Circadian and ultradian locomotor activity rhythms in the coastal dune beetle Pimelia depressa (Coleoptera: Tenebrionidae) from Rimel, Tunisia

Tenebrionidae beetles are a conspicuous component of arid and semi-arid environments. This study investigated locomotor activity rhythms in male Pimelia depressa collected from the coastal sandy dunes of Rimel Beach, Bizerte, north-eastern Tunisia. Locomotor activity was monitored under a natural light–dark cycle (LD), followed by constant darkness (DD). The results revealed a predominantly diurnal activity pattern, with both ultradian and circadian components under both conditions. Locomotor activity patterns varied considerably, ranging from unimodal to multimodal rhythms, irrespective of season or experimental condition. The circadian period was approximately 24 h under LD and lengthened to around 26 h under DD. Rhythms were most stable and clearly defined in spring under LD, whereas rhythm stability increased in winter under DD. Under entrained conditions, the main activity peak occurred around midday in all seasons, with the highest percentages recorded in winter (86%) and autumn (83%). Under DD, activity was concentrated around subjective dusk in winter (67%). The lowest overall activity levels were observed in winter under both experimental conditions. These findings highlight seasonal variation in locomotor activity and suggest behavioural plasticity in P. depressa in response to changing environmental conditions.