chlaenius species avoid cannibalism, possibly as a consequence of a behavioural display involving cerci interactions. This display was never recorded in conspecific larval interactions of the other species. Instead, cannibalism consistently occurred (20 to 50%) following aggressive interactions (1.2 to 2.6 attacks/event). The non-aggressive outcome in Chlaenius larval interactions contrasts with the high aggressiveness (35 to 85% of trials) shown by the other species. This suggests the presence of an inhibitory mechanism in the Chlaenius species. The cerci interaction probably inhibits cannibalism via intra-specific recognition; this behaviour may have evolved under the pressure of ecological factors (resource abundance and dispersion, larvae density, frequency of encounters). In Chlaenius spp. ecological constraints may have led to an inconvenient costs/benefits trade-off, while in other species they did not exert negative pressure on the occurrence of cannibalism.