Giant, stressed out, arthritic toads are taking over Australia

Invasive, yellow cane toads (Chaunus [Bufo] marinus) that weigh roughly 4.5 lbs each have been advancing across The Land of Down Under since 1935, and now occupy more than a million square kilometers, wreaking ecological havoc on native species as they go.

But toads on the invasion's frontlines, researchers from the University of Sydney have found, are stressed out by being foreigners in a foreign land.

The pressures of pioneering, and slogging an average of 130m daily during the rainy season, has led to spinal arthritis in 10 percent of the giant, poisonous toads. It's so severe in cases that the spine has fused. The arthritis is more common in individuals at the front of the pack, who tend to be larger and have longer legs.

The researchers explained the likely cause of the arthritis:

“Ironically, factors that have contributed to the toads' rapid spread across the continent also have rendered it susceptible to arthritis. First, cane toads are among the largest anurans. Larger body size provides advantages to a colonizer (e.g., a bigger animal can move faster, eat a wider range of prey, and withstand desiccation better and is less vulnerable to predators) but also increases susceptibility to arthritis [within lineages, larger species show a higher incidence]. Second, the process of invasion appears to have selected for larger toad body sizes on the invasion front, so an already large anuran has been selected for larger size. Third, behavioral adaptations to the invasion process itself [notably, the shift to a highly active lifestyle] place more stress on the locomotor system, including the spine. And fourth, biomechanical stresses are exacerbated by the evolution of longer hindlimbs, which deliver greater propulsive force with each hop.”

The study published in this issue of PNAS is the first to document arthritis in anurans and is a rarity in its study of the impact of invasion on the invaders. Most research focuses on the effects of the invaded.

PNAS study