Frogs Are Amphibians and Spend Time Both on Land and in Water.Female

Frogs are amphibians and spend time both on land and in water.Female frogs are vulnerable to predation by fish when they enter the water to lay eggs.A hypothesis has been proposed that frogs rely on chemical detection of predators in addition to visual detection.In other words, frogs detect the presence of predator fish by chemicals released by fish into the water.In designing an experiment to test this hypothesis, a scientist constructed two test ponds that were identical except for the presence of compounds released by fish.This condition was the only _______ that differed in the two setups used in the experiment.