Egg laying mammals and their distinction

It is very interesting to notice that in the animal kingdom though the mammals were considered as highly evolved they have the subclass ‘prototheria’ which has the egg-laying mammals, and are the most ancestral forms in the class ‘mammalia’. There are only five extant species grouped into two families and a single order, the ‘monotremata’. Despite bearing fewer species than most mammalian genera, the prototherians are unique and probably split from the lineage leading to other mammals sometime in the mesozoic. They retain many characters of their therapsid ancestors (for example, a complex hip structure, laying of eggs, have one opening for excretion and urination etc.). The skulls of monotremes are almost birdlike in appearance, with a long beak and smooth external appearance. Monotremes have several important mammalian characteristics, however, including fur (but they lack whiskers), a four chambered heart, a single dentary bone, three middle ear bones, and the ability to produce milk(lactate) among mammals’. This group looks like a combination of reptiles, birds and ‘true mammals’.

This bizarre array of combinations makes them very interesting and also very hard to pin down to one specific class. It is only based on modern genetics we are able to put them firmly within mammals because they have the same y-chromosome based determination of the sex of an individual. This group is mostly solitary with at least one species (short-beaked echidna) being territorial. Echidnas live on the ground and eat ants, termites and worms whereas platypuses spend most of their time in the water using their beaks to dig out worms from the stream beds. All extant species are exceptional diggers, using their powerful limbs to dig shelters. These burrows are a defence against predators as well as a great place to lay their leathery eggs. They can stay dormant for long periods of time if food sources are scarce. Very little is known about the mating habits of prototherians except that they are found in pairs during the breeding season.

Hearing, touch and smell are all important in prototherians. The platypus may even use electrical signals to find prey hidden underneath the stream or riverbed. A good sense of hearing is also important for echidnas as they forage along the forest floor looking for prey. Very simple sounds that can emanate from beaks are sometimes used but we do not know their functions well enough to discern whether they are mating calls or for protection of territory.

These are primitive creatures and have not had to evolve too much from the original ancestors so one is quite amazed to note that they have few protections against predation. Echidnas have spines, very much in the hedgehog fashion and functions in a similar way. Platypi (young platypus) has a bit of venom that it can inject from the spur of its hind-feet. Although, venom is a rarity for mammals to possess, it is not much against sophisticated predators like man. These animals are not aggressive in nature and will not attack if left unmolested. Due to human intervention and the wholesale destruction of habitat at least three species of echidna are at the brink of extinction. They are listed as critically endangered by the International Union for the Conservation of Nature (IUCN) redlist. Platypi were being hunted extensively for their fur but recent conservation efforts have helped steady their population.