An urban evolutionary island
In central Melbourne, the Yarra River flows, brown and busy. It seems an unlikely place for one of nature's most reclusive animals, the platypus (Ornithorhynchus anatinus). Yet, a small population persists here, separated from their upstream relatives. Genetic studies reveal these urban platypuses are an isolated group, cut off by weirs and other man-made barriers. This isolation has created a distinct genetic cluster, effectively a landlocked island in the middle of a metropolis.
This population faces pressures unknown to their counterparts in cleaner, more remote streams. Urban stormwater runoff introduces pollutants and causes sudden, high-velocity flows that can destroy riverbank burrows and wash away their food sources. A 2018 study found that 48% of the Yarra catchment's waterways provide 'very poor' habitat conditions for the platypus. The quality of the vegetation on the riverbanks, important for stable burrows where they raise their young (called puggles), is also in poor condition in the more urbanized lower reaches of the river. Despite these challenges, the platypuses' continued presence shows an adaptation to city life. Their numbers in the lower, urbanized sections of the Yarra have declined, and they are now considered a vulnerable species in Victoria.
A world of electricity and venom
The platypus navigates its murky, underwater world using a sense that few other mammals possess: electrolocation. When a platypus dives, flaps of skin cover its eyes and ears, and its nostrils seal shut. It hunts effectively blind and deaf, relying instead on its remarkable bill. This soft snout contains about 40,000 electroreceptors arranged in stripes, which detect the tiny electrical fields generated by the muscle contractions of its prey. By sweeping its head from side to side, it can pinpoint the location and distance of invertebrates like insect larvae and freshwater shrimp hiding in the riverbed.
This unique mammal is also one of the few that is venomous. The male platypus has a sharp, keratinous spur on each hind leg connected to a venom gland. The venom's composition changes during the breeding season and is potent enough to cause severe pain in humans. This adaptation is primarily used in competitive fights between males. The species is a monotreme, an egg-laying mammal, a trait it shares only with echidnas. This combination of ancient reproductive strategies and highly specialized sensory adaptations makes its survival in a modern cityscape a significant focus for urban ecologists and conservation biologists.