It was by a skin - sent by Captain John Hunter, governor of the penal colony in Sydney, to the Literary and Philosophical Society of Newcastle- upon-Tyne in 1798 - that the platypus first came to the attention of European biologists. The accompanying letter suggested that it was "a small Amphibious Animal of the Mole KIND".
It was another 34 years before direct evidence for the platypus's mammary glands was obtained - milk was seen oozing from them in a specimen accidentally killed. Then it took another 55 years before it was realised that the platypus laid an egg. It is an egg-laying mammal with a funny broad beak. The Royal Society, which published the key early descriptions, has maintained that tradition, and has just published nearly 200 pages of a symposium that it organised on the subject.
Much of the early development of the platypus takes place within the uterus and the embryo is quite advanced when the egg is laid. A clear head region and a well-defined trunk are already visible, with the early development of fore and hind limbs visible as bulges. The egg is incubated for about 10 days and then hatches. The hatchling has a special curved egg tooth in its upper jaw to let it break out of the shell and this is lost the following day. They look rather odd, like baby bears, and it is another few months before the definitive platypus features are established.
The bill is of great interest for it gives us clues about the psyche of the platypus, since it is a unique sensory organ. There seems to be nothing particular about the animal's hearing or vision. But the bill has a high density of receptors on its surface that can respond to both mechanical and electrical stimulation. These receptors together can provide the platypus with a complete three dimensional fix on its prey. The electrical detectors respond to the faint electric fields generated, for example, by shrimps. This signal is received before the mechanical receptors pick up the mechanical waves travelling through the water from the moving prey and because of their later arrival it is possible for the distance of the prey to be estimated by complex neural circuitry in its brain. With this equipment, it is capable of finding and eating half a kilo of freshwater invertebrates each night underwater with its eyes shut and ears closed.
The platypus enjoys surprisingly deep sleep which is not easily disturbed. But do they sleep like other mammals? Our sleep, like that of all mammals, is divided into so-called REM - Rapid Eye Movement - sleep and non-REM sleep. REM sleep is supposed to be when we dream while in non- REM sleep, neuronal activity is reduced. The function of these two types of sleep is not clear, but both are normally required. It has been suggested that REM sleep has evolved in relation to higher mental functions, but cats and armadillos, though they are not known for their intellectual achievements, have lots of REM sleep. Does the platypus, so much further down the evolutionary and intellectual scale (or so we think) have REM sleep? It does, showing typical periods of eye, head and neck twitching.
The platypus is apparently a solitary animal except during the breeding season. Though there are a few reports of males sharing a burrow there is no evidence of any social organisation. Males do have a a nasty pair of venom glands on each rear ankle which can cause severe pain to other platypuses, predators, and humans. They can be very aggressive when handled. With the arrival of settlers in Australia things have become tougher, and they are now killed by cars, dogs, foxes and plastic litter.
So what, one wonders, do they dream about? Shrimps? Like those who study them, it certainly is not money, but both have interesting lives.Reuse content