We induced early hatching using two types of cues. The earliest hatching in A. callidryas occurs in response to hypoxia at 3 days, whereas the onset of predator-induced, mechanosensory-cued hatching occurs later, usually at 4 days (Warkentin et al.,2017). To induce hatching of 3-day-old embryos, we used boiled, degassed water. In one developmental series,we tested all clutches for hatching competence at 3.75 days by submerging three eggs per clutch in hypoxic water in sealed glass vials. We removed tadpoles as soon as they hatched; eggs that did not hatch after 45 min were considered ‘not hatching competent’ and returned to air. We induced hatchingof 4-day-old embryos using mechanosensory cues delivered by jiggling embryos with a moistened blunt probe. As in the hypoxia-induced hatching protocol, a subset of each clutch was induced to hatch at 4 days and followed through development for comparison with siblings that remained in the egg.For specimen collection, tadpoles were euthanized with an overdose of buffered MS-222 before fixation.To prevent hatching of embryos during specimen collection, we chilled them on ice until they were unresponsive, then manually decapsulated them with forceps before preservation in ice-cold fixative.