Tadpoles rely on mechanosensory stimuli for communication when visual capabilities are poor

The ways in which animals sense the world changes throughout development.
For example, young of many species have limited visual capabilities, but
still make social decisions, likely based on information gathered through
other sensory modalities. Poison frog tadpoles display complex social
behaviors that have been suggested to rely on vision despite a century of
research indicating tadpoles have poorly-developed visual systems relative
to adults. Alternatively, other sensory modalities, such as the lateral
line system, are functional at hatching in frogs and may guide social
decisions while other sensory systems mature. Here, we examined
development of the mechanosensory lateral line and visual systems in
tadpoles of the mimic poison frog (Ranitomeya imitator) that use
vibrational begging displays to stimulate egg feeding from their
mothers. We found that tadpoles hatch with a fully developed
lateral line system. While begging behavior increases with development,
ablating the lateral line system inhibited begging in pre-metamorphic
tadpoles, but not in metamorphic tadpoles. We also found that the increase
in begging and decrease in reliance on the lateral line co-occurs with
increased retinal neural activity and gene expression associated with eye
development. Using the neural tracer neurobiotin, we found that axonal
innervations from the eye to the brain proliferate during metamorphosis,
with little retinotectal connections in recently-hatched tadpoles. We then
tested visual function in a phototaxis assay and found tadpoles prefer
darker environments. The strength of this preference increased with
developmental stage, but eyes were not required for this behavior,
possibly indicating a role for the pineal gland. Together, these data
suggest that tadpoles rely on different sensory modalities for social
interactions across development and that the development of sensory
systems in socially complex poison frog tadpoles is similar to that of
other frog species.