New research examining the diet of Japanese giant salamanders has revealed that these enormous river-dwelling amphibians experience a dramatic change in what they eat as they grow.
Endemic to Japan, the immense animals can reach up to 1.5 metres in length and weigh up to 25 kilograms. According to researchers at the University of Liège in Belgium, it’s roughly halfway through the salamanders' growth that their diet suddenly shifts, turning them from secondary consumers midway up the food chain into top predators.
The discovery was made following a study on 160 Japanese giant salamanders (Andrias japonicus) from the Ichi River watershed in Hyogo Prefecture.
By analysing the giant salamanders’ stomach contents, and measuring their carbon and nitrogen levels, the team were able to work out how their diets changed throughout their growth.
“The results reveal a spectacular dietary transition,” says researcher Clément Duret, from the Laboratory of Ecology and Conservation of Amphibians (LECA) at the University of Liège.
“Young, small salamanders feed mainly on aquatic insects. Their trophic position is similar to that of secondary consumers, an intermediate level in the food chain, as is the case with most salamanders.
“However, when individuals reach an average length of 62 centimetres, a sudden shift occurs in their diet.”
Once the salamanders grow beyond this threshold, the food they eat becomes much larger, with fish, frogs and freshwater crabs making up most of their diet.
In terms of mass, crabs occupy the majority of the space in the salamanders’ stomachs, but their nutritional value is low, explains Duret. “Conversely, fish, although less common in stomach contents, are a much more efficient source of energy for these predators.”
This dramatic change in diet places the salamanders at the very top of the food chain in the Ichi River ecosystem.
The animals’ gigantic size is a product of evolution, say the authors, who explain that morphological changes to the skull – including the enlargement of the jaw – allow larger salamanders to tackle prey that is too big for smaller, younger individuals.
Japanese giant salamanders are classified as vulnerable. Understanding how they behave and what they eat is vital for guiding conservation strategies – for example, preserving prey populations in the Ichi River, and indeed other waterways that they live in.
Learn more about the study: A giant's appetite: how body size drives the diet and trophic position of the Japanese giant salamander, published in the journal Oikos.
Top image: Japanese giant salamander in a Japanese river. Credit: Getty
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