Parental defence behaviour can reduce the probability of nest predation, but also incurs costs (e.g. less time for foraging). Under variable risk, parental strategies that adjust the amount of defence effort to the prevailing risk will be adaptive. Although nest predation is typically viewed as an all-or-none event, many nests are depredated only partially. Therefore, partial brood loss will indicate the occurrence of nest predation and, as a consequence, high predation risk. However, whether parents use loss of offspring as a cue of predation risk and increase their defence effort after partial brood loss has not been examined. The study examined whether a foundress of a paper wasp Polistes chinensis antennalis Pérez increases defence effort after the experimental removal of a larva from the nest. The foundress maintains a nest alone during the colony-founding stage. At this stage, larvae in the nest are often depredated by conspecific females of other nests. The foundress can chase off attacking females when it is on the nest, but it needs to leave the nest to gather resources such as food. For 30min after the experimental removal of a larva, foundresses increased the time spent on the nest by shortening the time spent on each off-nest activity and prolonging the interval between off-nest activities. The time spent on the nest then returned to what it was before the removal. These results suggest that foundresses use the loss of a larva as the cue of high predation risk and increase their defence effort in response.
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