Urban developmental environments alter tadpole phenotypes depending on origin

Urbanisation is rapidly altering environments with severe impacts on life on earth. While impacts from urbanisation are widespread, organisms with metamorphic life cycles are particularly likely to be affected, as they often undergo a major shift in lifestyle and habitat across life stages. Early life stages also likely undergo exceptionally high selection due to elevated mortality rates. Despite the potential importance of these early stages, we often lack insight into how urban conditions influence individuals early in their development. This study examined how exposure to urban environmental conditions alters developmental phenotypes in interaction with origin. We studied túngara frog tadpoles (Engystomops pustulosus) from urban and forest populations and conducted a reciprocal transplant between forest and urban treatments. Wild collected breeding pairs from urban and forest populations created clutches produced in the lab. Clutches were then split in two and exposed for 2 weeks to urban or forest developmental conditions. Tadpole developmental timing, morphology and vigilance behaviour were subsequently measured to assess the effects of origin and treatment, as well as their interaction. Urban developmental environments differ in water quality, are warmer, and have fewer predators compared to forest environments. Our findings demonstrate that tadpoles originating from urban populations have a faster developmental rate regardless of the environmental treatment. Tadpoles exposed to the urban environment have constrained growth compared to tadpoles exposed to the forest environment, independent of origin. Additionally, we find that urban origin tadpoles demonstrate increased behavioural plasticity in response to disturbance compared to forest origin tadpoles. Our results show effects of urban conditions on developmental phenotypes both within and across generations, most likely as an adaptive response to higher aquatic temperature regimes in urban areas. Divergence in juvenile growth and development may underlie divergence in adult phenotypes typical of adult túngara frogs found in urban and forest areas. These findings demonstrate the need to integrate early developmental stages to fully understand the evolutionary and ecological implications of urbanisation.