Researchers say cane toads may be rapidly adapting to urban environments – with changes seen in their poison glands and the length of their legs.
Distinguished Professor Lin Schwarzkopf, Head of Zoology and Ecology at James Cook University, was part of a team that examined how cane toads are changing in cities.
"Urban areas have been expanding rapidly, and this rapid expansion has caused drastic environmental changes over recent decades.
"Natural habitats, such as parks and gardens, are often isolated from each other by artificial barriers like buildings and roads and this isolation can make evolution quicker," said Professor Schwarzkopf.
She said many studies have examined the impacts of urbanisation on biodiversity, but scientists were only beginning to study urbanisation as an evolutionary force.
Professor Schwarzkopf said one instance of this was seen in toads.
The team collected more than 400 cane toads from three different sites in north Queensland and measured them.
"We found that the parotoid glands, which are the major anti-predator defence of toads, were smaller in urban than in rural populations.
The tibiofibula (a bone in the leg) length of males in urban populations was longer than those in rural populations, but females showed opposite trends," said Professor Schwarzkopf.
She said there may be fewer predators of toads in urban environments compared to rural environments and this may be why producing toxins was less important for city toads. There were fewer bird predators of toads in urban environments, but further work on predation rates is required to support this hypothesis.
"Fewer predators may mean urban toads have decreased their energy investment in anti-predator defences. If so, this change has occurred rapidly, within the past 85 years, as toads were introduced to north Queensland in 1935," said Professor Schwarzkopf.
She said a longer tibiofibular bone may allow males to move faster as they roamed further afield around fragmented urban environments.
"When toads roam around urban areas, they may have to move further in a single trip to overcome barriers such as roads and buildings. Given that male toads tend to move more often than females, males with longer tibiofibulas, and therefore better movement ability, may be favoured in urban environments.
"However, more sedentary females may not experience a similar benefit," said Professor Schwarzkopf. Further research on the causes of these changes in leg lengths is required to determine what may be driving them.
In any case, the results demonstrate that urbanisation drives the size, shape, and structure of invasive toads, suggesting they may rapidly be adapting to urban environments.
