Nocturnal soil-surface behaviour of a giant earthworm in the Black Forest, SW Germany

Abstract

The behaviour of the large, deep burrowing earthworm Lumbricus badensis, endemic to the southern Black Forest, was studied in its natural beech and spruce dominated forest habitat. Using night vision cameras, nocturnal activities at the soil surface were recorded of adult individuals, over six consecutive nights in early summer. Behaviour consisted of sub-midden activity (near-surface, worm unseen), foraging, and mating, which accounted for 60, 34 and 6% of activity, respectively. Less than 1% of activity was over surface dispersal. Sub-midden activity, likely consisting of litter incorporation, feeding and casting, usually started before all others and continued throughout the night. This activity may relate to the role of the midden as an external rumen where the incorporated, initially recalcitrant, litter becomes more palatable. Foraging started later in the night, and apart from collection of and feeding on small litter items, it also involved displacement of large objects such as spruce cones. Surface litter samples collected from paired areas of high- and low-density L. badensis showed that higher density areas contained significantly less beech leaves and woody litter. The five recorded mating events all occurred after midnight and involved pre-copulatory exploratory behaviour. Mating itself was unlike that previously reported for earthworms and involved no mutual ventral alignment and long-lasting close contact. Instead, individuals attached in turns to the partner's clitellar region with anterior segments and only for a short period, with an equally short intervening period between the two attachments. This unexpected observation warrants further in-depth study. Rainfall was shown to interrupt aboveground activity, which may also be reduced on cloudless, moon-lit nights. With climate change already affecting Black Forest ecology and the indications of detrimental effects of increasing summer droughts on L. badensis juveniles, there is some urgency to better understand the environmental controls and conservation needs of this enigmatic ecosystem engineer.