Throughout the world, invasive species remain one of the most serious threats to native wildlife. Perhaps the most infamous example of such an alien invasion is that of the cane toad (Bufo marinus) in Australia. Released in 1935 to combat sugarcane pests, cane toads proved themselves to be adept at reproducing rapidly in their new environment and, worse still, took quite a fancy to many of Australia's native wildlife, viewing them as a tasty snack. A recent study, by Rick Shine and his team at the University of Sydney, Australia, demonstrates that a barrier that once was thought would slow down the toads' expansion – low temperatures at high altitudes – is actually dealt with rapidly and successfully by the toads, their secret trick being incredibly rapid cold acclimation.
The study, published in Functional Ecology, noted that cane toads were being recorded up to 1100 m above sea level, with the animals experiencing considerably colder conditions than their native counterparts in central and southern America. Collecting animals from the field, Shine found that the critical thermal minima (CTmin), which identifies the temperature at which toads lose their righting reflex, was lower for high-elevation toads than for low-elevation toads – by a significant 2°C. However, a quick stay in a temperature-controlled laboratory abolished this difference in CTmin between the high- and low-elevation groups. This is because the ability to cope with the cold, it turns out, is achieved through acclimation, not adaptation. Surprisingly, a toad's CTmin in the laboratory was not affected by collection site, or 1 month's exposure to warm (24°C) or cool (12°C) conditions. Instead, a toad's CTmin was determined by the conditions it experienced over the previous 12 h, prior to experimentation.
These mountaineering toads at the forefront of their expanding range make rapid changes to their thermal tolerance, being able to adjust to cooler temperatures within only a few hours of exposure. Not a few months, a few weeks, or days even, but just a few hours. Such rapid physiological plasticity is highly impressive and suggests that many aspects of the toads' physiology warrant further investigation. What Shine and his team have achieved is to identify a mechanism by which an invasive species can continue to expand its population into significantly colder regions than were previously thought possible. Moreover, the cold acclimation that accompanies this expansion occurs at incredibly fast speeds. Shine explains, ‘It's the speed of the acclimation that is so incredible. Other amphibian species have evolved to tolerate low temperatures or acclimate over a time course of weeks, but the toads are really impressive.’
This study suggests physiological adaptations must be taken into account when predicting the range expansion of alien species and, in particular, the speed at which such adaptations can occur. For now, it looks like the cane toads will continue their march through Australia, with no thermal underwear required. Asked whether nowhere is safe from the toads, Shine says, ‘I think the polar bears are safe… at least for now’.
