A giant’s appetite: How body size drives the diet and trophic position of the Japanese giant salamander

In predators, growth often drives ontogenetic dietary shifts (ODSs),
leading to increasing trophic position (TP) with body size as growing
individuals gradually incorporate larger prey in their diet. In species
exhibiting extreme size variation, particularly those with gigantism, TP
may increase markedly with body size, as large individuals might gain
access to prey considerably higher in the food chain and inaccessible to
smaller conspecifics. This can ultimately lead to apex predator status in
the largest individuals. In this study, we investigated for the first time
ODSs in one of the world’s largest amphibians, the Japanese giant
salamander (Andrias japonicus). We combined stomach content and stable
isotope analysis (δ¹⁵N, δ¹³C) from 160 individuals across a broad size
range to quantify dietary patterns and TP changes. We found a non-linear
increase in TP with body size, from approximately 3.0 to 5.1, with a
marked inflection point at a snout–vent length of 39 cm. This threshold
corresponded to a clear dietary transition: from primarily consuming
aquatic insects, to feeding predominantly on fish, anurans, and freshwater
crabs. This transition likely reflects morphological and physiological
adaptations associated with gigantism, enabling the exploitation of large
prey. Our findings suggest that gigantism may be adaptive in predators
such as giant salamanders by promoting ecological opportunities, allowing
individuals to access high trophic levels through extensive growth and
ultimately function as apex predators. These results contribute to a
broader understanding of the ecological consequences of body size
evolution in predatory vertebrates, highlighting how extreme growth can
reshape species' ecological roles.