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The Amami tip-nosed frog is a battle-worn survivor of an invasion of mongooses on its island home. The mongooses left their mark on the species, leaving the frogs more skittish towards potential threats.

Small Indian mongooses (Herpestes auropunctatus) were introduced to Japan鈥檚 Amami Island in 1979 to control the island鈥檚 rat and pit viper populations. A handful spread out from a single starting point, eventually multiplying to 6000 individuals and infiltrating much of the forested island. They preyed on 鈥� and dramatically reduced 鈥� populations of native wildlife like the Amami tip-nosed frog (Odorrana amamiensis). Following a 20-year eradication campaign, most of the mongooses have now been removed and the frogs have rebounded.

The situation was a great opportunity to see if the invaders influenced the frogs鈥� evolution, says Hirotaka Komine at the Tokyo University of Agriculture and Technology.

Komine and his team searched the island for frogs. When they spotted one, they would approach and record how close they could get before the amphibian hopped away.聽They found that in places with greater impact from the mongoose invasion, frogs bounded away from potential threats quicker than frogs from less affected areas.

The results suggest that the frogs evolved a heightened wariness in the wake of the invasion and this fear has persisted even after mongoose eradication, says Komine. The mongoose density on the island has been extremely low for at least five years, and the maximum lifespan of a tip-nosed frog is three years, so the frogs that were tested have probably never seen a mongoose. This means the skittishness is probably a genetic change, not a learned behaviour.

This amplified fear may influence the island鈥檚 local ecology because it could change how the frogs acquire their own prey. Komine thinks the fearfulness may fade over time, though.

Measuring fear this way may not provide a perfect glimpse into mongoose-driven behaviours, says Alexandra Carthey at Macquarie University in Australia. 鈥淭his method relies on humans as a proxy predator, and I wonder whether that makes sense for remote prey populations that have never or rarely encountered humans,鈥� she says.

The results match how some native Australian mammals have adapted in the face of invasive foxes and cats, says Carthey.聽A century and a half after the predators were introduced and started attacking native marsupial populations, the remaining marsupials seem to have evolved anti-predator defences, like bolting at the first sniff of cat or fox odours.

鈥淲hile invaders often have dramatic negative impacts initially, they do not continue to have high impacts, because the persisting native species have learned or adapted to them,鈥� says Carthey.