Animals shed their skins for various reasons – from disposing of tough outer coatings that have grown too small to replacing worn out tissue that no longer serves its purpose. During these transitions, many animals are vulnerable to predators, while amphibians – which depend on their leaky skins to maintain a healthy internal balance of salts and water – could be at risk if their skins become even leakier, allowing salt to seep out while water soaks in. Intrigued by the challenges faced by skin-sloughing cane toads during their not-so-quick transition, Craig Franklin, Nicholas Wu and Rebecca Cramp from The University of Queensland, Australia, measured salt loss across the amphibian's skin and how the salt-transporting properties of the skin change when a toad replaces the old with the new.
Bathing cane toads in fresh water and measuring the water conductivity during the period between moults, in the 12 h preparation period before the moult, during the moult and after, Wu found that the salt loss rocketed almost 200-fold from 0.5 μS h–1 to 90 μS h–1 when the old skin had been sloughed off. However, when he checked the toads’ blood stats, the animals appeared to be unperturbed and had not lost essential salts, despite the increased skin leakiness. Puzzled, Wu measured the amount of salt flowing into the toad's body through specialised protein channels that pump salt against the natural gradient, and was impressed to see that the inward flow had doubled after the animal had shed its skin. And when he investigated the quantity and location of the pump proteins through the different stages of the moult, it was clear that the animals were producing more of the essential pumps and locating them deep in the skin after the old skin had been shed to maintain a healthy internal salt balance.
So, leaky cane toads install protein pumps to protect themselves from salt loss while they replace their skin. However, the team warns that other skin-sloughing amphibians may suffer if they begin replacing their skins more frequently in a bid to rid themselves of harmful skin infections, such as the deadly skin-attacking chytrid fungus, which threatens amphibian populations world-wide.
