As general rule, monotremes are nocturnal or crepuscular, and the best time to watch them is around dusk and dawn. They are active by night in order to avoid the heat of the day. However, daily activity is dictated to some extent by climate and the degree of disturbance. The two species whose range extends into temperate parts of Australia, the duck-billed platypus and the short-beaked echidna, are often active by day, especially in winter when the nights can be quite cold.

Cold weather and food shortages can induce the short-beaked echidna to enter short periods of torpor—a deep sleep, during which metabolic processes slow down and energy is conserved. The species is also the only monotreme capable of full hibernation. In most parts of the species' range, this is never necessary, but in the Snowy Mountains of New South Wales, winters can be sufficiently long and harsh so that echidnas spend up to four months asleep. They may wake periodically to investigate their surroundings, and even move to another den before going back to sleep. During hibernation, the echidna's body temperature may drop as low as 39.2°F (4°C), much lower than during shallow summer torpor. A hibernating animal uses very little energy, but even so, a long winter can take a serious toll on an individual's reserves of fat, and it may emerge in spring some 10-20% lighter than when it first began hibernating.

Detailed studies of the duck-billed platypus and the long-beaked echidna are hampered by the fact that these are rather shy and elusive animals. The same cannot be said of the short-beaked echidna, which is bold by comparison and especially tolerant of humans. In fact, Australian echidnas have few enemies and little to fear from any predator. They are too big to be threatened by cats or even foxes, and their sharp spines are usually sufficient to deter large dogs such as the dingo or birds of prey. If a large animal approaches too close for comfort, the echidna curls its body into a tight ball and raises its spines for protection. Should evasive maneuvers be necessary, the echidna can burrow extremely quickly, literally sinking into the ground until all that remains are a few protruding spines. Despite their lack of spines, adult platypuses are large enough to have few natural predators.

The platypus spends much of its time hidden away in waterside burrows, and emerges to feed only in the quiet hours after dusk and just before dawn. It tends not to travel far from home and usually slips straight into the water. When moving on land, the platypus uses a brisk but relatively inefficient waddle, but it is a superb swimmer and spends much of its time beneath the surface. Its perfectly streamlined body is propelled swiftly and silently through the water with the large webbed front feet. The back feet act as rudders and brakes, and the animal is able to twist and turn with a speed and agility comparable to that of a bird in flight. The platypus returns to the surface every minute or so to take a breath, but it does so silently and without a splash. At the start of each dive, it rolls forward in the water, its sleek form barely breaking the surface. Echidnas can swim, too—their spines help make them surprisingly buoyant and they can make rapid progress in water using a steady doggy paddle.

The monotremes are for the most part solitary animals, except for mothers with young. Single animals occupy a home range that may overlap with those of several others, but they are not territorial and show very little interest in each other except during the breeding season.

Less is known of the New Guinean long-beaked echidna than its two Australian cousins. It too is nocturnal and generally lives alone.

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