Trajectories of groups of Rana latastei tadpoles

Predation is a strong driver for the evolution of prey behaviour. To
properly assess the actual risk of predation, anuran tadpoles mostly rely
on water-borne chemical cues, and their ability to evaluate environmental
information is even more crucial when potential predators consist of
unknown alien species. Behavioural plasticity, that is the capacity to
express changes in behaviour in response to different environmental
stimuli, is crucial to cope with predation risk. We explored the defensive
behaviour of Italian agile frog (Rana latastei) tadpoles when exposed to
the chemical cues of two predators’ species, one native (dragonfly larvae)
and one alien (red swamp crayfish). Firstly, we observed if a sensible
life history trait (i.e. hatching time) might be affected by native
predatory cues. Secondly, we recorded a suite of behavioural responses
(activity level, lateralization and sinuosity) to each cue. For assessing
lateralization and sinuosity, we developed a C++ code for the automatic
analysis of digitally recorded tadpole tracks. Hatching time seemed not to
be affected by the potential risk of predation, while both predator
species and diet affected tadpoles’ defensive behaviour. Tadpoles
responded to predator threat by two main defensive strategies: freezing
and “zig-zagging”. While the first behaviour had been previously reported,
the analysis of individual trajectories pointed out that tadpoles can also
increase path complexity, probably to prevent predators from anticipating
their location. We also recorded a decrease in lateralization intensity,
which suggested that under predation risk tadpoles tend to scrutinize the
surrounding environment equally on both sides.