Ecological and evolutionary processes commonly result in morphological variation among larval amphibians. Variation in head shape plays a critical role in both food capture and predation risk in gape-limited salamanders, yet in situ studies of head shape variability are rare outside of cannibal morph assessments. We examined allometry differences in larval head width (HW) and snout–vent length (SVL) among three sympatric species of ambystomatid salamanders from 166 ponds in Missouri, USA: Ringed Salamander (Ambystoma annulatum), Marbled Salamander (A. opacum), and Spotted Salamander (A. maculatum). We also tested whether several abiotic and biotic factors would predict HW after accounting for SVL. We found that larval HW and SVL were strongly correlated for all species but that the strength of this relationship varied among species. For early-stage larvae, Marbled Salamanders showed isometric scaling relationships, whereas both Spotted Salamanders and Ringed Salamanders were allometric. For late-stage larvae, all three species showed allometric patterns. At a small SVL, HW of small Ringed Salamanders was greater than the other species. As larvae increased in SVL, Marbled Salamander HW increased most rapidly and eventually exceeded both Ringed Salamanders and Spotted Salamanders of a similar size. We also found that both abiotic and biotic factors predicted significant differences in HW corrected for SVL among species, including predator density, competitor density, and hydroperiod. Overall, variability in scaling relationships may provide ecological advantages to each species at different points in ontogeny and different biotic and abiotic factors may induce such variation in asymmetric ways among species.

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