Platypus and rakali are both top predators in Australian freshwater systems and probably compete to some extent for food. However, the size of prey that can be efficiently killed and consumed by an adult platypus is limited by the fact that its bill contains only rough grinding pads to process food (see Platypus diet and food consumption). In contrast, a water-rat has a formidable set of chisel-shaped teeth at the front of its mouth that are used to kill and dismember prey. Interestingly, the grinding surfaces of water-rat molars are relatively flat and smooth (Watts and Aslin 1981). Like the grinding pads of the platypus, the shape of rakali molars may serve to help process aquatic invertebrates.
In practice, little is known about the ecological and behavioural interactions between platypus and rakali. It’s sometimes been suggested that rakali may prey on young platypus, there there appears to be no supporting factual evidence. Coexistence of rakali and platypus has been documented in many places, so rakali clearly don’t exclude platypus automatically from freshwater environments (or vice versa). However, rakali are much more likely to occupy highly degraded habitats than the platypus, particularly in urban environments. This likely reflects the fact that rakali are willing to forage to some extent on land, enjoy human food scraps and can also dine on introduced fish species such as common carp Cyprinus carpio.
For details of a documented incident in which a platypus killed a rakali see Platypus News & Views edition 71 (2018 February) page 2.
Photo courtesy of B. Vickers
LITERATURE CITED
Watts CHS and Aslin HJ (1981) The Rodents of Australia. Angus & Robertson: Sydney.