Competition and response to predation risk: effects on morphological and behavioural defences in anuran larvae

Abstract Many freshwater organisms exhibit responses to conspecific density. Although a reduction in antipredator responses under high competition conditions is a well-documented pattern, most studies focus on species from temperate zones, in the case of anurans, primarily in those species that produce eggs in gelatinous masses or gelatinous strings. Research in subtropical regions remains limited. We investigated how competition intensity affects predation risk, shaping the morphological and behavioural defences in Physalaemus cristinae tadpoles, a prolonged breeder species that constructs foam nests. As a prolonged breeder, this species experiences sustained predation pressure over several months, influencing the dynamics of its intra- and interspecific interactions. Therefore, understanding this aspect of its life history is crucial for assessing the universality of this pattern across different ecological contexts. Initially, we assessed alterations in morphology, growth, and development. Variables included larval densities (4 and 8) and predator odour presence (with and without). We measured five morphological traits (body length, tail fin length, body depth, tail fin depth, tail fin muscle depth), growth, and development rates. Furthermore, we evaluated behavioural parameters: tadpole activity recorded before introducing predator odour and at 15, 30 and 45 min post-signal introduction, and tadpole location, calculated as the proportion of larvae in different areas based on the predator odour introduction point. At low densities, tadpoles exposed to predator odour displayed significant increases in all morphological traits compared to those not exposed. Conversely, at high densities, no differences were observed regarding predator odour. The results show that conspecific density can play a significant role in antipredator response. Tadpoles change morphology and behaviour in response to predator odour, but their response is dependent on tadpole density. The understanding of these complex interactions will enable the maintenance of long-term functionality and resilience in aquatic environments, which provide numerous ecosystem services.