Environmental and ecological factors influencing dive behaviour in the freshwater snake Acrochordus arafurae: a field-based telemetric study

Acrochordus arafurae is a fully aquatic, freshwater snake distributed throughout tropical Australia. To better understand the ecological factors influencing their behavioural repertoire, we remotely monitored field body temperature and diving in snakes free-ranging within their natural habitat. The body temperatures of A. arafurae exhibited a diel profile similar to the surface water temperature, and reflected the high proportion of time that snakes remained <1 m from the surface. The average dive depth was 0.62 m and 95% of dives had an average depth of 1 m or less. Snakes occasionally ventured into deeper water (>6 m), and there was a positive correlation between dive depth and duration. Average dive duration was 6.6 min and 84% of dives were terminated within 10 min, but all snakes performed dives >50 min during the 14-day observation period. We hypothesise that the dive behaviour was strongly influenced by predation pressure. The snakes partake in short dives within the aerobic dive limit to reduce the amount of time they need to spend at the surface on each breathing bout, reducing the risk of predation by birds. Predation is a strong selective force that might alter the time allocation during dive cycles.

Predator or prey? The dive response to aerial and aquatic predators of Arafura filesnakes

In air-breathing aquatic animals, predation is a strong selection pressure that may be reduced by modification to diving patterns. The risk of predation increases with time spent at the surface, thus shorter, more frequent surfacing events or longer, less frequent surfacing events would decrease predation risk. A reduction in time spent on the surface can be achieved by use of bimodal respiration, which is an ability to extend dive duration using dissolved oxygen to supplement aerially acquired oxygen. Air is a more efficient respiratory medium; however, under predation pressure, the cost of surfacing increases and the reliance on aquatic gas exchange should therefore increase. We tested whether the bimodally respiring filesnake (Acrochordus arafurae) changed its diving behaviour under simulated aerial (model bird) and aquatic (large fish) predation. Aerial predation did not alter dive or surface duration, percentage time surfacing or activity. However, a greater number of longer dives were observed with fewer long surface intervals, suggesting an increase in the use of aquatic gas exchange. The diel diving patterns (short night dives, long day dives) may provide an in-built antipredatory response to aerial predation. The threat of aquatic predation produced atypical antipredator behaviour, with longer surface intervals, shorter dives and increased activity, indicating that piscivorous filesnakes may have identified the predatory fish as prey rather than a predator.