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The invasive cane toads that live in Australia鈥檚 cities have developed different physical traits from those in the countryside, and these changes may be due to rapid adaptation to their differing lifestyles.

鈥淚n general, urban environments have less predators than rural environments,鈥� says at Yamagata University in Japan.

He and his colleagues collected nearly 600 cane toads (Rhinella marina) from 18 locations in and surrounding three cities in northern Queensland, Australia. Along with body length and mass, the researchers measured the length of the toads鈥� rear leg bones and the toxin-producing parotid glands behind their heads.

They found that city toads had smaller poison glands than their country cousins. The researchers think urban toads may be able to get by with a reduced chemical arsenal because there are fewer predatory birds in the city than in the countryside 鈥� something the team confirmed with bird surveys.

The leg bones in male city toads were longer than those in their rural counterparts. The fragmentation of urban habitat with buildings and roads, coupled with males鈥� relative propensity to wander, may be responsible, says Komine.

鈥淲hen toads move among habitat fragments, they may have to move further in a single trip to overcome such barriers鈥�, hence the longer hoppers, he says.

Komine says this is the first empirical study to demonstrate that an invasive species has responded to urbanisation with physical changes to its body shape.

Cane toads were introduced to Australia in 1935, so any divergence between city and country toads is quite recent. It is still unknown if the physical differences are heritable, evolutionary changes, or from more plastic environmental effects. Komine says this should be determined in future studies.

鈥淚nvasive species are often considered in the light of the economic or environmental damage that they cause,鈥� says at the Swiss Federal Institute of Aquatic Science and Technology in D眉bendorf. 鈥淏ut this study adds to the growing body of research that highlights their value for studying evolutionary patterns and processes.鈥�