Water in the bank

Natural History, May, 2007 by Nick Atkinson

For wallabies in Australia's Northern Territory, putting a nose in the river often leads to a lot more than just a refreshing sip of water. In some areas, saltwater crocodiles (which can also live in brackish waters) are common, lying nearly submerged in the water to ambush the thirsty and unwary. But the agile wallaby has found a way to get a safer drink, according to a study by J. Sean Doody, an ecologist at the University of Canberra, and two colleagues. Not only do they visit the river at times of the day when the "salties" are relatively inactive; the cunning marsupials have also figured out that it's safer to dig a drinking hole in the riverbank than to sip from the river directly.

A shallow pit in the soil a yard or more away from the river quickly fills with water. By recording wallabies' behavior at the drinking holes with motion-sensitive cameras and studying footprint patterns on the riverbank, Doody found that they much preferred the holes to the river.

He also discovered that the wallabies appear to respond to variable risk: where the crocodiles were numerous, the wallabies sited their holes farther from the water's edge and dug them deeper than where the crocodiles were scarce. Wallabies, it seems, are happy to invest extra energy to avoid becoming a hungry reptile's dinner. (Ethology)