Mammals of the Waterways

When the marsupials first scurried out of the forests of Antarctica and onto Australian soil, the mammalian fauna of the continent was completely different. A range of weird and ancient lineages scurried through the continent's verdant Paleocene forests, all thriving in an isolated environment at the end of the world where there were few competitors to hinder them. However, a metatherian newcomer would spell their doom. Over the next few million years, the marsupial descendants of a single common ancestor that arrived in the latest Paleocene would diversify and proliferate, and all of a sudden the isolated world that had allowed these more ancient mammals to survive was filled with competition. Soon, all other non-chiropteran mammals would be wiped out. All but one.

Despite their adaptability, there was one environment that presented a special challenge for marsupials - the water. Because their offspring spends the majority of development outside the body attached to a teat, any time spent in the water by a mother will kill the young very quickly. The only marsupial to have overcome this so far is the didelphine Yapok of South America, which developed strong muscles around the mouth of the pouch in order to keep it watertight when the mother is submerged. Australian marsupials never developed an adaptation like this, which meant that their domination did not spread to the waterways of the continent. In the absence of the marsupials, another mammal managed to survive - the platypuses. This ancient group, hailing from before the Cenozoic, would soon become the last lineage of non-therian mammals on earth, and even managed to reclaim the land later in the form of echidnas. Their body plan, perfected in the Mesozoic waterways, would experience very little change across the course of the Cenozoic (aside from the evolution of echidnas of course) and carry the lineage easily into the Holocene.

While they did experience some decline, the platypus survived man fairly well, and would continue on as they always have right up into the present in the mid Recurocene, remaining the dominant benthic foragers of the continent's waterways. Since the Holocene the platypus lineage has seen an increase in diversity, with three species now present, as well as an expansion in range, finally spreading north and claiming New Guinea. The Eastern Platypus (Ornithorhynchus familiaris) of eastern Australia and New Guinea is extremely similar to the Holocene platypus in both appearance and behaviour, having required little change in order to successfully fill out its niche. Variable in size, there are no subspecies of the Eastern Platypus but different populations have different body sizes depending on the latitude, with those from the southern end of their range being twice as large as those from New Guinea. But even the largest individual is smaller than any Grey Platypus (O. australis). Native to the Tasmanian Peninsula and some adjacent areas of the south-east, this southerly species can weigh up to three kilograms, and are descended from the equally large platypuses that inhabited then isolated Tasmania in the Holocene. Their larger body size helps contain warmth in the freezing southern waterways they call home.

The largest platypus of all however, at least in terms of length, lives on the other end of the continent. The vast Arafura Floodplain is the home of the most divergent platypus of all, the Floodplain Platypus (Dolichorostrum palustris). Reaching up to 80 centimetres in length, the rather elongate body of this species is a direct reflection of its habitat. Unlike the rivers and streams platypus usually inhabit, the floodplains are very densely vegetated, and so a bottom-feeder like a platypus must be able to weave and wiggle through debris as easily as possible, and the elongate, flexible body of the Floodplain Platypus allows this. During the wet season the floodplain sports a bounty of worms, molluscs, and crayfish, capable of supporting a high population of platypuses. However, everything changes once winter comes and the dry season commences.

The monsoonal rains that feed this lush environment cease with the coming of winter, and the landscape changes dramatically. The water recedes to pools and rivers, and the majority of the floodplain becomes covered in dry open grassland, a habitat completely unsuitable for platypuses. To cope with this, the platypuses employ a tactic reminiscent of many Australian frogs and turtles, digging into the mud while it is still wet. Once they are a metre or so below the surface they construct a small hibernation chamber in which they will spend the dry months. The ground around them hardens like rock, and this deep in the ground they are insulated from the winter sun's fierce heat, and so they can sleep peacefully right through winter, living off fat stored in their tails. Once rain comes and water floods the plains once more, the surrounding ground softens and the platypus awakens, quickly digging back up and commencing their regular behaviour.

The competition-free waterways of Australia not only provided a safe haven for monotremes, they would also present opportunities for other mammals that managed to reach the continent. Sometime in the latter half of the Miocene, a small rodent was cast adrift from south-east Asia and made landfall somewhere on Australia's northern shores. Here, in a land never trodden by land-bound placentals before, rodents would quickly diversify into a variety of new niches and forms, and it wasn't long before they began to take advantage of the lack of aquatic mammals in Australia's waterways. A whole new lineage of rodents adapted to life in the water sprung up - the water rats (Hydromys group). These almost otter-like murines are among the world's only truly carnivorous rodents, preying on everything from worms and snails to waterbirds and even other rodents. During the Holocene the group was far more diverse than platypuses, spanning six genera and a dozen species, and during the Recurocene it is no different. New Guinea, the birthplace of the group, remains the hotspot of diversity for the water rat lineage, with two-thirds of all species being endemic to this forested region. Six species extend into the rest of the continent, with only two being endemic - the Western Rakali (Hydromys occidentalis) of isolated south-western Australia and the Murray-Darling Rakali (H. odontocrassus), which is endemic to its namesake river basin. Both are descended from the Rakali or Australian Water Rat, which is the ancestor of four out of the six species found outside of New Guinea

The most extraordinary of all water rats is undoubtedly the Whiskren (Typhlomys tenuis). Possessing small arms and lacking any visible eyes or ears, this is the most aquatically adapted rodent on earth, only leaving the water to rest and raise their pups. It is descended from the bizarre and elusive Earless Water Rat, which had already begun to display many weird adaptions to its aquatic lifestyle. Whiskerns still possess functioning eyes, but they are tiny and hidden beneath a mass of fur, and so can only be used to sense light. Instead, their long, extremely sensitive whiskers guide their way underwater, able to detect the slightest movement. They have no external ears. Propelled by their huge webbed hindfeet and sensing the presence of prey with their whiskers, they emerge after dark in many of New Guinea's largest rivers for worms, small crustaceans and crayfish. They spend their day deep inside a burrow with an underwater entrance, which they dig out with their powerful incisors given that their arms are now too small to be used for digging. They can hold their breath and remain submerged for nearly half an hour, and when active they only really return to the surface to breathe.

Recently, another group of mammals on the other side of the placental evolutionary tree has begun to take to the water. In the aftermath of man's disappearance, competition between canids and felids was fierce as each family diversified and fought over niches in Australia's ecological vacuum. In eastern Australia, some cats began to avoid competition by foraging for prey in shallow streams and along riverbanks, pouncing on fish and waterbirds as they passed. The Rivercat (Felipotamus glaucos) is a descendant of those cats, and now inexplicably found in and along waterways. Far less specialised for aquatic life than platypuses or water rats, the Rivercat represents a species in transition, inbetween a terrestrial lifestyle and a semi-aquatic one.

Although still very reminiscent of land-hunting felines, the Rivercat is beginning to show the first signs of adapting to its new, waterbound lifestyle. The head and muzzle have elongated, the legs have shortened, the ears have developed the ability to close underwater, the webbing already present between the toes has expanded, and they can hold their breath comfortably for several minutes. Although they will prey on land-dwelling animals if the opportunity arises, most of their prey are aquatic animals like fish, waterbirds, frogs and crayfish, and sometimes water rats, platypuses, file snakes or baby crocodiles if they can catch them. They employ two main strategies when hunting. The first is to walk slowly along the waterside, watching the water intently for any sign of life. When prey is spotted, the cat will leap in to subdue it, aiming to land right on top of it and snatch it before it can escape. If they miss they may pursue their quarry for a short distance, but they are far slower swimmers than their prey and it will not take them long to give up. The second strategy is mainly employed to catch their favourite prey - waterfowl. Sighting a flock sitting on the surface, the cat will enter the water and dive down, hoping to approach unnoticed. Once within striking distance, it will ambush a bird from underneath, gripping with its claws and killing with a bite to the throat.

Rivercats occur across eastern Australia, New Guinea, and on the eastern and northern outskirts of the Arafura Floodplain. They are almost invariably found in wooded environments near rivers, lakes and wetlands. The size of a small-clawed otter, they are vulnerable to larger predators, and if threatened or spooked they will vanish under the water or up a tree. Largely crepuscular in habit, they sleep through midday and midnight, although they may be active throughout an overcast day or moonlit night. They sleep in dense waterside vegetation or, even better, a nearby rock crevice or small cave, but during the breeding season in winter a female will seek out a sheltered underground den in which to raise her young, often taking over the burrow of a platypus, water rat or marsupial. Here she gives birth to a litter of three to five kittens, which she raises on her own.