Christopher Auger

Anatomy and Physiology of Egg Laying Mammals

Egg-laying mammals are known as monotremes, and are commonly found only in one part of the world – Australia and New Guinea. There are only three species of this type of mammal, two of which are species of spiny anteaters called echidnas and the platypus. These three species belong to two families: Ornithohynchide and Tachyglossidea. In the early nineteenth century scientist considered these animals to be the link between modern mammals and reptiles.

The platypuses are an aquatic animal that feeds mostly from fresh water lagoons, lake and river bottoms. It has a duckbill shaped snout, a flat and broad tail, and webbed limbs. While the platypus is rarely observed on land, echidnas are a land-based animal. Like larger anteaters, they have a pointed snout and sticky tongues to catch ants and termites.

The male reproductive tract is similar to a normal mammalian reproduction system. However, while the female tract is similar to those of birds and reptiles, their reproductive systems produce both eggs and milk.

The reproductive systems of the male echidnas and platypus are similar to those of other mammals, but their testes are inside their body near the kidneys. This is often thought of as a reptilian trait. The tube goes from leads from the testes to the urethra, which ends in the penis. A small prostate gland produces seminal fluid. Platypus sperm was also found to have more reptilian characteristics than mammalian. Their sperm is long and slender with a filiform head much like reptiles (Griffiths, 1988). Mating in platypuses involves the male approaching the female from behind and curling his tail beneath her with his chest resting on her back.

While the male reproductive systems are very similar to that of other mammals, the female reproductive system is very much different. . The female reproductive tract is similar to those of birds and reptiles, however; their reproductive systems produce both eggs and milk. The eggs produced are very much like reptile eggs, but are unusual in that they exit the body through the same system used for the excretory systems (Dawson, 1983). The eggs then develop outside the mother’s body.

Their ovaries are like those of birds – in birds and platypuses both ovaries are present, but only the left is functional. Echidnas, unlike the platypus have two functional ovaries, but only one egg develops at a time. Once fertilized, the egg is transported to a uteri, it then enters the single urogenital sinus, then the cloaca, along with waste (Dawson, 1983). Unlike platypus, echidnas have a separate pouch where they incubate their eggs and nurse their young. The platypus also nurses it’s’ young, with milk secreted through large glands under their fur and not specific mammary glands. The milk produced from these glands is a “true milk” as it contains casein, whey proteins, carbohydrates, fat globules and minerals. Monotreme milk is very much like the milk produced by marsupials, and differs from the milk of placental mammals. This milk is especially high in iron, which is nectary because the liver of newly hatched monotremes is too small to store an adequate supply of iron (Griffits, 1988).

Monotremes eggs are similar to those of reptiles or birds but themselves have characteristics. The development of the egg is divided into three different stages. In the first stage the oocytes are 0.01 mm in diameter are located in the ovarian cortex. The outer covering is a single layer of cubiodal cells. This surrounds a thin layer, which surrounds the egg membrane.

At the second stage, the egg size is about 0.4 mm in diameter and the outer most covering of the egg is thicker. Fat droplets start to form; these will become the eggs yoke.

The last stage continues the development until the egg is about 3.9 mm. A layer two cells deep form a membrane now surrounds the egg. When